C1 and C3 are steps related to RAM.

What kind of RAM did you install there ? SDRAM ? EDO ? did you using matching pairs of EDO ?

At first I tried with the few sticks I had at hand, but in the end got my boxes out and tried about a dozen memory configurations (matched EDO and FPM pairs, SDRAM sticks from 32 to 256MB, PC66 to PC133), to no avail.

Then, following my hunch about the BIOS, I tried with a Soyo 5EHM image, and it posts! I have attached a floppy, and it seeks at the POST screen, so the super i/o must be somewhat compatible or maybe identical, but I can't get any further because it complains of keyboard error or KB not present. The keyboard is connected and it receives +5V and I didn't investigate further.

I then tried a LuckyStar P5MVP3 BIOS and it posts with this too, sans the floppy seek, only to stop at the same keyboard error.

Is there any use for trying to solve this error, or might it be caused by the fact that I'm using a BIOS that is perhaps too different to the original? I don't see much sense in using the board with another's BIOS, even if I remedy this somehow.

Here's a dump of the original/non-working bios, maybe someone more knowledgeable could identify the manufacturer.

Thanks for the dump !

It's apparently a ENPC EP-PI11. POST string is 06/01/1998-VP3-586B-W977-2A5LEEC9C-00
I don't know that brand but now there's a name to look for.

Edit : you can find the latest bios here, version 2.0A https://www.elhvb.com/mboards/enpc/bios/index.html
and manual here https://www.elhvb.com/mboards/enpc/manuals/index.html

all added here:
http://www.win3x.org/uh19/motherboard/show/7386

That 1.4 rev bios has a different designator. I hope the newer rev is a better match for the board. Also, make sure you add 72pin memory in pairs, not a single module.

Good job guys ! I never heard of ENPC before, interesting bunch of boards they made.
added: good thing the .DOC's are MS Word8 / Word97.

Glad to report a success, I'm deeply grateful for the assistance! Went straight to the 2.1A bios from UH19, everything works now.

This bios presents two options that I don't remember seeing on other SS7 boards:

POWER ON Function [Button Only/Keyboard 98/Password/Mouse Left/Mouse Right]
KBC input clock [6/8/12/16MHz]

Any of these might have caused the keyboard error when using 'foreign' bios images, with the second being more likely (I think it hints to a register that must be programmed with a proper value for the KBC to initialize properly).

Out of curiosity, what's the (hi)story with these manuals, how come they are both in PDF and Word formats, who collected them all? And all those bios versions for various boards from a never-heard-of brand... Mind: blown 😀

Evasive can tell you the history of www.elhvb.com.
As for manuals: many of the original manuals for ENPC boards were in Office 97 .DOC format.
Back in the late 80's and 90's many manuals were made and stored on websites using various versions of Word, Word Perfect, WordStar, etc. Not until the early 2000's did .PDF become the psuedo standard.
Many of those older manuals are in odd formats not easily viewed or converted to a newer more friendly format. Luckily the ENPC ones are in an easily viewed and easy to convert format created in late 1990's.

The mobokive part of elhvb.com (which are my full initials btw) is originally collected largely by Nic Daines, aka Hardware Junkie on motherboards.org. When his privately hosted Compaq P3 Xeon server was about to keel over I offered to host the files on my newly created domain with a hosting company that offered unlimited bandwidth and storage at the time. 20 years ago, that was rare to find. I have added some collected stuff of my own and partly the info from supportbios.info from the late Kuriaki who was collecting bioses and manuals too at the time.

It has been largely sitting there silent until quite recently when I became member of vogons. Now it is in the process of being integrated into UH19.

WordStar I can convert now. Corrupted old Office docs not so much alas.
If anyone comes across a complete manual for the ATC-1535/ATC-1545 board: yes please. The ones from their website are kaput.