This time I think I am correct....
I have searched on the net for a Rapid fire circuit, with positive commons, and I have found one
here.
It uses its own way to produce the auto-fire, but what interest me more is the part when the designer uses a
CD4066BE to isolate the contacts on the target computer.
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The CD4066BE is a DIL CMOS chip, and it seems a perfect option to isolate the EP from the joystick circuits. Now it only needs +5 and GND, and the control signals are on the TTL range, 0/+5v
I have modified the schematic to show only the applicable part of the schematic:
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Observe the pull-down resistors at the left side. They are necessary to maintain the switches open when the buttons are inactive, and also to avoid short-circuits when the buttons are pressed. Usually the computers joystick ports have these resistors included on the main PCB, but this circuit is emulating that to convert the port to keyboard matrix, the system used on the EP. In reality we don't know the voltage and intensity returned by the joystick circuit, but It seems that only +3,5v will trigger the switches, so enough.