Hey guys, so a few years back I picked up an early 90’s vintage generic lunchbox portable (PC-III Portable w/ Paper White VGA LCD). It had a “Topcat SX” motherboard in it (based on the VL82C320/i82C343 chipset and AMI Hi-Flex BIOS). At some point I cannibalized the BIOS EPROM because I needed a 27C512 for a piece of test gear I was repairing. Long story short, when I pulled the system out to start using it again last week, I realized I’d lost the ROM dump I’d taken of the BIOS! So, I figured I’d just throw the board in the parts pile and pickup a better board on eBay. Well, I happened upon a basically identical board for $10 shipped, the only difference being it was an SX-20 instead of an SX-16 and having SIMM holders instead of SIPP sockets; I figured at that price, why not, at least I’d have a second nearly identical board full of parts!

So the new board mostly works, however when running a memory test (via Microscope 7) I’m getting random reboots. Sometimes it makes it through two full passes, sometimes it makes it 20 seconds. I’ve tried different RAM and setting the DRAM refresh timings to the minimum settings in the BIOS but nothing seems to help. I’ve scoped the RAS/CAS and other lines on the memory bus and the DRAM *is* being refreshed like it should be. I’ve tried scoping the RESET and IO CH CK lines on the ISA bus to see if an NMI is being generated when the reset happens, again, nothing. Replaced the keyboard controller with the chip from the old board, no change. I’ve scoped the +5V and +12V rails and there’s no dips when the reboot happens. I’ve tried wiggling the SIMMs, heating and cooling various chips, wiggling connectors and cards, tapping the machine, etc. to try and cause the reboot but I can’t. Clocks look good (both directly from the output of the oscillator cans and the OSC and BCLK lines on the ISA bus).

If I leave the machine sitting at the prompt, it won’t reboot. It only seems to happen when accessing conventional memory above about 100k.

I’m wondering if one of the two chipset ICs might be bad. I can swap over ones from the old board, but before I do that I’d like to know if maybe there’s something I’m overlooking. (I mean, for all I know it could be one of the 74-series chips between the RAM and chipset.)

High Resolution Motherboard Photo: http://timb.us/images/pciii.jpg