Xerox Phaser 6121MFP Service Manual page 100

Theory of Operation
Duplex Unit Drive
2-58
The print engine's Exit Roller, driven by the Fuser gear assembly during one-sided
operations, feeds media into the Duplex Unit. For duplex operation, the duplex
option lever disconnects drive to the Exit Roller, and engages the Duplex
Switchback Motor.
This reverses the Exit Roller feeding the media into the Duplex Unit.
After entering the Duplex Unit, two Transport Rollers, driven by the Duplex Unit
Transport Motor, move media through the system.
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Phaser 6121MFP Service Manual