CA2071048A1 - Telephone dial-inbound data acquisition system with demand reading capability - Google Patents

Abstract

TELEPHONE DIAL-INBOUND DATA ACQUISITION SYSTEM WITH DEMAND READING CAPABILITY

Abstract A telephone dial-inbound data acquisition system with demand reading capability. The system has particular application for remotely reading utilitymeters. A remote data unit (RDU) dials up a host computer on a periodicbasis. The RDU is assigned a callback start time by the host computer. TheRDU calculates an actual callback time based on the callback start time and arandomly generated time delay. The RDU dials up the host computer via a modemwhen the local time, as shown by an on-board real-time clock at the RDU, equalsthe calculated actual callback time. Meter reading data is then transferred tothe host computer. The RDU also contains circuitry for detecting a pollingsignal generated by the host computer. Upon detection of this polling signal,the RDU immediately dials back the host computer and transfers utility meterreading data to the host computer. The RDU will also immediately dial back thehost computer if an alarm condition occurs, e.g. tampering or a low batterycondition. The RDU contains circuitry for characterizing the on-hook andoff-hook electrical characteristics of the telephone line to ensure that theRDU does not interfere with normal operation of a customer's telephone line.

Description

2~71048 83.102/2029 TELEPHONE DIAL-INBOUND DATA ACOUISITION SYSTEM WITH DEMANP READING CAPABILITY

Back~round of the Invention Field of the Inve~tion The invention relates to the field of remote data acquisition and, more particularly, to a system for periodically communicating data acquired by aremote data unit over a dial-up telephone line to a central computer and havingdemand reading capability.

Prior Art There have been various attempts to provide remotely interrogable data acquisition units for the purposes of remotely and automatically reading datastored at these units. For example, in the utility industry, a remote dataunit (RDU), also known as a meter interface unit (MIU), can be located at acustomer's home or manufacturing site for accumulating utility consumption data(electricity, water, or gas) and for communicating this information back to acentral computer located at the utility's home office. This type of systemenables the utility to automatically and remotely read a customer's utilitymeter without having to send a person to the customer's location to physicallyread the meter.

Prior art automatic meter reading systems have generally used one of three forms of communication media. These are radio (RF), two-way interactive cabletelevision (CATV), or dial-up telephone lines.

An example of an RF system is shown in U.S. No. 4,614,945, in which a mobile van interrogates an RF transponder attached to a meter to take the meterreading. However, this system suffers from the drawback that a vehicle must bedriven within a few thousand feet of the meter in order to take its reading.-Another drawback is that the RF transponder located on the meter is notsuitable for use indoors due to attenuation of the low power 900 MHz signal - ,, . ,. .~ .

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207~048 used for communications. This limits the use of this device to meters whichare located outdoors. In many areas of the country, however, meters(particularly water meters) are located indoors in basements.

The second type of system, which uses two-way interactive coaxial cable, is exemplified by U.S. Pat. Nos. 4,504,831 and 4,707,852. In these systems aremote data unit located at a customer~s site periodically acquires utilitymeter consumption data and transmits it back to a central computer over a cabletelevision coaxial cable. One drawback to this system is that it can onlyoperate with a two-way interactive cable system since signal amplification isrequired fro~ the customer's location back to the cable head-end. Also,two-way cable systems are relatively uncommon and therefore their use inautomatic meter reading systems is limited.

Because of the foregoing limitations of RF and cable systems, the most effective remotely interrogable automatic meter reading systems in existencetoday utilize the dial-up telephone network as the communications medium.Telephone-based automatic meter reading (AMR) systems generally fall into twocategories: so-called telephone "dial-outbound" systems and telephone"dial-inbound" systems.

A telephone dial-outbound system is exemplified by U.S. Pat. No.
4,582,152. In this system communication is established from a central (host)computer to a remote data unit located at the customer's site and connected toutility meters. Access to the RDU is made through a special subscriber testtrunk which enables the host computer to be connected to a particular RDUwithout ringing a customer's telephone. This system has the advantage that thehost computer can dial and access any RDU in the system at any time. Thisenables a utility to make a meter reading at-will, for example, when a customerhas his service disconnected for the purposes of rendering a final bill.However, this system has the drawback of requiring special test trunk accesscircuitry which has to be installed at each telephone exchange within autility's service area. This special circuitry enables the host computer tocommunicate with the RDUs connected through the particular exchange withoutputting a ringing signal on the customer's line. Another disadvantage is thatthis particular arrangemeilt requires the cooperation of the local telephone company to purchase and install the special circuitry at each exchange.

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. 3 . 2071~48 An alternative to a telephone dial-outbound system is the telephone dial-inbound system. An example of such a system is shown in the above-mentioned U.S. Pat. Nos. 4,504,831 and 4,707,852. In one embodiment, apseudorandom number generator is provided at the RDU which is used to ran~omlyselect a callback time for the RDU. When this callback time occurs, the RDU"wakes-up", dials the host computer's telephone number via 8 modem provided inthe RDU and, upon making the connection with the host computer, transmits meterreading data back to the host. Randomization of the callback times is requiredsince there can be several thousands of RDUs in a system. If all RDUs were tocall back at the same time, only one would actually get through. By spacingthe frequency of callback times sufficiently (e.g. once every thirty days) andusing an appropriate pseudorandom number generation algorithm, the callbacktimes can be spread out over the thirty day period, thus minimizing collisionsin callback times. If a collision does occur, the RDU is programmed to pausefor several seconds and retry the connection. If no connection is made after apredetermined number of retries, the RDU is reset.

One drawback to the aforementioned system is that throughput of the system is not optimized. Because of the nature of the randomizing sequence, it ispossible that after an RDU is reset, it will call back in as few as two minutesor in as much as thirty days after the previous callback time. Without knowingthe approximate time of reporting at the central computer, the database ofmeters which are expected to report in is difficult to manage. Furthermore,utilities generally try to schedule a reading of a customer's meter to fall onapproximately the same day every month, so this system does not lend itself toaccommodating a utility's usual meter reading practices. In addition, there isno way for the utility to call up the RDU and take an immediate (on demand)reading of a customer's meter.

Other telephone dial-inbound systems have attempted to improve the throughput of data transmission through various techniques. For example, inU.S. Pat. Nos. 4,241,237 and 4,455,453, an RDU is programmed to call the hostcomputer at a precise real time, as indicated by an onboard real-time clock inthe RDU. At the designated time, the RDU seizes the telephone line andtransmits meter reading data to the host computer. The host computer thensends a signal containinp the desired next real time of callback to the RDUwhere it is stored. The host computer also sends a synchronization signal to . ; : ~ .................................... :. :: - : :
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207~0~8 force the real time clock at the RDU to synchronize with a master clockassociated with the host computer.

Similarly, U.S. Pat. No. 4,056,684 shows a telephone dial-inbound system for remote alarm monitoring. Alarm monitoring circuitry at a customer's homeperiodically dials into a host computer to indicate proper operation of thesystem. The host computer downloads a desired time interval to the remotealarm monitoring unit. The remote alarm monitoring unit, which has a clock,then waits the appropriate amount of time, as indicated by the time intervaldata sent from the host unit, until it calls back again.

One drawback to the foregoing systems is that some allowance must be made for drift in the onboard real time clock at the RDU. This drift can be up to200 parts per million. Over a thirty day period, this implies a possible driftof several minutes plus or minus the nominal desired callback time. Because ofthis, the host computer has to set aside a several minute block of time foreach RDU to call back to avoid collisions due to onboard clock drift. Forexample, with a two minute interval callback spacing, the maximum number ofRDUs which can phone on a single line in a thirty day period without a chanceof a collision occurring is 21,600. Such a system cannot accommodate manylarge utilities, some of whom have well over 100,000 customers.

One common problem faced by designers of telephone dial-inbound data acquisition systems is the requirement that the RDU not interfere with thenormal operation of a customer's telephone. Not only can such interference beannoying to a customer, who may find he cannot use his telephone when the RDUis attempting to access the host computer, but it can be potentially lifethreatening if the customer needs to use the phone to dial the police or firedepartment in an emergency. Therefore, many telephone companies require anysort of auxiliary device which is connected in parallel with the customer'stelephone not interfere in any way with the normal operation of the telephone.Preferably, the RDU must be fail-safe in this regard - the RDU must not underany circumstance accidentally seize the line while a customer is using hisphone, and must immediately release the line if the customer picks up thetelephone handset to make a call.

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In order to meet these requirements, some sort of "off-hook" detection circuitry must be enployed. Circuitry for detecting whether a customer'stelephone i9 on-hook or off-hook i9 shown, for example, in U.S. Pat Nos.
4,469,917, 4,578,534 and 4,847,892. However, these circuits merely detectwhether the voltage or other electrical characteristic of the telephone linehas dropped below a predetermined level which is assumed to indicate an"off-hook" condition. However, the line voltage is dependent upon the type oftelephone system, the particular electrical characteristics of the customer'sline, the loading on the telephone system, and the type of equipment connected,thus it can vary significantly. Because of these factors, these types ofcircuits may falsely indicate an ~off-hook" condition when, in fact, no suchcondition exists, or fail to detect the customer attempting to gain access tothe line.

Other types of dial-inbound telephone-based automatic meter reading systems utilize a so-called "polling" or "interrogate and callback" scheme. InU.S. Pat. No. 4,469,917 a remote data unit is placed in an "alert" conditionfor a short period of time each day during which it waits for a single ringingsignal to be transmitted from the host computer to the RDU. The first ring isintercepted so as not to ring the customer's telephone. If no subsequent ringi9 heard within a predetermined time period, e.g. five seconds, the RDU then immediately dials back the central computer and transmits the data to the hostcomputer. In U.S. Pat. No. 4,345,113 the first incoming ring on a customer'sline is intercepted without ringing the phone. The RDU then answers the phoneand waits to hear a special tone sent by a host computer. If this tone isheard within a predetermined period of time, the RDU sends its data back to thehost computer. If the tone is not heard, the RDU assumes the call is a normalone and disconnects the RDU.

