The Vectrex has two analogue inputs representing the horizontal and vertical motion of the joystick. These expect to see between -3 and +3 volts, depending on the position of the stick. When the stick is pushed left, pin 5 reads -3V, and when pushed right, pin 5 reads +3V. Pin 6 is the vertical pot, and is negative when the sitck is down and positive when the stick is up. When the stick is released, it springs to the centre and both voltages are approximately zero.



As luck would have it, the potentiometers used in the PSX joysticks are a pretty close match for the original Vectrex ones. The resistance of the linear track is 9.4k ohms, and the wiper moves over the whole of this range as the stick is moved from one side to the other, much the same as the original.



Unfortunately, the Y-track is connected the opposite way round to what the Vectrex expects. The PCB traces need to be cut and rewired to accommodate this.



I used the left hand stick for both X and Y directions, leaving the right hand one disconnected. To get the desired voltage range of -3 to +3 volts, resistors of about 3k ohms need to be added to each of the track ends. I used surface mount resistors which I soldered directly to the PCB by using a pen-knife to scrape a bit of the green solder resist off in the area i wanted to place it. This method is quite fiddly and time-consuming, so wiring in ordinary resistors might be easier.



I also wanted to have the option of using the D-pad. To avoid having a switch, I wired it in parallel with the potentiometers and used resistors to limit the current it bleeds in. (See circuit diagram.) 4.7k ohm resistors between the D-pad contacts and the voltage rails will result in about +/-3 volts at the Vectrex input when the pad is pressed and the stick is centered. The Vectrex won't be harmed if the analogue inputs go beyond this range, so there's no need to switch out the joystick wipers.



Lastly, there is a puch switch under each joystick in the PSX. Rather than waste this, I decided to wire it in parallel with button 4 on the main board. Never found I actually used it though, so don't know if it's really worthwhile.

