Lexmark Optra C710 Service Manual page 62

5016-001
Operation of the motor
When the leading edge of the paper reaches the paper pass sensor
in the duplex upper unit, the upper unit motor goes into high speed
mode (270m/sec). At this time, the duplex lower unit motor rotates in
the normal direction. The rotation is transmitted to the gears in the
lower unit as shown. When the leading edge of the paper reaches
the paper pass sensor [S3], the duplex upper unit motor stops
rotating.
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Service Manual