Lexmark C720 Service Manual - Page 79

5024-001 Duplex Theory of Operation The duplex unit allows the printer to print on both sides of a sheet of paper. The duplex unit uses two motors to move the paper from the paper exit of the printer through the duplex unit. The paper is fed back into the printer through the attached lower feed unit. The duplex sequence of paper movement is as follows: 1. Paper exits the printer through the top instead of the paper exit bin. This is accomplished by an upper solenoid (DSOL-U) in the duplex unit moving the upper shutter assembly directing paper into the duplex unit. 2. The upper duplex transport motor 2 (DPM2) and rollers feed the paper into the duplex unit. 3. The paper is stopped, by the duplex registration roller, for staging. The paper passes over a paper sensor (PT5) which indicates a paper present signal. 4. The lower duplex motor 1 (DPM1) is energized and the paper is directed from the duplex registration roller through the lower shutter switching the direction of the paper to the paper reverse and transport unit. 5. The lower shutter direction is changed by the lower duplex solenoid (DSOL) allowing the paper to be driven by the duplex motor 1 (DPM1) into the lower feed unit transport rollers. 6. The paper passes over another paper sensor (PT4) for a paper present signal. 7. The lower feed unit transport rollers are driven by the lower feed transport clutch (DPKCL) and delivers the paper to the printers registration roller. 8. The paper is then printed on and passed out of the printer through the top output bin. General Information 1-49