Posted By
TLC
on 2022-11-28
06:09:45
|  Re: 3rd joystick on SIDcard: original SIDcard and NAE/CAE
I couldn't reproduce this on my Plus/4 + old Synergy SID card + TAC-2 setup. Bit 6 appeared to be stable all the time during the test.
Looking up this part on the schematics (http://solder-synergy.de/plus4/hardware/makers/sid.gif), one'd think that $fd80 is pretty much supposed to return "1"'s for every unconnected port bits, given the 8*3.3k pull-up resistor pack RP's presence (upper right corner on the schematics).
Maybe, Q6 of the 74LS245 bus driver is missing it's connection to the respective pin of RP on this particular SID card. (Explanation: when a TTL input is left floating, it's typically on a weak high level by default - which can be pulled "low" by some electric noise very easily, i.e. by sinking very small currents. That is, if Q6 is missing it's 3.3k pull-up to +5V i.e. if this pin is left floating, then it can well pick up electric noise/crosstalk, and report 0's on some semi-periodic basis.) Ed.: similar behaviour could be also a result of Q6 having been accidentally tied to some other active signal during manufacturing (--> solder joints, slightly wrong PCB, whatever), which'd then interfere with this (otherwise unused) input.
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