HP LaserJet 4 Service Manual - Page 118

Photosensors Photosensors, monitored by the DC Controller, are responsible for detecting the presence of paper, paper motion, and timing. (See Table 5-2.) Photosensor PS2 senses paper in the PC tray; photosensor PS4 senses paper in the MP tray. The message PC LOAD or MP LOAD is displayed when either the PC tray or the MP tray is selected as the primary tray and is out of paper. PS6 and PS7 (respectively) sense the presence of paper in the Lower Cassette and the Envelope Feeder. Paper must be loaded into the primary paper tray before printing can resume. The message PC EMPTY or MP EMPTY is displayed when either the PC tray or the MP tray is out of paper and is not selected as the primary paper tray. LC EMPTY or EE EMPTY is displayed when the Lower Cassette or the Envelope Feeder is empty. The EMPTY message is only a reminder that the paper tray is empty; it is not necessary to load paper to continue printing from another tray. The Paper Control PCA, Lower Cassette Assembly, and the Envelope Feeder communicate with the DC Controller through serial data lines. The condition of the printer engine photosensors PS2, PS4, PS5, the optional Lower Cassette PS6, and the optional Envelope Feeder PS7 is encoded into a digital logic stream and read by the DC Controller on line SLI. PS1 detects paper at the Input Feed Roller and is used in jam detection and registration. PS3 detects the presence of paper in the Fuser Assembly, and is used in jam detection. PS5 terminates laser operation when it detects the end of the sheet. This allows the printer to print images on odd-sized media fed from the MP tray without contaminating the Transfer Roller. Note The LaserJet 5 printer uses different tray designators and associated messages as follows. LJ 4 / 4 Plus MP Tray PC Tray LC Tray MP/PC/LC LOAD MP/PC/LC EMPTY LJ 5 Tray 1 Tray 2 Tray 3 TRAY x LOAD (x=1,2, or 3) TRAY x EMPTY (x=1,2, or 3) 5-6 Functional Overview