Have you tried reading your 128KB dump with a hex editor to see if it's two 64KB files that are exactly the same? I seem to remember when I was writing the chip for the IDE-XT, that since I could only get the 32KB chip that I had to pad the size of the file written to fill the whole chip? It's been several years since I did that. But if your BIOS is a unique 128KB BIOS image, definitely share that. I would first check to see if it's two instances of the BIOS stacked one right after the other, after that? I'm not sure what to say.

At a second look it appears the 1st 64KB are the Video BIOS of the on-board Cirrus Logic VGA controller and the System BIOS itself is in 2nd half.

Attached is my BIOS dump.

As far as I can tell it look OK but would appreciate a second opinion. I really don't understand my my board doesn't even beep, i.e. memory error, etc. it's like life-less.
I checked all the voltages, tried different jumper configs for the CPU configuration, installed/removed RAM, disabled the on-board VGA.

CPU gets warm and has proper 33 MHz clock on its clock pin. I triple checked now for broken traces due to battery damage and at this point I'm pretty sure I got them all repaired.

Seems to me I'm the only one that has problems getting this board even to show any form of life 🙁

A little late to the thread here but I have been working on a device for addressing issues with these sockets/processors. Hope it helps.

Socket 1/2/3 Voltage Interposer Tweaker (Alpha)

Does anyone else have this Motherboard and is able to take a dump of its BIOS?
https://theretroweb.com/motherboards/s/biosta … 3F-umc498f-2vlb

If so, please upload to "TheRetroWeb.com" project (formerly: UltimateRetro.net). That would be fantastic.

I checked all jumper settings, voltages, clocks, etc ... but still don't even get a POST code on my ISA card 🙁

Is someone able to confirm if NONE of the jumpers for the CPU config in the lower right of this Motherboard, the system will still at least show some POST codes?
I would assume there should be POST codes even if the CPU config is wrong, right?
I tried different CPU configs for my Intel 486 DX/33 and DX2/66 (and yes, I tested multiple different CPUs), but never get any POST code whatsoever.

I can confirm the CPU is getting a nice and clean 33 MHz clock input. What other signals should I check next?

magicvoodooable wrote on 2022-08-09, 23:52:

At a second look it appears the 1st 64KB are the Video BIOS of the on-board Cirrus Logic VGA controller and the System BIOS itse […]
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At a second look it appears the 1st 64KB are the Video BIOS of the on-board Cirrus Logic VGA controller and the System BIOS itself is in 2nd half.

Attached is my BIOS dump.

As far as I can tell it look OK but would appreciate a second opinion. I really don't understand my my board doesn't even beep, i.e. memory error, etc. it's like life-less.
I checked all the voltages, tried different jumper configs for the CPU configuration, installed/removed RAM, disabled the on-board VGA.

CPU gets warm and has proper 33 MHz clock on its clock pin. I triple checked now for broken traces due to battery damage and at this point I'm pretty sure I got them all repaired.

Seems to me I'm the only one that has problems getting this board even to show any form of life 🙁

Hi magicvoodooable,

About the BIOS for this board, your BIOS dump looks OK to me as well. These AMI BIOSes use a Little-Endian 16-bit checksum on the BIOS core, the upper 64KB part of the 128KB dump. When I let my hexeditor do this calculation, it comes up with the correct 16-bit outcome of zero.

I also tried your TMS27PC010A dump on 86Box and got a hang and bootloop at POST code D4h. This is exactly as expected and I got the same on another AMI WinBIOS for the PCChips M912 which uses the same UMC8498 chipset.
POST D4h occurs very early during POST and is where the BIOS jumps to code in Shadow RAM after the BIOS ROM is decompressed and copied there. As 86Box doesn’t know the UMC8498 chipset, it can’t control Shadow RAM and the jump at POST D4h will end up in oblivion. 😉

But your BIOS dump runs and produces POST codes and has a correct checksum. So I’m convinced it is sound and that your board must have a hardware problem…

@nathanieltolbert: Your BIOS dump is only 64KB and is missing the Video BIOS for the onboard VGA chip. Also the checksum is incorrect, so I’m afraid it is a corrupt BIOS. 🙁
I believe it is a decompressed Shadow RAM copy of the upper 64KB of the original BIOS and can’t be used to put in an EPROM. Maybe a good idea to have it pulled from The Retro Web. 😉
Can you make a full 128KB dump from your EPROM chip with an EPROM programmer, so I can compare it with magicvoodooble’s BIOS?

Cheers, Jan

Hi Chkcpu!
Thank you so much for your efforts and feedback! That is invaluable to me knowing the BIOS should be OK.

Would you have any clue/hint what I should be checking next? Voltages seem all OK and the CPU receives the 33MHz clock. Is there any documentation somewhere describing the overall power-up process of such a 486 system? I even tested another 486 Motherboard with the same PSU to confirm the PSU isn't the issue.

On the POST card (plugged into ISA slot) I see RESET is being released (i.e. first ON, but then turns OFF) as well as the CLK LED lights up. However, the 7-digit displays stay at "---"
Also as mentioned in my previous post, I tried different Intel 486 DX/DX2 CPUs ... no luck.

Again, thank you very much for your BIOS analysis!