A light pen is an optoelectronic device whose light-sensitive portion is placed in proximity to a CRT. As the electron beam sweeps past the light pen, a trigger pulse is generated which can be enabled to latch the horizontal and vertical beam positions. There is no hardware bit to indicate this trigger, but this can be determined in the two ways as shown in chapter 8, "Interface Hardware." Light pen position is usually read during blanking, but MAY be available prior to that. CONNECTOR PIN USAGE FOR LIGHT PEN INPUTS ---------------------------------------- PIN MNEMONIC DESCRIPTION HARDWARE REGISTER/NOTES --- -------- ----------- ----------------------- 1 Unused 2 Unused 3 Unused 4 Unused 5 LPENPR* Light pen pressed See Proportional Inputs 6 LPENTG* Light pen trigger VPOSR, VHPOSR 7 +5V 125ma max, 200 ma surge Both ports 8 Ground 9 Unused * Note: depending on the maker, the light pen input may be either. _____________________________ | | VPOSR read only | | DFF004 |_____________________________| _____________________________ | | VHPOSR read only | | DFF006 |_____________________________| _____________________________ | | BPLCON0 write only | | | DFF104 |_|_|_|_|_|_|_|_|_|_|_|||_|_|_| 15 3 0 ^ |_ _ _ _ light pen enable _____________________________ | | POTINP read only | | DFF104 |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_| ^ 15 | 0 | PEN PRESS = POT0X |_ _ _ _ _ _ _ _ | ___________ PORT 1 / 5 \ | | o o o o o -|- - \ 6 / \ o o o o / \|______/ light pen _ _ _ _| _ _ _ _ _ | | _v_ _ _ latches V & H positions Figure E-3: Light Pen
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