A digital copy machine includes an image reading unit IR, an image memory, an image formation unit PRT, and a circulation type sheet transportation mechanism having a transportation path of a length that can accommodate n (n.gtoreq.2) sheets for reversing the side of the sheet having a copy image...http://www.google.de/patents/US5703693?utm_source=gb-gplus-sharePatent US5703693 - Digital copy machine allowing duplex copying in short time through novel recirculation timing 
