Posted By
TLC
on 2021-06-01
07:30:32
|  Re: Let’s JoyTest! :)
@Luca - basically, this problem affects the keyboard port (K0...K7 inputs) of the TED. The cause is probably ESD (not proven yet, but, likely). The damage likely happens when someone swaps a joystick between the joystick ports. (Also, accidents can play a role. I myself killed at least two TEDs' keyboard ports while experimenting / fixing... the last case being a quite unlikely one by any measure).
The ports we could see so far ( --> BSZ also had his share of history with/concerning broken TED keyboard ports  ) broke in a "special" way. In a standard TED, reading the keyboard port is a 2-step operation; writing to $ff08 captures K0-K7 to an internal temporary latch, reading from $ff08 reads from the latch. In a TED with a "broken" keyboard port, the latch becomes "transparent" (roughly speaking), that is, writing to $ff08 no longer has any effect, and reading $ff08 always reads a recent K0-K7 state. How-why this happens is yet unknown. We also don't know yet, whether this is "characteristic" behaviour, i.e. if all or most broken TED keyboard ports fail like that.
The direct symptoms usually affect the readability of the joysticks. They either no longer work (at all), or, there is crosstalk, and/or stutter, depending on code. A very typical symptom: in Basic, clicking the fire button of joystick in port 2 gives a "Shift + Run/Stop" combination (rather than Shift alone).
Recent experiments addressed "how exactly" the broken keyboard port works. (...This evolved out of the unlucky event of me having wrecked my second TED  , and discussing ideas of how that likely happens / works / could be worked around. We both had already known about the "transparency" phenomenon, and had also known for a fact, that, with a broken TED, the joysticks could still keep working to some extent, but had no idea how that was at all possible under given circumstances). The result didn't turn out to be complicated, yet, it's somewhat difficult to explain (as it goes all down to the electronic level of how the joystick ports work). I merely created the mentioned proof-of-concept code snippets to support (or, reject) what I had suspected after evaluating some measurements. BSZ then did all the heavy lifting to incorporate the results into his tester. At this point, the question is rather the likely proportion of affected machines, and, whether the TED keyboard port fails consistently (i.e. whether a common fix could be at all feasible).
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