Dead IBM PC 5150 motherboard - help troubleshooting
ard at p850ug1.demon.co.uk
Wed Jan 23 18:27:56 CST 2008
> 5) Clock input to 8088 is dead-on 4.77xxxxxx MHz (my scope shows all the
> digits, but I don't remember them :-) -- I think the 4.77MHz is close
It is. The CPU is clocking...
> 6) One thing that seems odd is that the ADxx (Address/Data) lines seem
> "stuck". The scope does not show a waveform, and measuring with a meter
> shows some lines right at +5VDC, some at about +4VDC and some at 0VDC.
> Perhaps the processor is indeed HALTed???
What are the S lines doing? That's S0...S2 (pins 28, 27, 26 of the CPU in
maximum mode, which is how the 8088 is configured in the 5150). According
to the data sheet, S2/ = 0, S1/ = 1, S0/ = 1 is the halt mode status.
It's certainly wrong for the 8088 AD lines to be stuck. Do they do
anything, even briefly, after a reset.
IIRQ, the PC bios writes the POST error codes to port 0x60 (not 0x80 as
in ATs and later machiens). That's port A of the on-board 8255, and I
_think_ that port is configured as an output and the keyboard
shift-register connected to it tri-stated so that you can read the error
code on those port lines. See what (if anything) they're set to.
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