Amstrad LP-1 light pen

The LP-1 Lightpen by Amstrad

The LP-1 is the official Amstrad lightpen (with software from Amsoft, and hardware from Trojan).

The CAD-MASTER seems to be the same hardware (but marketed directly by Trojan, and with different Software included).

Technical

Connects to the Joystick port (not to the Lightpen input on the Expansion Port).

 Input:
 Light Sensor --> Keyboard.Row9.Bit1 (aka Joystick Down) (0=Light, 1=No Light)
 Output:
 Keyboard.Row9 is constantly held LOW during lightpen access
 Keyboard.Row6 is constantly held HIGH during lightpen access
 Keyboard.Colums aka PSG.Ext.PortA are configured as inputs aka HIGH-Z

The above LOW and HIGH (or HIGH-Z) levels seem to be misused as power supply GND and 5V. There seems to be no power-supply required for the CPC version (unlike the ZX Spectrum version, which connects to cassette EAR port, and which requires an external 9V battery).