Vegge wrote on 2020-03-27, 05:24:
So protected mode cpu fails. So from what I can read it's toggled with the reset line from the kbc? I checked the reset line during the supersoft testing. Always low at cpu, and always hi at kbc.
80286 can enter protected mode at any time, but cannot exit it. The only way to do so is to reset the CPU (same goes for the NPU). So the AT boards have a special feature in the keyboard controller chip - it can toggle the reset line so the code can do a soft-reset when required. This process also uses a flag in the CMOS data, and in the status of the keyboard controller, to tell the difference between a normal reset and a mode switch.
So if the test fails but we have no other info about what exactly went wrong, it can be a number of things:
- CPU being bad (that'd be the first one I've heard about but I suppose it can happen)
- 8042 keyboard controller being bad or wrong kind - these have internal internal ROM with program but since the mobo did work for a while I doubt it's the wrong chip
- CMOS data getting corrupted somehow (even with weak battery should not happen on PSU power, but might be a bad diode or solder?)
There seems to be some PAL devices near your 8042, not sure what for, possibly weak? These do go bad sometimes and good luck finding a replacement. Though it might be possible to read them out and use a GAL replacement, but then again if you can read them out then they still work and don't need replacing.
And then there's the 74612 memory expander/controller. First time I see one of those. It's probably used to address the HMA and/or provide EMS banks. See, there's one more aspect of 286 - it has more address lines than 8088/86 so the A20 line must be masked to provide 100% compatibility with XT machines. This is also the job of the 8042 keyboard controller and could be part of the protected mode test. Again, could be related to the PAL chips.
Since you now have something booting with display, I'd start with removing chips and seeing what that does. '612 first, then 8042, then the PALs.