U.S. Pat. No. 4,578,534 describes a telephone dial-inbound data acquisition system in which an RDU detects and intercepts a single ringingsignal and causes the RDU to call back the host computer if such a single ringis detected. If more than one ring is detected, the RDU shuts down and allowsnormal operation of the customer's telephone.

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2~7~0~8 U,S. Pat. No. 4,847,892 describes a telephone based data acquisition system in which an RDU has a real time clock and calls a host computer on aperiodic basis, e.g. onca a month. In addition, once a day, the RDU is placedin a "standby" mode for a call from the host computer. In this "standby" mode,if a single ringing signal is detected the RDU immediately calls back the hostcomputer to transmit its current data. In this fashion, scheduled periodicdata transfers take place without any action of a utility or the requirementthat the host computer call up the 2DU to take the reading However, the RDUis capable of being accessed on a daily basis during the predetermined "window"during which the RDU is in its "standby" mode. This enables the utility totake a daily reading of a utility meter.

Systems such as those described above, wherein the RDU interrepts, absorbs or in any way eliminates the first ring do, in fact, interfere with thecustomer's normal telephone service. Customers in effect pay for ringingservice and each ring carries incremental value. For example, an elderlyperson may have difficulty getting to the telephone in time to receive a callif one ring is eliminated. Therefore, it is not likely that telephonecompanies would allow installation of such systems on a broad scale.

Systems such as those shown in U.S. Pat. Nos. 4,469,917 and 4,578,534 which require the host computer to first call up the RDV, the RDU to bealerted, and the RDU to hang up and immediately call back the host computer areinefficient. This is because it can take upwards of thirty seconds to completeeach dial-up circuit, i.e. placing the call from the host computer to the RDUand then placing the call from the RDU back to the host computer. This,combined with the necessary overhead for transmitting data from the RDU back tothe host computer, means that a typical reading cycle can take up to ninetyseconds. This substantially reduces the throughput (number of RDUs accessedper unit time) of the system. In addition, these systems all rely upon sometype of single ring detecting circuitry which, in an effort to not annoy acustomer, typically eliminates the first ring, so that only the second andsubsequent rings will actually be heard by a customer if the ring was notintended for alerting the RDU. Such systems also have the disadvantage thatthe single ring alerting circuitry could fail in the off-hook mode afterdetecting the first ring, thereby rendering the customer's telephone useless.

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2~7~l 048 The type of telephone dial-inbound meter reading system where the RDU
automatically calls the host at a predetermined time could be the mostefficient in terms of throughput, if it were not for the problems associatedwith clock drift. Although various attempts have been made to overcome thisproblem, none have achieved sufficient efficiency to enable a large number ofutility meters, e.g. 100,000 or more, to be efficiently read during a one monthperiod on a single incoming telephone line. In addition, with the heavycapital expense associated with installing a telephone-based automatic meterreading systems, utilities generally not only wish to be able to read theirmeters on a periodic basis (e.g. monthly) but also to take on-demand readingsof a meter when necessary to address billing complaints or conduct specialreads. This is particularly true in urban areas where there is a largemovement of households where connect and disconnect calls are frequent,necessitating taking a meter reading on a particular day to prepare a finalbill for a customer.

Since telephone-dial-inbound automatic meter reading RDUs must be capable of operation during periods in which telephone line power may fail or not beavailable, they normally require some sort of onboard battery for backuppower. While the use of some types of batteries, such as lithium batteries, incombination with low power consumption integrated circuit devices, haveextended the time period over which an RDU may operate without needing abattery change, it is a very desirable feature that a utility be alerted whenthe battery power for a particular RDU begins to fall below a criticalthreshold. In addition, utilities have indicated the desire to know if the RDUor its associated utility meter is being tampered with. In either of theforegoing cases, low battery voltage or tampering indication, it would bedesirable that the RDU immediately dial the host computer and identify theproblem and its nature.

Summary of the Invention The present invention overcomes the various drawbacks of prior art telephone dial-inbound data acquisition systems while providing many desirablefeatures.

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2~71048 More particularly, in a preferred embodiment the invention concerns an apparatus for transferring data over a telephone network comprising a remotedata unit (RDU) and a host computer, with the remote data unit and hostcomputer being interconnected over a dial-up telephone network. The remotedata unit includes means for storing a desired callback start time and a realtime clock for generating an indication of real time. The remote data unitalso includes means for generating an offset time interval in a pseudorandommanner and means for adding the offset time interval to the desired callbackstart time to generate an actual callback time. This actual callback time isthen continuously compared with the real time generated by the real time clockin the RDU. The call placement process is then begun by the remote data unitwhen the real time equals the actual callback time. The remote data unit thentransfers data from the remote unit to the host computer. Subsequently, thehost computer transfers a new callback start time to the remote data unit.Communications between the remote data unit and the host computer are thenterminated.

In the normal mode of operation, i.e. transmitting meter readings, the actual co unication time between an RDU and the host computer is only two tothree seconds. Since system throughput is a critical issue for a large utilitywith hundreds of thousands of meters, it is important to allow as many RDUs aspossible to report in a very short time period. With the foregoingarrangement, the remote data unit will call back during a predetermined"window" of time. The start of this time window is the callback start timetransmitted from the host computer to the remote data unit during a priorcommunications session. The offset time interval, which is generated in apseudorandom manner at the remote data unit, is then added to the callbackstart time. The resultant actual callback time (the callback start time plusthe offset time interval) is then used as the next time of callback to the hostcomputer. With this arrangement, several remote data units can be assignedidentical callback start times, while the offset time intervals randomlygenerated at each remote data unit will cause the actual callback times todiffer slightly during a predetermined window of time. This enables multipleRDUs to call a host computer during a relatively short (on the order of one andone-half minutes) "window" of time, while minimizing the chances of collisionsbetween each call. The number of RDUs sharing a timeslot and thepseudorandomization algorithm can be selected to optimize channel loading.

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20710~8 This dramatically raises the throughput of the system over prior art systemswhere each RDU calls back during a fixed or limited range of times.

Throughput of the present invention may be further improved by having the remote data unit calculate the next actual callback time while it is still incommunication with the host computer and transferring this actual callback timeindication to the host computer. The host computer may then compare thisactual callback time with callback times of other remote data units within thesystem. If the host computer detects a conflict or uneven distribution ofactual callback times, it can then signal the particular RDU, while it is stillin communication with the host computer, to generate another offset timeinterval and calculate a new actual callback time until an acceptable actualcallback time is indicated by the remote data unit.

In the preferred embodiment, the offset time interval is generated in a pseudorandom fashion by utilizing the "hundredths of a second counter" of thereal time clock provided at the remote data unit. When the remote data unitaccesses the telephone line, the count value of the hundredths of a secondcounter of the real time clock is sampled. Since the actual time that theremote data unit accesses a line will vary slightly, due to the amount of timeit takes to dial up the host computer through the telephone network, thehundredths of a second digits, which are free running in the real time clock,will result in a random number between "00~ and "99". Since there is noattempt to synchronize this counter with the host clock, the hundredths of asecond count value of every RDU in the system will be random with respect toevery other RDU. This number is then used as the number of seconds for theoffset time interval for the next scheduled callback time. The host computermay download the next desired callback start time, to which the offset timeinterval is edded, or the remote data unit may be programmed to use the samecallback start time for the next reporting period as used previously. Thisapproach maximizes the efficiency of the RDU program design by avoiding complexand lengthy mathematical randomization formulas.

Alternatively, the remote data unit may include means for mathematically computing a pseudorandom reporting time and means for comparing thispseudorandom time with a real time clock at the remote data unit. When thepseudorandom time report equals the real time, the remote data unit seizes the ?~

2D 7.1 048 telephone line, dials the host computer, and transfers data from the remoteunit to the host computer. While still in communication with the hostcomputer, the remote data unit then calculates a new pseudorandom time signaland transmits it to the host computer. The host computer contains a table ofallowable callback times and compares this pseudorandomly generated timesignal, which is indicative of the next desired callback time for the remotedata unit, with all allowable callback times for all remote data units in thesystem. If the desired callback time is acceptable, the host computerackncwledges this and the remote data unit then uses this time as its callbacktime on the next call. If this time is not acceptable, the host computer soindicates and causes the remote data unit to recompute a new callback time, andthen checks the availability with the host computer to see if the time isacceptable.

In this alternative embodiment, in addition to the desired start time, the host computer may also send an indication to the remote data unit of a range ofallowable times so as to constrain the generation of the pseudorandomlygenerated time signal to the allowable range.

Other features of the present invention include circuitry for detecting an alarm condition at tne remote data unit and, in response to such alarmcondition, immediately accessing the telephone line and transferring a signalindicative of this alarm condition to the host computer. This alarm condition,for example, may be an indication of voltage of a battery powering the remotedata unit dropping below a predetermined limit. Alternatively, the batterycondition can be checked and reported to the host computer during a regularlyscheduled communications session. The alarm condition can also be anindication of tampering with the remote data unit or a utility meter associatedwith the remote data unit. Preferably, ta~pering is detected by applying aperiodic pulsed current to a conductive loop and detecting the continuity ofthis loop. If there is an indication of a lack of continuity, which couldoccur through the breaking of a conductive meter seal, the opening of a tamperswitch, or the movement of a mercury switch, the alarm condition is generated.Pulsing the current on the loop has the advantages of conserving battery powerat the remote data unit and being more noise immune. This is because a higherpulse of current may be applied to the conductive loop for a short period oftime for the same size battery than would be practical if a continuous signal - . . ` . ~ ,. . . `.

