Dead IBM PC 5150 motherboard - help troubleshooting

[jim s]

Richard Beaudry wrote:
> <snip>
> 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???
>   
On several of the CPU's if you hold the processor reset, you actually 
get the reset vector address out.  So on the 8088 8086 80186, you get 
FFF0 out.  I don't know if 8088 has all 16 lines out, or is multiplexed, 
but on the 186 we could troubleshoot a board we had with a 186 on it for 
a support processor by setting a flipflop that held it reset, and then 
look at the lines with a scope.

Jim