Troubleshooting a 286.. oscope level!

tony duell ard at
Sat Oct 3 12:26:08 CDT 2015

> > Hello everyone,
> >
> >  I'm working to troubleshoot a 286 laptop style computer. I've kind of hit the end
> > of my knowledge and wondering if anyone has any insight.

Make/model? (Just in case anyone knows it and knows if it has any quirks).

> >
> >  The computer in question I've never seen run. So I don't know normal behavior. 
> > There were a few caps inside that were leaking electrolyte, I cleaned it all up and
> >  replaced them. A few traces look a little bit corroded but test fine.
 > >
> >  First thing I'm thinking, is did the BIOS eproms loose a bit or two of data from 
> > age? Bit rot? I did read off the two BIOS chips (high and low pair I assume.) I can
> >  see text like Copyright Pheonix Technologies 1988, but I can see that for instance
> >  the first character of the text Copyright is wrong, it's a P in one file and 9A in the
> >  other.

That would worry me. If one bit of the BIOS program (as opposed to copyright messages)
is wrong then the machine could well behave very oddly. Having found that I would want to
get a known-good dump of the BIOS into EPROMs.

That's an odd failure too. Most times true bit-rot turns 0's into 1's. Now assuming that first
character should be 'C' (or 0x43), then in one case it becomes 'P' (0x50) and in the other it
becomes 0x9A. In neither case is that simply setting bits that should be cleared. So perhaps 
not bit-rot but some other failure of the memory device.

> >
> >  Second thing I hooked up Oscope and cut on computer. It never does a floppy seek.
> >  When I poke around the 27c256 EPROM I see constant activity on all address lines,
> >  and all datalines. This includes the OE pin as well. Would a normal runnng computer
> >  hit the BIOS that much?

OE/ might be something as simple and MemRd/ (Memory Read). Is it bringing CS/ (chip select)
low that often?

A machine of that vintage may or may not 'shadow' the BIOS ROM -- that is copy it into RAM for
faster access. If it does, then it probably won't access the ROM once the copy is complete (but is
it completing the copy -- and detecting it has gone through all the locations ?). If it doesn't (which
is actually quite likely) then it will be running a program from ROM to set up I/O devices and attempt
to read the boot disk. Which means much of the processor activity _will_ be reading the BIOS ROM.

> >
> >  Any thoughts appreciated!

> When you drop into BIOS, is anything actually set?  Can you set the time, and get it to stick?
> I have a Twinhead 386sx/16 I bought new, the only thing I've used it for in the past 20 years
>  is a serial terminal, and every time I go to boot it, I have to drop into BIOS, and configure
>  things, as the BIOS battery is long dead.

Are you getting confused between the BIOS (the I/O drivers and bootstrap in EPROM) and the 
BIOS parameter table (often called the 'BIOS' or 'CMOS' (as it is stored in battery-backed
CMOS RAM) by the PC crowd)? I was under the impression that the OP's machine didn't
produce any display (does it? If so, what?) and doesn't respond to the keyboard.


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