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11 ' were applied to the loop, and because any noise induced in the loop is ignoredexcept during the sampling period in which the pulse is applied.

The invention further includes circuitry for detecting whether the telephone line is in an on-hook or off-hook condition. This circuitry includesmeans for detecting the electrical characteristics associated with on-hook andoff-hook conditions for a particular remote data unit and other devices andtelephones connected to a customer's telephone line. For example, the off-hookcondition detecting circuitry can include means for detecting the tip-to-ringvoltage on the telephone line and disconnecting the remote data unit from thetelephone line when this voltage drops below a predetermined value. Theoff-hook condition detecting means also contains circuitry for blocking accessto the telephone line by the remote data unit if the tip-to-ring voltage fallsbelow a predetermined value. In this fashion, the remote data unit isprevented from seizing the telephone line if an off-hook condition is detected,e.g. where a customer is using a phone. The RDU will also immediatelydisconnect from the host computer if it detects that the customer has taken thephone off-hook. Both of these features are important because a remote dataunit must not interfere with the normal operation of a customer's telephone andmust make the telephone immediately available in the case of emergencies.

Prior art on-hook/off-hook detection circuits generally only to 104k to see whether the voltage on the telephone line has fallen below a certain fixedthreshold value, where this threshold value is considered to indicate anoff-hook condition. However, it has been found that not only does the normal~on-hook" voltage in a dial-up telephone network vary considerably fromcustomer line to customer line, but the amount of voltage drop at a particularcustomer site may vary due to the number of phones or other devices such asdata sets that are interfaced with the telephone line and which draw currentfrom the line. By using analog to digital conversion circuitry in conjunctionwith a microcomputer dedicated to monitor line characteristics, the presentinvention provides the capability to adapt the RDU to numerous telephone lineenvironments. The present invention provides a more accurate and reliableoff-hook condition detecting scheme by establishing and storing an initialbaseline of actual on-hook and off-hook voltages for a particular telephoneline to which the RDU is connected and then monitoring the line voltage for 2071~48 ` 12 -changes which result in the ratio of the baseline on-hook or off-hook voltageto the respective monitored on-hook or off-hook voltage falling below a pre-determined value. This system not only prevents accidental seizure or failureto release in the presence of an off-hook condition, but also prevents the RDUfrom being locked out from access to the the telephone line if the telephoneline voltage drops below a predetermined value, even though this predeterminedvalue does not actually indicate an off-hook condition. This method alsoallows the RDU to "learn" its respective telephone line characteristics andadapt its operating limits accordingly, thereby greatly enhancing itscapability of operating under varying telephone line conditions. With priorart off-hook detection circuits which merely detect whether the telephone linevoltage is above or below a predetermined level, the RDU could falsely believethat the customer's telephone was off-hook even though it was actually not, dueto the supply voltage fluctuating below the preset level. The presentinvention also improves upon prior art methods by providing the capability ofdetecting an off-hook condition during pulse dialing.

A further feature of the present invention is that the remote data unit includes circuitry for detecting a predetermined number of normal ringingsignals applied to the telephone network. In response to the detection of suchpredetermined number of ringing signals, the remote data unit sccesses thetelephone network and establishes communication between the host computer andremote data unit. This arrangement enables a utility to dial-up the remotedata unit on demand to take a meter reading at any time. The ring signaldetection circuitry may be programmed to respond to one or more normal ringswhich would be heard by a customer. Preferably the number of rings is one. Ifmore rings than the predetermined number are detected, the RDU assumes it is aregular telephone call and takes no further action. If a single (or otherpredetermined number) unanswered ring is detected, the RDU then immediatelycalls back the host computer to transmit its data. This allows polling of theremote data unit at any time by a utility. The first ring is not interceptedas in the case of sooe prior art units, but is allowed to ring a customer'stelephone. If a customer does not pick up the telephone, then the RDU willinitiate its callback as described above. If the customer picks up histelephone, assuming that it is a call for him, a utility company operator wouldinstruct the customer to allow his phone to ring one time unanswered, so the~eter can be read automatically. The first ring is not intercepted, absorbed : .
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` - 13 - 2071 048 or in any way eliminated since some telephone companies do not find thispractice to be acceptable. Furthermore, some types of single ring detectioncircuits which intercept the first ring could potentially fail in an off-hookcondition, thereby rendering a customer's telephone inoperative.

An additional feature of the present invention is the provision of an expansion bus at the remote data unit. The expansion bus accepts auxiliarycirc~it cards for direct communication with the remote data unit. Theseauxiliary circuit cards could implement additional desirable features, such asadditional memory for the RDU, demand recording capabilities, time of usebilling capability, energy management controls, pulse accumulator, radiofrequency data link, or the like.

The circuitry of the present invention is designed using low power integrated circuitry to minimize battery drain and thereby prolong the periodof time between battery replacement. Furthermore, it is contemplated that thecircuitry of the remote data unit may be placed in a separate enclosure remotefrom the utility meter. Alternatively, the circuitry for the remote data unitmay be integrated into the mechanical components of a meter register (e.g. awater, gas, or electric meter).

Brief Description of The Drawinv Fivures These and other features and advantages of the present invention will be described in more detail in the following detailed description of the preferredembodiments when taken in conjunction with accompanying drawing figureswherein:

Fig. 1 illustrates the overall arrangement of a remote data acquisition system utilizing the principles of the present invention;

Fig. 2 shows a utility meter incorporating an onboard remote data acquisition unit;

Figs. 3, 4 and 5, when taken together, constitute a block diagram for the remote data unit of the present invention;

2071 0~8 - 14 .

Figs. 6, 7 and 8 are detailed schematic diagrams of the blocks shown in Figs. 4, and 5, respectively;

Fig. 9 illustrates a low battery warning circuit which can be incorporated into the power supply shown Ln Fig. 7; and Fig. 10 shows the arrangement of expansion bus ports for the remote data unit shown in Figs. 3-8.

DETAILED DESCRIPT~ON OF THE PREFERRED EMBODIMENTS

Fig. 1 shows an overall arrangement of the remote data acquisition system of the present invention. A host computer 1, which may be an IBN PC orcompatible or other type of computer, is connected to dial-up telephone line 3via a telephone modem 4. Telephone line 3 is connected to a local telephoneexchange 5. Telephone exchange 5 is connected to other similar exchanges in awell known fashion. Each telephone exchange 5 is connected to a customer sitevia local line 6. At each customer site, there is provided a remote data unit(RDU) 7 whose construction and operation will be described in more detailbelow.

Each RDU 7 is connected via a communications channel 9 with a utility meter, such as a water meter 11, electric meter 13 or gas meter 15.Communications channel 9 may comprise wires, coaxial cable, optical fiber, aradio frequency link, or the like. Communications channel 9 enablescommunication between the register associated with a utility meter 11, 13, orand its associated RDU 7, so that the amount of water, electricity, or gas indicated by these meter registers may be communicated to its associated RDU7. As will be described in more detail below, each RDU 7 is capable of readingup to eight different meters or sources of data signals at a time.

Each RDU 7 is connected in parallel to local telephone line 6. Associated with each line 6 is a customer's telephone or other type of telephone data set17.

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207~0~8 Preerably, the utility meter registers associated with meters 11, 13, or are of the "absolute encoder" type; i.e. encoder wheels associated with the meter register display encode the reading of the display so that when circuitryassociated with the meter display is interrogated, the encoded register readingcorresponds with the amount visually shown on the register display. Such anencoded register and associated circuitry is shown in U.S. Pat. No. 4,085,287.

Alternatively, a meter register may count pulses generated by the motion of ~he measuring element of its associated meter 11, 13, or 15. These pulsesmay be accumulated in a memory dev$ce associated with the meter register oraccumulated by its associated RDU, as described in more detail belo~.

In this fashion, data indicative of the consumption of water, electricity or gas can be co unicated to an RDU 7.

As described in more detail below, each RDU 7 contains circuitry for automatically and periodically dialing up host computer 1 via telephone line 3and 6 for communicating this consumption data back to host computer 1. EachRDU 7 further includes circuitry which is responsive to a ringing signalinitiated by host computer 1 to cause RDU 7 to immediately call back hostcomputer 1 at times other than its periodic automatic callback time. Thisenables a utility to take a reading of utility meters 11, 13, or 15 on demand.

Fig. 2 illustrates an alternative embodiment wherein the circuitry associated with each RDU 7 is integrated within the same enclosure 19containing the utility meter register associated with utility meters 11, 13, or15. This integrated RDU/register l¢ has the benefit of eliminating anadditional enclosure which would otherwise be required for an RDU 7 shown inFig. 1. It also eliminates the need for a separate communications channel 9between an RDU and its associated utility meter and minimizes unit cost.

The circuitry forming RDU 7 (or the integrated version 19 as shown in Fig.
2) is shown in block diagram form in Figs. 3, 4, and 5. Figs. 6, 7, and 8 aredetailed schematics of the correspondingly numbered blocks shown in Figs. 3, 4,and 5.

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2~71~8 Block 21 is the telephone line transient suppression circuit. Circuit 21 is connected to local telephone line 6 and serves the purpose of rectifying thetelephone line voltage to allow either polarity of the tip-to-ring connectionto function identically. Connected to transient suppression circuit 21 ishook-switch circuitry 23 which is activated by "back end" microprocessor 51.Hook-switch circuitry 23 is activated whenever microprocessor 51 determinesthat it is time to initiate a callback to host computer 1. Such a call onlyoccurs after telephone line 6 is tested to see whether it is available (thatis, if an on-hook condition is detected). Line testing occurs whenmicroprocessor 51 activates hook-switch circuit 23. This initiates thepowering up of the circuitry shown in Figs. 3 and 6 to test the line anddetermine that it is available (on-hook).

