Posted By
TLC
on 2004-08-23
06:29:02
|  Re: Mouse emulation
The only way to get a 1350/1351 to work with the Plus/4 joystick ports is using a bus driver chip ("gate") as interface circuitry.
The problem is: whilst the C64 joyports expect the joystick inputs to be shorted to ground (to signify when that particular direction is pressed), the Plus/4 expects them to be shorted to SELECT' (a low-active output that goes low only when the joyport is selected by the CPU). Common Plus/4 joystick interfaces therefore simply connect the Plus/4 Select' outputs to the joystick's GND point.
The 1351 needs the common ground point, since it's an active device (the chip inside will obviously fail to work if either GND or +5v or both are missing). Therefore, the "SELECT' routed to GND" way simply doesn't work here. One has to supply both GND and +5v to the mouse, and then gate the mouse direction outputs to the Plus/4 joyport inputs according to SELECT'.
(Another possibility: drop the gate idea completely, and wire the interface cable directly (GND to GND, etc...). Will work, but the mouse will interfere with the keyboard (and the other joyport), since it will no longer be possible to select either the keyboard, joy port 1 or joy port 2 in the keyboard scan process exclusively).
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