HP LaserJet 4 Service Manual - Page 133

At the beginning of each sweep (before the beam reaches the drum), the beam is reflected off the beam detect mirror. The momentary pulse of light is sent to the DC Controller, where it is converted to an electrical signal used to synchronize the output of data (VDO) for one sweep (scan line). This pulse, referred to as the Beam Detect signal (BD), is used to diagnose problems with the laser diode or scanner motor also. After the writing station, the drum surface has an invisible (latent) electrostatic image. Portions of the drum not exposed to the laser are still at the uniform negative potential (placed there by the primary charging roller), but those portions exposed to light have been discharged to ground, to form the latent electrostatic image. Figure 5-11 Drum Signals Functional Overview 5-21