If telephone line 6 is in an on-hook condition, microprocessor 51 proceeds normally to place a telephone call to host computer 1. However, if telephoneline 6 is determined to be in an off-hook condition, ~icroprocessor 51 takescertain steps to prevent RDU 7 from initiating such a call.

More particularly, microprocessor 51 controls the status of hook-switch circuitry 23 by clocking data into local telephone line microcontroller 31.Data is clocked into microcontroller 31 indicating what the tip-to-ring voltageon the telephone line should be. Local microcontroller 31 is powered upthrough power supply 27. The tip-to-ring voltage is sensed through line sensacircuit 29. The output of line sense circuit 29 is applied to an A-to-Dconverter incorporated as part of microcontroller 31. The A-to-D convertergenerates a digital representation of the tip-to-ring voltage and compares to aprogrammable reference level representative of an off-hook condition. Thereference level can be a ratio of the previous result of measuring thetip-to-ring voltage during installation, or it can be remotely programmed bythe host computer. Prior to seizing the line, RDU 7 compares an average of thecurrent line voltage to this reference level, and if greater or equal to, theline is assumed to be "on-hook" and available for use by RDU 7. If thisvoltage is less than the reference level, the line is assumed to be "off-hook"and therefore, unavailable for use at that time.

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:.. ,- :., 207~048 Microcontroller 31 then activates a second hook-switch circuit 35. Hook-switch 23 and second hoo~-switch 35 provide reduntant protectlon from RDU 7going off-hook accidentally. This arrangement prevents either microcontroller31 or back-end microprocessor 51 from independently causing RDU 7 to seizetelephone line 6 and go into an off-hook condition. Both microcontroller 31and microprocessor 51 must be activated at the same time and activate theirrespective hook-switches 35 and 23 before telephone line 6 can be seized.

Watchdog timer 47 also prevents RDU 7 from remaining in an off-hook condition for more than a predetermined amount of time, e.g. four minutes. Ifwatchdog timer 47 has not been reset by microprocessor 51 within thispredetermined time period, power supply 41 is turned off causing RDU 7 to shutdown. This prevents RDU 7 from accidentally locking up and seizing telephoneline 6 in the event of a component failure or error.

Modem signal coupling transformer 37 couples signals from modem 57 to the telephone lines 6 via hook-switch 23 and transient suppression circuit 21.Data signals are applied from microprocessor 51 to modem 57 in a well knownfashion. These data signals may be, for example, data indicative of a utilitymeter reading, an alarm condition, a low battery indication, tamperingindication, or the like.

An important feature of the present invention is that after micro-controller 31 and microprocessor 51 have caused RDU 7 to seize telephone line6, microcontroller 31 continuously monitors the telephone line voltage via linesense circuitry 29 in order to determine whether a telephone or other data setdevice 17, which is connected in parallel to telephone line 6, has goneoff-hook. Most telephone companies require that any auxiliary device connectedto the telephone line, such as RDU 7, automatically and immediately disconnectfrom the telephone line in the event a customer wishes to use the telephone orother telephone data set. Ideally, this should be done quickly and without anyinterference to the customer's normal use of his telephone. This alsorepresents a safety feature in the event the customer needs to use h~stelephone in an emergency, e.g. to telephone the police or fire department.

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Line sense circuitry 29 in combination with the circuitry of micro-controller 31 accomplishes this by continuously monitoring the voltage oftelephone line 6 and calculating an average of the voltage on the line todetermine when any changes have occurred which would indicate another deviceconnected to lines 6 going off-hook. More particularly, after RDU 7 has seizedthe line, it again measures the tip-to-ring voltage. This measurement is thenused as a basis for determining subsequent customer off-hook conditions. TheMIU continually samples and measures the tip-to-ring voltage and if thisvoltage falls to a specific ratio of the original measurement (for example 75~)or less, RDU 7 assumes a parallel off-hook condition has occurred. To improvethe reliability of the process, and to reject spurious noise on the line, RDU 7averages the measurements and uses the running average to determine thetelephone line status. Additionally, if the tip-to-ring voltage falls belowthe minimum operating requirement of the off-hook detect circuitry (29 and 31),the circuitry will signal this to microprocessor 51 as an off-hook conditionprior to shutting itself down. If such a condition is detected,microcontroller 31 also signals this via optocoupler circuit 33 to the back-endmicroprocessor 51. At this point, both microcontroller 31 and microprocessor51 send signals to respective hook-switches 35 and 23 to immediately releasetelephone line 6 so that RDU 7 appears to be on-hook. This can be performed inless than 300 milliseconds so that by the time a customer has brought thehandset of his telephone 17 to his ear, he will only hear silence. Thecustomer must depress the hook-switch on his or her telephone to get a dialtone, but this is a typical and normal requirement.

The use of two independently controlled hook- switch circuits 23 and 35 ensures that RDU 7 will operate in a fail-safe condition. If eitherhook-switch should somehow fail in the "off-hook" condition, the other onewould still be capable of placing RDU 7 back in an on-hook condition.Furthermore, both microcontroller 31 and microprocessor 51 have the ability todetect whether their respective hook-switch circuits 35 and 23 are cyclingproperly between the on-hook and off-hook states and can disable RDU 7 fromfurther operation if a fault has occurred, or can signal host computer l that afaulty condition exists when RDU 7 is next scheduled to call back host computerl. While characterizing and testing the line voltage is preferred, detectingother electrical characteristics of the telephone line, e.g. line current ortip-to-ring resistance, can also be employed to achieve similar results.

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20710~L8 Another important feature of the present invention i9 the ability of RDU 7 to immediately respond to a polling or interrogation signal applied to the RDU7 over telephone line 6. The polling signal is defined as a single ringingsignal on telephone line 6 which is unanswered after five seconds.Alternatively, instead of a single ring, RDU 7 may be configured via a field-installed ~umper to respond to any number of unanswered rings for use in anapplication where RDU 7 is the only telephone device coupled to line 6 or whereline 6 is a leased line. In either case, whether it is a single ring ormultiple rings, the operation of RDU 7 is the same.

Where RDU 7 is configured to respond after a single ring, this ringing signal is detected by power supply circuit 27 which is activated and drawsminimal direct current from the telephone lines 6. The amount of current isnot enough to cause an off-hook condition but is sufficient to operate ringdetect circuitry 25, line sense circuit 29 and microcontroller 31. This amountof current is a maximum of three milliamps, and the circuitry of the presentinvention actually draws less than one-half milliamp.

Once power supply 27, ring detect circuit 25, line sense circuit 29, and microcontroller 31 are powered up, microcontroller 31 monitors the ringingsignal on telephone line 6 to determine whether the ringing signal is a validone and that it is at least 500 milliseconds long. Microcontroller 31 thencontinues to monitor telephone line 6 for five seconds after the ringing signalstops to see whether the telephone line is in an off-hook condition, as sensedby line sense circuit 29, indicating that someone has picked up a telephone ordata set 17 associated with the telephone line 6. If an off-hook condition isdetected, this is communicated to microcontroller 31 which then causes powersupply 27 to shut down. Power supply 27 will also deactivate if more than thepredetermined number of rings (e.g. one) is sensed by ring detect circuitry 25and microcontroller 31.

Assuming that a single polling ring is sensed, and telephone line 6 is otherwise in an on-hook condition, microcontroller 31 then assumes that it hasreceived a legitimate polling signal. Microcontroller 31 then causes theremaining circuitry shown in Figs. 4, 5, 7 and 8 to be powered up via ringindicator/optocoupler circuit 33. This signal "SRA" is applied to the back-endpower supply 41 and microprocessor 51. This causes microprocessor 51 to take a 207104~

reading of the encoded register associated with utility meters 11, 13, or 15via meter interface ports 53 (typically four in number). This data is thenformatted as a serial data stream and applied to modem 57 which includes atelephone dialing circuit for dialing the telephone number of the host computer1. The modem signal is applied to the modem signal coupling transformer 37.Microprocessor 51 also causes hook-switch 23 to be activated and micro-controller 31 causes the second hook-switch 35 to be activated. This allowsRDU 7 to seize telephone line 6 and receive the signal from modem 57 to becoupled via coupling transformer 37 to telephone line 6. Data is then sentfrom RDU 7 to host computer 1 via local telephone line 6 and the dial-uptelephone line 3. Upon completion of the sending of data, microprocessor 51causes power supply 41 to be turned off and hook-switch 23 to be returned tothe on-hook condition. Microcontroller 31 is also signaled to place the secondhook-switch 35 in the on-hook condition and to inactivate the front end powersupply 27 to take RDU 7 off-line from telephone line 6. This single ringpolling feature does not require the customer to be at home if the utilitywishes to poll the customer's RDU, as is the case with some types of prior artRDUs.

Ring detection circuitry 25 also includes a feature whereby a customer can answer his telephone within the first five seconds after a single ring appearson the telephone lines and if the telephone is returned to the on-hookcondition before the five seconds expires, this will still indicate a valid onering polling signal. This increases the probability of a single ring pollingsignal cutting through and activating RDU 7.

The foregoing arrangement has the advantage of ensuring a reliable detection of a polling signal applied to a customer's RDU without interferingin any way with the customer's use of his telephone. Additionally, unlike someprior art single ring detection circuits, the single ring detection circuit 25of the present invention does not attempt to suppress or "capture" the firstringing signal applied to a customer's telephone lines. Some prior art devicesdo this in an effort not to annoy the customer with a single isolated ring.However, a number of telephone utilities will not authorize such ring detectdevices which suppress the first ring on a customer's line, either as a matterof policy or because it has been found that such circuits can interfere withthe operation of other telephone devices or data sets coupled to a customer's :

telephone line. Further, eliminating a single ring can make it more difficultfor elderly or handicapped people to answer the telephone.

