HP 8550mfp Service Manual - Page 182

Paper pick-up Paper pick-up begins when the print signal is sent. The pick-up roller is lowered to the paper level, and the main motor (M4) rotates the roller. This process feeds the paper into the printer from the input trays.
Tray pick-up - When the pick-up motor is in normal rotation, the tray 2 pick-up roller, feed roller 1, and separation roller are driven. The paper is then picked up from the tray 2 pick-up roller. In the case of reverse rotation, the tray 3 pick-up roller, feed roller 1, and separation roller are driven. The paper is then picked up from the tray 3 pick-up roller. • Multi-feed prevention mechanism - During paper pick-up from tray 2 or 3, the separation roller prevents multi-feeding. • Tray 2 last page detection - The last-page sensor (PS29) consists of a light emitter and light receiver. When paper is present in tray 2, the light from the light emitter is reflected by the paper and received by the light receiver. However, when the last page is picked up, the light from the light emitter is transmitted inside the printer and does not reach the light receiver. The controller board signals the formatter that there is no paper in tray 2 and stops formation of the next image. The tray 2 last-page detection prevents the imaging drum and the transfer drum from getting dirty by detecting that no paper is available before the next image is written. 180 Chapter 5 - Theory of operation EN