Information provided by: Andrew Gale
For a simple joystick doubler, you need two 9-way D plugs, and one
9-way D socket. Simply connect pins 1, 2, 3, 4, and 6 on the socket to
exactly the same pins on BOTH plugs. Then connect pin 8 on the socket
to pin 8 on the plug for joystick#1, and pin 9 on the socket to pin 8
on the plug for joystick#2 (NOT pin 9, notice). And that's it.
HOWEVER! You will find that you get joysticks clashing with each
other - if one joystick is pushing up-left and the other joystick is
pushing up-right for example, then it appears that BOTH joysticks are
pushing up, left and right all at the same time. How can you solve
this? There are two methods: firstly you can try adding 5 diodes to
each D plug: connect the anode to the direction pin on the joystick
port (the socket) and the cathode to the relevant direction pin on the
plug. Do this for every direction pin, and connect pins 8 and 9 on the
joystick port as before. I've had limited success with this - one port
worked but the other was temperamental. It might be better to use
germanium diodes rather than silicon diodes since the voltage drop is
lower. A far better solution is method two: use two hex tristate
buffers with /OE fed from STROBEL1 or STROBEL2 (pins 8 and 9) on the
joystick port. I shan't explain how to do it here, but if you're brave
enough to give it a go then you can probably figure it out for
yourself. Perhaps I'll post the circuit diagram up somewhere... (pin
5 is Ov, and pin 7 is +5v)