This polling mode of operation enables a utility to individually poll or interrogate a particular RDU and get an immediate, real time, meter readingfrom its associated meter register. This is especially beneficial when autility needs to take a customer's meter reading on other than the usuallyscheduled day or time, such as when a customer has a billing complaint or ismoving out of a home or apartment and a final meter reading must be taken. ~-In addition to the polling mode of operation, ~DU 7 also may be activated in three other ways. RDU 7 may be programmed to periodically call hostcomputer 1 on a predetermined schedule. The RDU may also be awakened to callback to the host computer if an alarm condition is detected (e.g. tampering ora low battery). RDU 7 may also be activated through an external servicerequest "XSR" applied from an auxiliary circuit card or other external devicecoupled to expansion por~s 55 of RDU 7.

In the automatic callback mode, RDU 7 is initially set up with a desired callback start time which is stored as a digital representation of hours,minutes, and seconds in real time clock 45. This desired callback start timeis monitored by microprocessor 51 to ensure that it is a valid time (e.g.February 31st would not be accepted). If it is not a valid time, RDU 7 willrequest host computer 1 to upload another time until a valid callback starttime is accepted by RDU 7.

Once this callback start time has been stored, a free running counter associated with real time clock 45 counts in increments of lOOths of a secondand is sampled. The sampled lOOths of a second digits are used as the amount(one second per count) of offset time interval to be added to the previouslystored callback start time. The free running lOOths of a second countercontinuously counts between "OO" and "99". Due to inherent, random delays inthe amount of time it takes to establish communications with the host computer,the value of the lOOths of a second counter when it is sampled will result inthe random samplin~ of a number between "OO" and "99". This number is then added to the seconds position of the previously stored callback start time.Thus, for example, if the callback start time were set initially for 1:10 a.m.

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" - 22 - 207 ~. 0 ~8 and the lOOths of a second counter associated with real time clock 45 weresampled at ~50n, fifty seconds would be added to the callback start time togenerate an actual callback time of 1:10:50 a.m. If the lOOths of a secondcounter were sampled, for example, at 175n, seventy-five seconds would be added to the callback start time to generate an actual callback time of 1:11:15 a.m.Thus, the free-running lOOths of a second counter associated with the real timeclock 45 can generate an offset time interval in the range of 0-99 seconds inessentially a pseudorandom manner without burdening the microprocessor withrandom number computations.

The actual callback time (the initial callback start time plus the offset time interval) is continuously compared with the real time as generated by realtime clock 45. When these two times are the same, real time clock 45 generatesa signal I~LN" which turns on the back-end power supply 41. This, in turn, causes the rest of the circuitry associated with RDU 7 to be powered up. Thefront end microcontroller 31 checks telephone line 6 via line sense circuitry29 to ensure that telephone line 6 is in an on-hook condition. Microprocessor51 then applies a signal "TEST" to hook-switch 23 and microcontroller 31applies a signal to second hook-switch 35 to place RDU 7 off-hook.Microprocessor 51 then initiates a dialing sequence via modem 57 and couplingtransformer 37 to dial the telephone number associated with host computer 1.During this process, line sense circuitry 29 continues to monitor the voltagesassociated with telephone line 6 to determine whether a telephone or other dataset 17 of a customer is being taken off-hook. If such a condition is detectedat any time, microcontroller 31 will cause RDU 7 to be immediately deactivatedso that a customer may use his telephone or other data set normally. Linesense circuitry 29 also will prevent RDU 7 from waking up and attempting toseize the telephone line at its scheduled callback time if it senses thattelephone line is already in an off-hook condition, indicating that a customeris presently using his telephone or data set. In such a case, RDU 7automatically schedules a new reporting time based on a retry algorithm.

Assuming that telephone line 6 is free for use, RDU 7 then dials host computer 1 at the time indicated by the sum of the callback start time and therandomly generated offset time interval. Once a communications link has beenmade between RDU 7 and host computer 1, RDU 7 then proceeds to send data in aserial fashion indicative of various types of information it has acquired ;, ;: - :::: : :::, . : -: , , . . :; :. -20710~8 and/or stored. This information can be, for example, data indicative of themeter reading of a utility meter, such as meters 11, 13, or 15. Otherinformation, such as the status of devices coupled to expansion bus ports 55,the voltage of battery B, (see Figs. 4 and 7) may also be transmitted at thistime.

Upon the completion of transmission of data from RDU 7, host computer 1 may send an indication of the next desired callback start time to RDU 7 forstorage in real time clock 45. The pseudorandom offset time interval is thenimmediately calculated, as described above, and added to the new callback starttime to generate the next actual callback time. This actual callback time isthen transmitted as data from RDU 7 to host computer 1 which then compares itwith other acceptable times it has stored in computer memory. If this nextactual callback time is acceptable to the host computer, an acknowledgmentsignal is sent to RDU 7 and RDU 7 then proceeds to disconnect itself fromtelephone lines 6. If this next actual callback time is not acceptable, asdetermined by host computer 1, host computer 1 will send a "veto" signal to RDU7 requesting it to regenerate a new actual callback time. If RDU 7 receivessuch a request, it samples the free running counter associated with real timeclock 45 again to generate a new offset time interval. This offset timeinterval is then added to the previously uploaded callback start time togenerate a new actual callback time. This new actual callback time is thentransmitted back to host computer 1 which, again, compares it with its table ofacceptable callback times. In this mode of operation, this process will berepeated as many times as is necessary to have an acceptable actual callbacktime generated by RDU 7.

The foregoing arrangement has a number of advantages over prior art devices which require the RDU to callback at a precise time. As explainedearlier, such devices are sub;ect to problems associated with clock drift whichcan amount to several minutes over a thirty day period. Because of this, thehost computer must schedule each RDU to phone in at times which aresufficiently spaced apart to minimize the chances of two RDUs calling in at thesame time. For example, with a two minute interval callback spacing, themaximum number of RDUs which can phone in a twenty day billing period without achance of a collision occurring is 14,400. However, many large utilities havein excess of 100,000 customers. For such prior art devices to be usable with ' ':

2071048 - `

utilities having such a large customer base requires either the use of multiplehost computers, which is expensive, or the use of multiple incoming telephonelines and a multiplexer, also an expensive solution. In the previous example,such a prior art system would require at least 7 incoming telephone lines tosupport a customer base of 100,000.

The arrangement of the present invention can increase this throughput by at least a factor of ten, i.e. from one call every two minutes to a call everytwelve seconds. In normal operation through a local telephone exchange, andusing DTMF (Touch Tone~) dialing, it can take anywhere from 3-10 seconds todial the host computer number and have it answer. A few seconds are requiredfor the modem of RDU 7 and the host computer to establish communications. Twoor three seconds are then required for RDU 7 to send its data to host computer1 and for host computer 1 download the next desired callback start time.Disconnection time generally amounts to under one second.

Under average conditions, it takes approximately 15-20 seconds total for RDU 7 to dial up host computer 1, transfer its information, receive the nextdesired callback start time and disconnect itself, however, only about 5-10seconds of this is the actual usage time of the host computer's line. Thisimplies that nine or more RDUs on average may be assigned an identical callbackstart time by host computer 1. This is because these nine or more RDUs willcall back at different random times within a 99 second "window" beginning atthe callback start time assigned by host computer 1. Thus, in an average case,nine or more RDUs may telephone back within a predetermined 99 second timeinterval as opposed to one RDU telephoning back within a 120 second timeinterval as is the case with prior art RDUs which only call back at exact,preassigned scheduled times. Thus, the present invention can handle utilityaccounts with over 100,000 customers or meters without the necessity of usingadditional host computers, additional telephone lines or multiplexers. In thesame twenty-day interval, the maximum number of RDUs which can report in wouldexceed 175,000, using a single telephone line. Multiple telephone lines couldbe utilized for redundancy and to increase throughput, but the number of linesrequired would be only a fraction of the number required for prior art systems.

Instead of downloading the next callback start time during each communication session, the initial default callback start time, as currently .,, ::
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` - 25 - 20710~8 stored in RDU 7 may be reused, This i9 especially useful where a ut~litydesires its RDUs to phone in meter readings at approximately the same time on aparticular day each month, In the unlikely event that an RDU 7 dials up host computer and gets a busy signal, RDU 7 uses a retry algorithm with programmable intervals to determinethe next callback attempt time. If RDU 7 is unable to establish communicationswith host computer 1 after several tries, RDU 7 will default to attempting tocall the host once per day.

Host computer 1 contains a table of all the identification numbers for RDUs connected to its system so that if a particular RDU does not call in atits scheduled time, the utility will be alerted to this fact and then can pollany RDU whose data has not been transmitted in accordance with the single ringpolling scheme previously discussed.

As an alternative to the foregoing scheme, it is possible to use the free running counter of real time clock 45, and/or the free-running countersintegral to the microcontroller 51, to generate a callback time in apseudorandom fashion. This callback time is then compared with the timeindicated by the real time clock. When this pseudorandomly generated callbacktime and the real time bear a predetermined relationship to each other, the RDUis awakened and dials up host computer 1. As in the previous embodiment, thispseudorandomly calculated next time of callback can be transmitted to hostcomputer 1 prior to termination of a communication session between RDU 7 andhost computer 1. Host co~puter 1 can then determine whether this time isacceptable and; if not, cause RDU 7 to generate another callback time until onewhich is acceptable to host computer 1 is generated. Host computer 1 can beprogrammed to send a range of allowable callback times to RDU 7 so as to constrain the generation of the next callback time to fall somewhere withinthis range. This latter embodiment produces essentially the same results asthe previously discussed embodiment, except that it is a more random processbecause the host computer 1 does not send an explicit indication of the nextcallback start time in this latter embodiment.

