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Tell me about the word, the word from the IRD

Towmater

Experienced Member
Joined
May 14, 2017
Messages
208
I can’t get a 286 to post. There is no corrosion. The board only has ISA slots. After trying two ISA/PCI POST cards, all I get is an unlit IRDY LED, although it is unclear if the IRDY signal only refers to PCI signals. (Searches for IRDY error only results in discussions about POST cards.) There are no hex codes displayed. There is no signal on a known-working VGA card.

I’ve tried burning new Phoenix BIOS ROMS, tested all socketed 74 logic in a Mini-pro, checked clocks with an oscilloscope, see normal activity on two of the three CPU’s that I’ve tried, and am fast running out of options.

I’ve ordered a set of chip sockets, assuming that I will need to remove and test each soldered-in IC, as well as an eBay Motorola clock/BIOS setting chip, but if you can suggest anything that I can test before desoldering and likely butchering a lot of traces, I’d love to hear what procedures you might follow to troubleshoot mainboards.
 

modem7

10k Member
Joined
May 29, 2006
Messages
10,310
Location
Melbourne, Australia
The board only has ISA slots. After trying two ISA/PCI POST cards, all I get is an unlit IRDY LED, although it is unclear if the IRDY signal only refers to PCI signals. (Searches for IRDY error only results in discussions about POST cards.)
IRDY# is a signal on the PCI bus, and not on the ISA bus.

My modern POST card is shown at [here].
In its manual, the description of the IRDY LED includes, "LED that turns on an off when there is a message."
If I plug the card into an ISA bus, I see the IRDY LED lit, and expectantly, the LED flickers a little during the POST's RAM check.
Using a logic probe, I tried to see if there is a direct correlation of the flashing to one of the 'control signal' pins in the ISA slot, but I could see no correlation.
There was partial correlation, but nothing direct.
I then traced where the IRDY LED goes to, and found that it connects via a resistor to chip U2 (a GAL).
So maybe my POST card is synthesising an IRDY-like signal from multiple pins in the ISA slot.

If your POST cards are different to mine, then when they are fitted to an ISA slot, the expected IRDY LED behaviour may be different.
Although you should be able to establish the 'norm' by use in a known working motherboard.
 

Towmater

Experienced Member
Joined
May 14, 2017
Messages
208
Should be "and unexpectedly"
ironically, modem7, you led to the solution. For one brief, shining moment, after replacing the roms with the originals, the machine booted. I replaced the bios sockets with double-wipe versions and the machine has worked consistently since. Thanks!
 

modem7

10k Member
Joined
May 29, 2006
Messages
10,310
Location
Melbourne, Australia
Only about three months ago, there was a member here (Carles20VT) who identified a faulty IC socket for a BIOS ROM.

I had to replace one on an IBM 5160 motherboard that I had been using for many years as an experimental and research board. The IBM BIOS ROM at U18 was very frequently swapped out for other ROM's. I had started to notice intermittent starting failures after I swapped the ROM. A socket replacement fixed the problem.
 
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