HP Color LaserJet 8500 Service Manual - Page 203

Pick-up and feed system When the 2,000-sheet input unit is loaded with paper and the paper tray is closed, the paper stack lifts into position under the pick-up, feed, and separation rollers. This operation is detected by the PS34 sensor in the 2,000-sheet input unit's pick-up assembly. The presence of paper in the paper tray is detected by the PS33 sensor. The paper level is detected by switches SW601 and SW602. The paper size is detected by switches SW701 through SW704. When the EPH controller board on the printer sends an input command to the controller board PCA on the 2,000-sheet input unit, the paper deck driver runs the pick-up motor to rotate the pick-up, feed, and separation rollers. As the pick-up roller turns, the paper feeds into the VTU and passes through the PS31 and PS32 sensors. If the paper fails to reach the PS31 and PS32 sensors in the VTU within the allotted time, the controller board PCA on the 2,000-sheet input unit assumes that a paper jam has occurred. The 2,000-sheet input unit stops operating and reports the jam to the EPH controller board on the printer. A paper jam message appears on the printer control panel. EN 2,000-sheet input unit 201