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In addition to dialing up host computer 1 on a regular scheduls, RDU 7 is programmed to call host computer 1 if any alarm conditions are detected. Thesealarm conditions may be, for example, an indication of tampering with RDU 7 orits associated utility meter or the detection of a low battery level of batteryB.

Tamper detection is performed by tamper detect circuitry 49. Tamper detect circuitry 49 is turned on periodically (e.g. every 16 seconds) bywatchdog timer circuitry 47 which contains a crystal oscillator operating at 32kHz. Uatch dog timer circuitry 47 is powered by 3 volt power supply 43 whichoperates off the approximately 7 volt lithium battery B shown in Figs. 4 and 7.

When tamper detect circuitry 49 is turned on, a 20 millisecond pulse of 1.5 milliamp current is applied to lines Ll and L2. Lines Ll and L2 constitutea closed conductive loop. This conductive loop, for example, may run through aclosure seal of RDU 7 or a meter register associated with registers 11, 13, or15. If the enclosure for RDU 7 is opened or a register associated with utilitymeters 11, 13, or 15 is removed, the conductive loop Ll, L2 will be broken.Tamper detect circuitry 49 checks the continuity of lines Ll, L2 by looking fora return of the pulse applied to the loop. If no such pulse is detected at thetime the pulse is applied, the tamper detect circuitry sends a tamper signal topower supply 41 which causes RDU 7 to be awakened and to dial up host computer1. RDU 7 then transmits a tamper indicating signal to host computer 1 to alertutility personnel of the occurrence of an alarm condition at RDU 7 or itsassociated utility meter.

More particularly, any time that tamper detection loop Ll, L2 is broken, the pulsed tamper signal is no longer shunted to the ground and therefore isdetected by tamper detect circuit 49. A voltage is applied to the base of Q9of tamper detection circuit 49, which begins to conduct as well as Q10 whichconducts in phase with the applied pulse to discriminate noise from causing afalse tamper indicating signal. If the signals are in phase, then the tamperindicating signal at the collector of Q9 is pulled to a lower level which isthen applied as tamper signal to power supply 41. This signals microprocessor51 that a tamper condition has occurred to initiate an immediate callbacksequence. When there is continuity in the loop defined by line Ll, L2, whichlines are attached to the pins of jumper J3 of tamper detect circuit 49, the . . . : .

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The use of a short, pulsed tampering signal applied to lines Ll, L2 has several advantages. First, pulsing the conductive loop Ll, L2 only once every16 seconds conserves battery power without significantly eroding the securityof the system. The tamper pulse, although occurring at precise intervals,would effectively appear random to someone external to the RDU, since it wouldbe difficult to detect the occurrence of the pulse in the very-low-impedanceloop Ll, L2. It would be virtually impossible for someone to break the tamperdetecting loop, tamper with RDU 7 or its associated utility meter, andreconnect the tamper detecting loop without detection during the 16 secondinterval between pulses. An additional advantage is that by using a short, buthigher energy pulse, it is easier to detect the continuity of tamper loop Ll,L2 where the length of the loop is great (e.g. up to 1000 ft.). This makes thetamper detecting scheme of the present invention less likely to be affected bynoise and provides a good signal-to-noise ratio for detecting the pulse appliedto tamper detecting loop Ll, L2 over much longer loop lengths than would bepossible with a continuous direct current scheme, and without draining theonboard battery B of RDU 7 significantly.

In addition, RDU 7 may be programmed to immediately dial up host computer l if the supply voltage of battery B falls below a predetermined thresholdindicating that battery B needs to be replaced.

Normally, whenever RDU 7 is activated and dials up host computer 1, the voltage of battery B is read and stored at host computer 1. This occurswhenever RDU 7 activates its regularly scheduled callback time, it has beenpolled by host computer 1 or a tamper indication has been detected, or a devicehas been connected to local diagnostics port 59 or an external port devicerequest service. This occurs during the first few tenths of a second after RDU7 is powered up and before it turns on modem 57 and seizes telephone line 6.The voltage measurement of battery B is made while RDU 7 is only drawing about8 milliamps from battery B. Subsequently, after RDU 7 has seized telephone - , . ~. .

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20710~8 line 6 arld activated modem 57, which increases the current drain to 15-20 milliamps, another battery measurement is taken and stored.

Once communication with host computer 1 has been established, these two measurements are transmitted to host computer 1, where the status of battery Bcan be determined using an appropriate algorithm based upon these two storedmeasurements, This algorithm may be selected by the operator of host computer1 based upon the particular type of battery employed, its dischargecharacteristics, its age, ambient temperature variations, or the like. Byperforming the calculations at the host computer, this enables the algorithm tobe modified to better approximate a desired battery life cycle voltage curve sothat a battery is not replaced too soon or too late. The foregoing arrangementwhere battery voltage is measured at several load points is significantly moreaccurate than taking a single voltage reading under load as is the case withmany prior art devices.

Alternatively, a circuit of the type shown in Fig. 9 may be connected to battery B. This circuit continuously monitors the battery voltage and comparesit with a voltage reference. When the detected battery voltage falls beforethe voltage reference, this ~riggers a switch and generates a low battery alarmsignal to turn on power supply 41 and initiate a callback sequence by RDU 7.While this embodiment does slightly increase the standby current drain onbattery B, it does have the advantage of immediately alerting an operator athost computer 1 if the operating voltage of battery B should fall below acritical threshold at some time between its regularly scheduled callback times.

RDU 7 may also be caused to dial up host computer 1 by means of a diagnostic or test signal applied to local port 59 or by a dial up requestsignal applied by an auxiliary circuit connected to expansion bus ports 55.The application of a diagnostic/test signal to local port 59 enables thefunctioning of RDU 7 to be tested when it is first installed in a customer'ssite or if a problem is later suspected with its circuitry. As shown in Fig.lO, expansion bus ports 55 are arranged in the same housing as contain in RDU 7in a daughter board/mother board configuration similar to as is used inpersonal computers. This enables auxiliary circuit boards to be added to RDU7. These additional auxiliary circuit boards may contain additional memory, - . - , . :
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perform demand recording functions by sampling inputs from meter interfaceports 53, perform load survey functions, accumulate pulses from a pulser typeutility meter, a PTZ (pressure/temperature/density) corrector for a gas meter,contain a radio data link for communication with a transmitter/receiver locatedat a utility meter, or the like. Any of these auxiliary circuit cards may bedesigned to generate a wakeup signal to power supply 41 to turn on RDU 7 andinitiate a dial up sequence back to host computer 1.

The expansion bus is designed to handle eight signals, including power and ground signals. One of the power signals may be supplied directly from batteryB for expansion bus auxiliary devices which require a continuous source ofpower. Regulated five volt power is also supplied via power supply 41 fordevices requiring a regulated five volt supply. The remaining bus lines are aserial data line, a serial clock line, an external service request XSR line, anexternal enable line for selecting one of four expansion bus ports to talk to.A line labelled "VXA" is a daisy chain analog input which can be connected tothe analog-to-digital converter of microcontroller 31 to allow reading ananalog voltage off the bus. This selection of signals serve to minimi~e thecomplexity of the attached function modules.

Typical operation of RDU 7 will now be described. A normal scheduled callback is initiated by the real time clock 45 reaching its designated actualcallback time (the callback start time plus the pseudorandomly generated offsettime interval) and causing power supply 41 to be turned on to power up the restof the components of RDU 7. Once power has been applied to RDU 7 andmicroprocessor 51 has determined that real time clock 45 was indeed the reasonthat it was powered, microprocessor 51 records the battery voltage and theninitiates a call to host computer 1.

Microprocessor 51 retrieves the host computer's telephone number from internal memory, asserts the test line which is line A shown in Figs. 3 and 6.This controls switch-hook circuitry 23 to test the voltage of telephone lines 6by powering up front end power supply 27, front end microcontroller 31 and theremaining circuitry of the RDU front end shown in Figs. 3 and 6.

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207~0~8 Once tbis is accomplished, microcontroller 31 signals its ready condition back to microprocessor 51 via optocoupler circuit 33. Microcontroller 31 thenbegins testing telephone line 6 to make sure that it is in an on-hookcondition. Nicroprocessor 51 sends a command by toggling data bits throughhook-switch circuitry 23 which are read through signal conditioning circui~ 39and into microcontroller 31 to tell microcontroller 31 what the expectedon-hook line voltage should be. Microcontroller 31 then begins to test thevoltage of telephone line 6 by doing an analog-to-digital conversion todetermine whether the telephone line voltage is in fact what it should be foran cn-hook condition. It will be recalled that during this testing procedure,power supply 27 and hook-switch circuitry 23 will draw a minimal amount (e.g.less than 3 milliamps) of current from telephone line 6 so as to not signaltelephone exchange 5 that a device has gone off-hook on telephone line 6. Ifan off-hook condition is detected by line sense circuit 29, microcontroller 31takes this information and outputs a continuous signal to microprocessor 51 viaoptocoupler 33. Microprocessor 51 then causes hook-switch 23 to be releasedand turns off power supply 41.

If telephone line 6 is determined to be on-hook, a momentary signal is sent by microcontroller 31 to microprocessor 51 via optocoupler 33. Micro-controller 51 also activates a second switch-hook 35 which now causes RDU 7 topull more than 20 milliamps from telephone line 6 indicating to the telephoneexchange 5 that a call is about to be made over line 6.

