My old recapped classic has some ADB gremlins I'm trying to debug.
More specifically, the symptoms are that the system starts to behave as if the "a" key were being held down. Sometimes this can be interrupted, but usually not. This can occur even if the keyboard is not plugged in (and just in case the mouse might have been the culprit, I've confirmed that it happens with multiple mice).
Although the problem starts with the stuck "a" key thing, eventually the mouse begins to behave erratically as well, and before long it becomes uncontrollable, pausing or making jerky jumps across the screen when you try to move it. All of this happens reliably except when it doesn't---it seems to be a good days/bad days sort of thing.
Theory time:
I am moderately certain that the ADB scancode for the "a" key is 0x00 [1], which is a suspicious coincidence. I have learned from the same resource a bit about how the bus works, and given that ones and zeros on the bus are both signals that start low and finish high [2], I wonder whether it might be difficult for the bus transceiver chip to "hear" a bunch of zeros from the keyboard on accident (to say nothing of the start and stop bits that ADB communication requires). Rather, based on these schematics (for the SE) [3] , I wonder whether the VIA may be receiving spurious interrupts from the transceiver---and if the transceiver had not shifted any bits into the VIA when that happens, then the VIA might assume that it was hearing 0x00 from the keyboard. If this in turn occurred just as the computer was polling the bus for news from the keyboard, then perhaps you'd see the issue.
(Note that the high-order bit in that keycode means "key down", which means that the computer would only have to hear a 0x00 once in a while in order to think that the user had started pressing down the "a" key.)
Has anyone run into this before?
[1] https://archive.org/stream/apple-guide-macintosh-family-hardware/Apple_Guide_to_the_Macintosh_Family_Hardware_2e#page/n343/mode/2up
[2] https://archive.org/stream/apple-guide-macintosh-family-hardware/Apple_Guide_to_the_Macintosh_Family_Hardware_2e#page/n351/mode/2up
[3] https://archive.org/stream/apple-guide-macintosh-family-hardware/Apple_Guide_to_the_Macintosh_Family_Hardware_2e#page/n331/mode/2up
More specifically, the symptoms are that the system starts to behave as if the "a" key were being held down. Sometimes this can be interrupted, but usually not. This can occur even if the keyboard is not plugged in (and just in case the mouse might have been the culprit, I've confirmed that it happens with multiple mice).
Although the problem starts with the stuck "a" key thing, eventually the mouse begins to behave erratically as well, and before long it becomes uncontrollable, pausing or making jerky jumps across the screen when you try to move it. All of this happens reliably except when it doesn't---it seems to be a good days/bad days sort of thing.
Theory time:
I am moderately certain that the ADB scancode for the "a" key is 0x00 [1], which is a suspicious coincidence. I have learned from the same resource a bit about how the bus works, and given that ones and zeros on the bus are both signals that start low and finish high [2], I wonder whether it might be difficult for the bus transceiver chip to "hear" a bunch of zeros from the keyboard on accident (to say nothing of the start and stop bits that ADB communication requires). Rather, based on these schematics (for the SE) [3] , I wonder whether the VIA may be receiving spurious interrupts from the transceiver---and if the transceiver had not shifted any bits into the VIA when that happens, then the VIA might assume that it was hearing 0x00 from the keyboard. If this in turn occurred just as the computer was polling the bus for news from the keyboard, then perhaps you'd see the issue.
(Note that the high-order bit in that keycode means "key down", which means that the computer would only have to hear a 0x00 once in a while in order to think that the user had started pressing down the "a" key.)
Has anyone run into this before?
[1] https://archive.org/stream/apple-guide-macintosh-family-hardware/Apple_Guide_to_the_Macintosh_Family_Hardware_2e#page/n343/mode/2up
[2] https://archive.org/stream/apple-guide-macintosh-family-hardware/Apple_Guide_to_the_Macintosh_Family_Hardware_2e#page/n351/mode/2up
[3] https://archive.org/stream/apple-guide-macintosh-family-hardware/Apple_Guide_to_the_Macintosh_Family_Hardware_2e#page/n331/mode/2up
