HP Color LaserJet 5500 Service Manual - Page 204

Multiple-feed prevention The separation roller prevents multiple sheets of paper from entering the paper path. If the pickup roller picks only one sheet of paper, a clutch in the separation roller allows the paper to feed by rotating in the same direction. However, if the pickup roller picks multiple sheets, the separation roller rotates in the opposite direction from the feed roller, pushing the extra sheets back into the cassette. Only the top sheet of paper is fed into the printer. Figure 5-38. Multiple-feed prevention illustrates this procedure. Figure 5-38. Multiple-feed prevention Overhead transparency detection In order to prevent print quality problems, the printer prints on overhead transparencies (OHT) at a slower speed than on plain paper. The printer has a horizontal registration/ transparency sensor in the pickup unit that detects an OHT and signals the DC controller to slow the motors accordingly. (This sensor also detects the side-to-side alignment of paper, which is useful for duplex printing.) The horizontal registration/transparency sensor uses a light transmitter and receiver to detect the presence of plain paper. When plain paper passes through the sensor, the light is blocked, and the OHT signal changes from low to high. Consequently, when the signal is not blocked, but paper is detected by the paper leading edge sensor, the DC controller determines the media to be an OHT and adjusts the feed speed accordingly. Figure 5-39. Overhead transparency detection illustrates this process. 182 Chapter 5 Theory of operation ENWW