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Very quick repair of post code 07 of a random mainboard with a Pentium CPU (maybe also 486, 386, ???)

W

Wobblestone

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Jul 22, 2023
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Hi,
I had to post this. Although for the experienced guys here, this might not even be worth posting.
Surprisingly I did not find much about this, so maybe it might help others to do a very easy fix on a retro mainboard.

What was the problem?
The other day I pulled out my trusty working testbench, since I had to check out some ISA cards. This testbench consists of a mainboard with a Pentium CPU. The mainboard doesn't matter, but for sake of completeness it is a Soyo SY-5VA.
One day before I have removed the clock chip, which was a Dallas, on which I already soldered a new CR2032.
I switched it on and... it did not boot. The post card stopped at post code 07. The previous sequence was C0 - C1 - C6 - 0C - C3 (for some seconds) - C5 - <too fast to follow> - 06 - 07.
There it stopped. No beeps, nada.
Ok, out with the RAM. It almost instantly stopped, and beeped. Ok, thoses beeps indicate no RAM.

Then I gave up, I was in a hurry and no time for that shit.

Today I pulled out that thing again. And it instantly hit me.... I removed the clock chip. An IBM PS/2 SX55 with 386SX, which I serviced some weeks ago, did not boot at all without a clock chip. After inserting the Dallas (without new battery on it), it at least booted. The same with the Commodore PC40 with 80286.

Ok, in with the clock chip - and it again booted into DOS

Conclusion: If you have an old mainboard which used to work fine, and you've removed the clock chip, it seems to be very likely, that it will not boot! Insert again the clock chip - and it doesn't matter if the battery is good or not - and it should work again.
 
I really hate those Dallas All in one clock/battery/sram chips. Thankfully there are alternatives and you can still buy brand new manufactured ones from Digi-Key, Mouser and even Amazon. I just bought 2 yesterday that were made in the later half of 2023. Of course they will last about 5 - 6 years and then you have to replace again. I like the replacements with coin cells added for easy replacement. It won't boot probably because it needs the SRAM that is included in the module for settings. I seem to recall that it may be checked during POST?
 
Brand new ones DS12887, may not be compatible with the original DS1287 units, so be aware.

One reason that a 286+ won't boot with the Clock/RAM chip removed is obvious if you look at the BIOS listing for the PC AT. When such a system boots up, it checks for a byte in the clock RAM that says "how did I get here?"
If the answer is "you were checking the protected mode function of the CPU and did a hard reset to resume in real mode", the boot process will continue. Otherwise, it will fail.
 
It won't boot probably because it needs the SRAM that is included in the module for settings. I seem to recall that it may be checked during POST?
Click to expand...
I'm by far not an expert. But it seems to check that the clock chip is there at post, since it stops without that chip. Empty battery is not an issue. It is just annoying, since it always forgets the settings for harddiscs and so on.
When such a system boots up, it checks for a byte in the clock RAM that says "how did I get here?"
If the answer is "you were checking the protected mode function of the CPU and did a hard reset to resume in real mode", the boot process will continue. Otherwise, it will fail.
Click to expand...
Oh cool, that's the answer. Thank you very much! (y)
 
Chuck(G) said:
Brand new ones DS12887, may not be compatible with the original DS1287 units, so be aware.
Click to expand...
I have yet to find that but I have only used them on Pentium Gen motherboards so it is good to know. The datasheet states they are direct replacements for the DS1287 but yeah I am sure there are some cases anyway. The last one I replaced was on a gigabyte motherboard. Interestingly when pulled out of the socket the silkscreen indicated an ALI part number. M5818 IIRC. I am assuming this is a clone of the 1287?


Chuck(G) said:
One reason that a 286+ won't boot with the Clock/RAM chip removed is obvious if you look at the BIOS listing for the PC AT. When such a system boots up, it checks for a byte in the clock RAM that says "how did I get here?"
If the answer is "you were checking the protected mode function of the CPU and did a hard reset to resume in real mode", the boot process will continue. Otherwise, it will fail.
Click to expand...
Nice. I knew there was some mechanism so thanks for the detailed explanation.
 
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