Exchange 5 then generates a dial tone and microprocessor 51 checks modem 57 to see if in fact a dial tone is present. If so, microprocessor 51 dials asingle DTMF (Touch Tone~) digit and tests line 6 again for a dial tone. If thedial tone signal is still present, microprocessor 51 proceeds to pulse dialtelephone line 6 by toggling hook-switch circuit 23. On the other hand, if thedial tone signal is not present after the first pulse tone has been applied,indicating that the telephone exchange switch is DTMF tone compatible, micro-processor 51 will dial the rest of the DTMF digits indicative of the telephonenumber of host computer 1 through modem 57. Microprocessor 51 monitors throughmodem 57 the progress of the call and various timing parameters includingwaiting for an answer tone on the receive signal path.

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,: ~ - : :. : ::: -.: ~ : . . :: . : -20710~8 Once an answer tone has been establishet from the modem associated with host computer 1 and verified it as being valid, modem 57 tunls on its carr1er(e,g. 1200 baud) which signals the host computer modem to switch its carrier to1200 baud receive. Once that receive carrier is present on line 6 and isdetected by modem 57 and communicated to microprocessor 51, the communicationlink is considered established between ~DU 7 and host computer 1. Datacommunications between RDU 7 and host computer 1 consists of an initial singlecommand sent by host computer 1 to RDU 7 requesting RDU 7 to identify itself(e.g. send its ID number or serial number) and to command RDU 7 to take meterreadings from the registers associated with utility meters 11, 13, or 15. RDU7 then takes the meter readings through meter interface ports 53. If anyspecial meters or devices are connected through any of the expansion bus ports55, RDU 7 will also read meters or those devices. The data is then sent backto host computer 1 as a serial data stream using commonly available dataformats, such as ASCII or the like.

Host computer 1 also has the capability of asking RDU 7 to send data from each meter interface port and expansion bus port separately, rather than as onecontinuous data stream. This speeds up the communications process insituations where a utility does not wish to read all of the available ports, orfor other specialized mses.
., Once data transfer has been completed, host computer 1 then downloads a signal indicative of its current real time as generated by its onboard realtime clock. This host computer clock time is downloaded to RDU 7 to reset thetime being kept by real time clock 45. This resynchronizes the real time clockof RDU 7 with the time being kept by host computer 1. Host computer 1 then downloads the next callback start time. RDU 7 generates the offset timeinterval in a pseudorandom manner, adds it to the callback start time andcalculates an actual callback time from the sum. This actual callback time istransmitted back to host computer 1 which determines whether it is anacceptable time through comparison with a table of such times stored in itscomputer memory. If the proposed actual callback time is acceptable, the hostcomputer 1 indicates this to RDU 7 which hangs up the telephone line and goesinto its idle state. If the proposed actual callback time is not acceptable,the host computer will ask RDU 7 to generate a new offset time interval and .. . .
~:. , ,; . , . : , . :~::: , . .: . :, :

.' ' , ' ! . . ' ' :, ':: . , . ' " '' ' ' . i ' ::: : ' '' ' 2n7~8 calculate a new actual callback time until an acceptable time i9 indlcated bythe RDU 7.

While the present invention has been described in considerable detail, it will be understood that various additions and modifications will occur ~o thoseskilled in the art, while still falling within the scope of the invention asdefined in the appended claims.

" ` ' ' '' ' . ` . ' . ' ., , " "' ' " ` ' ~ ' ' ' ' , ` ~ ,'.',' ' ' ' , , '.' ''~' ' ~ " ` , ,' ' ' ' ~ ' ' ~ . , .

Claims (73)

1. Apparatus for transferring data over a telephone network comprising:

a remote data unit; and a host computer, said remote data unit and host computer being interconnected over a telephone network;

said remote data unit including:

a) means for storing a callback start time;

b) real time clock means for generating an indication of real time;

c) means for generating an offset time interval in a pseudorandom manner;

d) means for adding said offset time interval to said callback start time to generate an actual callback time;

e) means for comparing said real time with said actual callback time;

f) means responsive to said comparison between said real time and said actual callback time for accessing said telephone network when said real time equals said actual callback time; and g) means for transferring data from said remote unit to said host computer;

said host computer including means for transferring a new callback start time to said remote data unit while said host computer and remote data unit are in communication with each other.

2. The apparatus of claim 1 wherein said remote data unit further includesmeans for transferring an indication of a subsequently generated actual callback time to said host computer while said host computer and said remote data unit are in communication with each other.

3. The apparatus of claim 2 wherein said host computer includes means forcomparing said subsequently generated actual callback time with a table of allowable times, said host computer further including means for sending an indication back to said remote data unit of whether the actual callback time is acceptable as shown by said comparison with said table of allowable times and, if so, said actual callback time is accepted as the next time of callback for said remote data unit and, if not, said host computer sending an indication to said remote data unit to cause said offset time interval generating means to generate a new offset time interval, said host computer storing an indication of the acceptable time of callback for said remote data unit in said table of allowable times.

4. The apparatus of claim 1 wherein said offset time interval generatingmeans includes means for constraining the generation of said offset time interval within a predetermined range.

5. The apparatus of claim 1 wherein said remote data unit includes means formonitoring the voltage of a battery powering said remote data unit and for transferring an indication of said battery voltage to said host computer when said remote data unit and host computer are in communication with each other.

6. The apparatus of claim 1 wherein said remote data unit includes means fordetecting an alarm condition and, responsive thereto, for immediately accessing said telephone line and transferring a signal indicative of said alarm condition to said host computer.

7. The apparatus of claim 6 wherein said alarm condition is an indication ofvoltage of a battery powering said remote data unit dropping below a predetermined limit.

8. The apparatus of claim 6 wherein said alarm condition is an indication of tampering.

9. The apparatus of claim 8 wherein said remote data unit includes a remoteconductive loop and means for periodically applying a current to said loop and for detecting the continuity of said loop and, responsive to an indication of a lack of continuity, generating said alarm condition.

10. The apparatus of claim 1 wherein said remote data unit includes means for detecting whether said telephone line is off-hook.

11. The apparatus of claim 10 wherein said off-hook detecting means includesmeans for measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, means for measuring an average value of an electrical characteristic of said telephone line at a predetermined time, means for comparing said initial value and said average value and for disconnecting said remote data unit from said telephone line when said comparison differs by a predetermined amount.

12. The apparatus of claim 10 wherein said off-hook detecting means includesmeans for measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, means for measuring an average value of an electrical characteristic of said telephone line at a predetermined time, means for comparing said initial value and said average value and for preventing access to said telephone line by said remote data unit if said comparison differs by a predetermined amount.

13. The apparatus of claim 10 wherein said off-hook detecting means includes means for storing said initial value.

14. The apparatus of any one of claims 11, 12 or 13 wherein said electrical characteristic is the voltage of said telephone line.

15. The apparatus of claim 1 wherein said remote data unit includes anexpansion bus arranged to receive auxiliary circuit cards for direct communication with said remote data unit.

16. The apparatus of claim 1 wherein said remote data unit further includesmeans for detecting a predetermined number of ringing signals applied to said telephone network within a predetermined time period and, responsive thereto, for accessing said telephone network and establishing communication between said host computer and remote data unit.

17. The apparatus of claim 1 in combination with a gas, electric or water meter.

18. The apparatus of claim 17 wherein said meter includes an absolute encoder in communication with the remote data unit.

19. A method for transferring data from a remote data unit to a host computer over a telephone network comprising the steps of:

in said remote data unit:

a) storing a callback start time;

b) generating an indication of real time;

c) generating an offset time interval in a pseudorandom manner;

d) adding said offset time interval to said callback start time to generate an actual callback time;

e) comparing said real time with said actual callback time;

f) accessing said telephone network when said real time equals said actual callback time;

g) transferring data from said remote unit to said host computer;
and in said host computer:

h) transferring a new callback start time to said remote data unit while said host computer and remote data unit are in communication with each other.

20. The method of claim 19 further including the step of transferring anindication of a subsequently generated actual callback time from said remote data unit to said host computer while said host computer and said remote data unit are in communication with each other.

21. The method of claim 20 wherein said host computer further performs the steps of:

a) comparing said subsequently generated actual callback time with a table of allowable times;

b) sending an indication back to said remote data unit of whether said actual callback time is acceptable as shown by said comparison with said table of allowable times and, if so, said actual callback time is accepted as the next time of callback for said remote data unit and, if not, said host computer sending an indication to said remote data unit to cause the said generation of another offset time interval; and c) said host computer storing an indication of the accepted time of callback for said remote data unit in said table of allowable times.

22. The method of claim 19 wherein said step of generating said offset timeinterval further includes the step of constraining the generation of said offset time interval within a predetermined range.

23. The method of claim 19 wherein said remote data unit monitors the voltageof a battery powering said remote data unit and transfers an indication of said battery voltage to said host computer when said remote data unit and host computer are in communication with each other.

24. The method of claim 19 wherein said remote data unit further performs thesteps of detecting an alarm condition and, responsive thereto, immediately accesses said telephone line and transfers a signal indicative of said alarm condition to said host computer.

25. The method of claim 24 wherein said alarm condition is an indication ofvoltage of a battery powering said remote data unit dropping below a predetermined limit.

26. The method of claim 24 wherein said alarm condition is an indication of tampering.

27. The method of claim 26 further including the steps of periodicallyapplying an electrical current to a remote conductive loop and detecting the continuity of said loop and, responsive to an indication of a lack of continuity, generating said alarm condition.

28. The method of claim 19 wherein said remote data unit further performs the step of detecting whether said telephone line is off-hook.

29. The method of claim 28 wherein said step of detecting whether saidtelephone line is off-hook includes the steps of measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, measuring an average value of said off-hook electrical characteristic at a predetermined time, comparing said initial value with said average value and disconnecting said remote data unit from said telephone line when said comparison differs by a predetermined amount.

