HP 4550 Service Manual - Page 127

Last paper detected The printer detects the last page of the print job before writing the next image. This prevents toner particles from smearing on the photosensitive drum and ITB. To detect the last paper, the last paper sensor (PS3) monitors the rotation of the last paper detection roller on the multi-purpose tray. If there are two or more sheets in the multi-purpose tray, the last paper detection roller does not rotate when one sheet is picked up. Once the last paper is picked up, the last paper detection roller rotates. PS3 monitors this rotation, and a warning message is sent to the control panel. Figure 5-28 Laser paper detected Overhead transparency detection Overhead transparencies (OHT) are detected by the OHT sensor (PS301) located at the front of the registration roller. Light is emitted from PS301 and passes through the paper path, reflected by the reflective mirror, and returned to PS301. As paper passes over PS301, the light emitted from PS301 is blocked by the paper and thus does not return to PS301. Transparencies are detected when the light passes through the OHT which is reflected by the prism, and returns to the sensor. The printer is then switched to OHT mode and changes the paper path speed during transfer and fusing. Figure 5-29 Overhead transparency sensor C7085-90921 Chapter 5 Theory of operation 125