HP Color LaserJet 9500 Service Manual - Page 166

Image mask control The image mask control is used to avoid the laser beam emission on a non-image area during periods other than the unblanking period. When the CNT0 and the CNT1 signals are high, and the CNT2 signal is low, all of the signals from the CPT to the laser driver IC, LD1, and LD2 are turned off automatically. The sample-hold circuit then enters the image mask state. During the image mask state, LD1 and LD2 do not emit light even if the VDO1, /VDO1, VDO2, and /VDO2 signals are sent. /BD /TOP margin masking Top and bottom signal Right and left margin masking T2 signal T1 89 ms 2 mm T3 2 mm Figure 45. Image mask control 2 mm 2 mm Horizontal synchronous control The horizontal synchronous control aligns the writing position in the image horizontal direction. The ASIC generates the unblanking signal in the main CPU based on the BD input (/BDI) signal that the Beam detect PCA sends. Then, the ASIC controls the laser control signals and forcefully emits light from LD1 and LD2 during the unblanking period. A small, fixed mirror (Beam detect mirror) is located at the scanning ending position in the optical path of the laser beam. The Beam detect mirror reflects each laser beam and the beams are sent to the Beam detect PCA in the laser/scanner unit. By detecting the laser beam, the Beam detect circuit PCA generates a /BDI signal. Based on the /BDI signal, the ASIC in the DC controller generates a horizontal sync (/BDO) signal. After inputting the /BDO signal, the formatter outputs the video signals to the DC controller to align the starting position in the image horizontal direction. 164 Chapter 5 Theory of operation ENWW