burton@parcvax.Xerox.COM (Philip M. Burton) (07/10/87)
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I need help with a difficult and frustrating hardware problem on my
IBM PC AT (6 MHz, model 068). The system has an IBM EGA, an
IBM disk controller, and an AST ATvantage card for RAM and for
serial and parallel ports.
The system reboots spontaneously, but only very intermittently.
The problem first showed up during a recent heat wave.
If you are a hardware expert or a PC BIOS expert, please read.
Because this problem is so intermittent, the repair shops
really can do nothing.They suggest swapping boards or
running diagnostics. I've run the IBM Advanced Diagnostics
for memory, motherboard,etc., for 24 to 48 hours at a
time. Of course, this turned up nothing.
I've also opened up the system and pushed down on all the memory
chips and other socketed chips. Several people have suggested
that I use a heat gun and freeze spray approach, but engineers
at work have warned me against doing that unless I REALLLLLYYY
know what I am doing. Which I don't.
For $750, plus labor, an IBM dealer will swap out my
motherboard and I get another 6 MHz (whoopie!!) board.
A refurb 8 MHz board is $2100, but I can't do a swap. Some deal!!
And, my problem may not even be on the motherboard. Naturally,
I'd like to fix my problem inexpensively by getting a chip replaced.
This system is used at home, mainly by my wife for her
business. Typically, it is used several days a week, for
several hours at a time, then switched off. If I use it in
the evening, I turn it back on. The system unitis in a
well-ventilated area, mounted vertically, "tower" style.
There seem to be three related problems, which first showed up
during the recent heat wave, when t was over 90 F ambient:
1. The system would simply not boot up at all. The screen
remained blank, as if the monitor was turned on but not the system
unit. After 10 or 20 minutes, sometimes with a few power-on
cycles, the machine would boot up normally, and work properly.
This problem appeared only a few times, and then disappeared.
2. The system reboots spontaneously, often when there is no
keyboard or disk activity. Several people I talked with said
that this problem was common with clones, but not with
genuine IBM AT's.
This problem usually occurs when the machine has been on less
than one hour. Rebooting restores normal operation.
3. The system appears to freeze up, and then shows the message:
System Error 109
on the top line of the display. A power cycle is necessary to clear
up the system to normal operation. At times, the system reboots
spontaneously before displaying this message.
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The AT Service manual lists several different error codes for the
motherboard, but doesn't identify the cause. All it says is to
replace the motherboard, which I certainly don't want to do.
The AT Technical Reference manual seems to indicate that Error 109
is related to "Low Memory Chip Select" and this is an error
condition that occurs only during Power On Self Test. I've tried
to read the BIOS and POST listings but I can't understand what
I'm reading. I can't read schematics at all!!
The most direct reference to Error 109 is on page 5-82 (Test 7)
of the Reference Manual. There is a section of code with the comment:
Low Meg Chip Select Test
Test that a write to Address 1B0 0000 does not write to
B000:0, or 1B0 8000 does not write to B8000:0.
The addresses B000:0 and B8000:0 are video memory, and 1B0 0000 is
part of the AT's protected memory, which I have installed.
If you are willing to help, I can save you some scanning through
the BIOS code because I've marked all the pages of the BIOS code
that I thought might apply to this problem.
Ideally, a combination of hardware and software help will
narrow the problem down to a specific chip on my motherboard,
which I can get swapped out inexpensively.
----------------------
If you can help me in any way, please message me at:
burton.osbunorth@xerox.COM
or
<backbone sites>![seismo|ucbvax]!burton.osbunorth@xerox.COM
If the above don't work, try:
<seismo|ucbvax or other backbone sites>!hplabs!parcvax!burton.osbunorth
or
<seismo|ucbvax or other backbone sites>!hplabs!parcvax!burton
or
hplabs!parcvax!burton@ucbvax.berkeley.EDU
or call me at
408 737-4635
Thanks in advance. I'll try to return the favor somehow.
Phil Burton
Xerox Corporation
This is a personal matter, with no official connection to Xerox Corporation.