My intention was always to connect a PC style analogue joystick to a Playstation. This would then be used instead of the left thumb joystick on a Playstation controller pad. However it occured to me that a simpler - but still quite useful project would be a good place to start.
Many older Playstation games don’t actually require an analogue stick - but will use the directional buttons on the left hand on the pad. This project was then to interface an Atari 9 pin digital joystick to an existing Playstation controller pad to operate instead of these directional buttons:
The basis of project is the PCB from an old Joytech pad. I had hoped the directional buttons would be simple pull to ground types - but in fact they were implemented with 2 buttons on each of 2 control lines. Pulling the line high would activate one button and pulling it low would activate the opposite button. The additional circuitry therefore consisted of 2 pairs of transistors (a PNP and an NPN in each pair) and a 4049 CMOS hex inverter to buffer and invert the NPN transistor selects so than all the control lines were pull to ground. The fire button (X key on the pad) was already a pull to ground - so it was directlly connected to the fire pin on the 9 pin socket.
I have now soldered it up on stripboard - and its been tested with a Playstation 1 and also my Playstation/PC interface.
The Playstation 1 was 100% happy with it - but it sometimes lost sync with the Playstation/PC interface. I need to investigate this further - but its likely to be due to the long cable lengths affecting the clock signal.