The schematic was released in german magazine "Schneider Magazin, 07/1988", apparently using 5 diodes and a 4.5V battery.
CPC Joy Port Atari-ST Mouse Amiga Mouse JoyUp Pin1 --|>|------------- Pin1 XB Pin1 YA JoyDown Pin2 --|>|------------- Pin2 XA Pin2 XA JoyLeft Pin3 --|>|------------- Pin3 YA Pin3 YB JoyRight Pin4 --|>|------------- Pin4 YB Pin4 XB JoyFire2 Pin6 ------------------ Pin6 Left Button Pin6 Left Button JoyFire1 Pin7 ------------------ Pin9 Right Button Pin9 Right Button JoyFire3 Pin5 --|>|------------- Pin8 GND Pin8 GND JoySel1 Pin8 ------------------ Pin8 GND Pin8 GND Batt4.5V (-) ------------------ Pin8 GND Pin8 GND Batt4.5V (+) ------------------ Pin7 +5V Pin7 +5V
The Atari-ST adapter is supported by G-Paint 2.0.