It may be worth mentioning that microcomputer light pens tend to be somewhat interchabeable. They need either +5V, +12V, or both as power, The actual light pen signal is a pulse that occurs when the light pen detects light from the CRT screen, it's almost always TTL level, and the only possible problem here is that it's the wrong polarity, which can be trivially fixed with a 74x04 inverter. If there's a pushbutton switch on the light pen, most likely it just grounds the appropraite wire when pressed. DEC lightpens (at least the one on my GT40) are different. They're just the phototransistor, the amplifier, etc, is inside the display. Microcomputer lightpens tend to put the electronics in the pen or its interface box. -tony