30. The method of claim 28 wherein said step of detecting whether saidtelephone line is off-hook includes the steps of measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, measuring an average value of said off-hook electrical characteristic at a predetermined time, comparing said initial value with said average value and preventing access to said telephone line by said remote data unit when said comparison differs by a predetermined amount.

31. The method of claim 28 further including the step of storing said initial value.

32. The method of any one of claims 29, 30, or 31 wherein said electrical characteristic is the voltage of the telephone line.

33. The method of claim 19 wherein said remote data unit further performs the steps of:
(a) detecting a predetermined number of ringing signals applied to said telephone network within a predetermined time period, and responsive thereto;

(b) accessing said telephone network and establishing communications between said host computer and remote data unit.

34. Apparatus for transferring data over a telephone network comprising:

a remote data unit; and a host computer, said remote data unit and host computer being interconnected over a telephone network;

said remote data unit including:

a) timing means for generating a time signal in a pseudorandom manner;

b) real time clock means for generating an indication of real time;

c) means for comparing said real time with said pseudorandomly generated time signal;

d) means responsive to said comparison between said real time and said pseudorandomly generated time signal for accessing said telephone network when said real time bears a predetermined relationship to said pseudorandomly generated time signal; and e) means for transferring data from said remote data unit to said host computer, said transferring means further transferring a subsequently generated pseudorandom time signal generated by said timing means to said host computer;

said host computer including means for comparing said subsequently generated pseudorandom time signal with a table of allowable times, said host computer further including means for sending an indication back to said remote data unit of whether the time indicated by said pseudorandomly generated time signal is acceptable as shown by said comparison with said table of allowable times and, if so, said time is accepted as the next time of callback for said remote data unit and, if not, said host computer sending an indication to said remote data unit to cause said timing means to generate another pseudorandomly generated time signal, said host computer storing an indication of the acceptable time of callback for said remote data unit in said table of allowable times.

35. The apparatus of claim 34 wherein said host computer further includesmeans for sending an indication to said remote data unit of a range of allowable time, said timing means being responsive to said range of allowable times to constrain the generation of said pseudorandomly generated time signal to said allowable range.

36. The apparatus of claim 34 wherein said remote data unit includes means formonitoring the voltage of a battery powering said remote data unit and for transferring an indication of said battery voltage to said host computer when said remote data unit and host computer are in communication with each other.

37. The apparatus of claim 34 wherein said remote data unit includes means fordetecting an alarm condition and, responsive thereto, for immediately accessing said telephone line and transferring a signal indicative of said alarm condition to said host computer.

38. The apparatus of claim 37 wherein said alarm condition is an indication ofvoltage of a battery powering said remote data unit dropping below a predetermined limit.

39. The apparatus of claim 37 wherein said alarm condition is an indication of tampering.

40. The apparatus of claim 39 wherein said remote data unit includes a remoteconductive loop and means for periodically applying a current to said loop and for detecting the continuity of said loop and, responsive to an indication of a lack of continuity, generating said alarm condition.

41. The apparatus of claim 34 wherein said remote data unit includes means for detecting whether said telephone line is off-hook.

42. The apparatus of claim 41 wherein said off-hook detecting means includesmeans for measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, means for measuring an average value of an electrical characteristic of said telephone line at a predetermined time, means for comparing said initial value and said average value and for disconnecting said remote data unit from said telephone line when said comparison differs by a predetermined amount.

43. The apparatus of claim 41 wherein said off-hook detecting means includesmeans for measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, means for measuring an average value of an electrical characteristic of said telephone line at a predetermined time, means for comparing said initial value and said average value and for preventing access to said telephone line by said remote data unit if said comparison differs by a predetermined amount.

44. The apparatus of claim 41 wherein said off-hook detecting means includes means for storing said initial value.

45. The apparatus of any one of claims 42, 43, or 44 wherein said electrical characteristic is the voltage of said telephone line.

46. The apparatus of claim 34 wherein said remote data unit includes anexpansion bus arranged to receive auxiliary circuit cards for direct communication with said remote data unit.

47. The apparatus of claim 34 wherein said remote data unit further includesmeans for detecting a predetermined number of ringing signals applied to said telephone network within a predetermined time period and, responsive thereto, for accessing said telephone network and establishing communication between said host computer and remote data unit.

48. The apparatus of claim 34 in combination with a gas, electric or water meter.

49. The apparatus of claim 48 wherein said meter includes an absolute encoder in communication with the remote data unit.

50. A method for transferring data between a remote data unit and a host computer over a telephone network comprising the steps of:

(a) generating at the remote data unit a time signal in a pseudorandom manner;

(b) generating at the remote data unit an indication of real time;

(c) comparing said real time with said pseudorandomly generated time signal;

(d) accessing said telephone network when said real time equals said pseudorandomly generated time signal;

(e) transferring data from said remote data unit to said host computer;
(f) transferring a subsequently generated pseudorandom time signal generated by said timing means to said host computer;

(g) comparing at said host computer said subsequently generated pseudorandom time signal with a table of allowable times and sending an indication back to said remote data unit of whether the time indicated by said pseudorandomly generated time signal is acceptable as shown by said comparison with said table of allowable times and, if so, accepting said time as the next time of callback for said remote data unit and, if not, said host computer sending an indication to said remote data unit to cause another pseudorandom time signal to be generated; and (h) storing at said host computer an indication of the accepted time of callback for said remote data unit in said table of allowable times.

51. The method of claim 50 wherein said host computer further sends anindication to said remote data unit of a range of allowable times, said remote data unit being responsive to said range of allowable times to constrain the generation of said pseudorandomly generated time signal to said allowable range.

52. The method of claim 50 wherein said remote data unit monitors the voltageof a battery powering said remote data unit and transfers an indication of said battery voltage to said host computer when said remote data unit and host computer are in communication with each other.

53. The method of claim 50 wherein said remote data unit further performs thesteps of detecting an alarm condition and, responsive thereto, immediately accessing said telephone line and transferring a signal indicative of said alarm condition to said host computer.

54. The method of claim 53 wherein said alarm condition is an indication ofvoltage of a battery powering said remote data unit dropping below a predetermined limit.

55. The method of claim 53 wherein said alarm condition is an indication of tampering.

56. The method of claim 55 further including the steps of periodicallyapplying an electrical current to a remote conductive loop and detecting the continuity of said loop and, responsive to an indication of a lack of continuity, generating said alarm condition.

57. The method of claim 50 wherein said remote data unit further performs the step of detecting whether said telephone line is off-hook.

58. The method of claim 57 wherein said step of detecting whether saidtelephone line is off-hook includes the steps of measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, measuring an average value of said off-hook electrical characteristic at a predetermined time, comparing said initial value with said average value and disconnecting said remote data unit from said telephone line when said comparison differs by a predetermined amount.

59. The method of claim 57 wherein said step of detecting whether saidtelephone line is off-hook includes the steps of measuring an initial value indicative of an off-hook electrical characteristic of said telephone line, measuring an average value of said off-hook electrical characteristic at a predetermined time, comparing said initial value with said average value and preventing access to said telephone line by said remote data unit when said comparison differs by a predetermined amount.

60. The method of claim 57 further including the step of storing said initial value.

61. The method of any one of claims 58, 59, or 60 wherein said electrical characteristic is the voltage of the telephone line.

62. The method of claim 50 wherein said remote data unit further performs the steps of:

(a) detecting a predetermined number of ringing signals applied to said telephone network within a predetermined time period, and responsive thereto;

(b) accessing said telephone network and establishing communications between said host computer and remote data unit.

63. Apparatus for detecting an off-hook condition of a telephone line comprising:

a telephone data set coupled to the telephone line;

means, coupled to the telephone line, for measuring an initial value indicative of an off-hook electrical characteristic of said telephone line;

means coupled to the telephone line, for measuring an average value of an electrical characteristic of said telephone line at a predetermined time;

means for comparing said initial value and said average value; and means, responsive to said comparison, for causing said telephone data set to be established in an off-hook condition if said comparison differs by a predetermined amount.

64. The apparatus of claim 63 wherein said electrical characteristic is the voltage of said telephone line.

65. The apparatus of claim 63 including means for disconnecting said telephonedata set from said telephone line when said comparison differs by said predetermined amount.

66. The apparatus of claim 63 including means for preventing access to saidtelephone line by said data set when said comparison differs by said predetermined amount.

67. The apparatus of claim 63 including means for storing said initial value,means for calculating acceptable values from said initial value and storing said acceptable values in a table, and means for comparing said average value of said electrical characteristic of said telephone line with said table of acceptable values to determine whether an off-hook condition has occurred.

68. The apparatus of claim 63 wherein said value representative of an off-hook electrical characteristic of said telephone line is programmable.

69. A method of detecting an off-hook condition of a telephone line, comprising the steps of:

measuring an initial value indicative of an off-hook electrical characteristic of said telephone line;

measuring an average value of an electrical characteristic of said telephone line at a predetermined time;

comparing said measured initial value and said average value to each other; and establishing a telephone data set coupled to the telephone line in an off-hook condition if said comparison differs by a predetermined amount.

70. The method of claim 69 wherein said electrical characteristic is the voltage of said telephone line.

71. The method of claim 69 including the step of disconnecting said data setfrom said telephone line when said comparison differs by a predetermined amount.

72. The method of claim 69 including the step of preventing access to saidtelephone line by said data set when said comparison differs by a predetermined value.

73. The method of claim 69 including the steps of:

storing said initial value;

calculating acceptable values based upon said initial value;

storing said acceptable values in a table; and comparing said average value of said electrical characteristic of said telephone line with said table of acceptable values to determine whether a off-hook condition has occurred.