That's a weird resolution, it sounds like a custom setup possibly?
You can get similar BIOS files for Advantech SBCs here, my PCM-4825 has the same CT65550 chip: http://ftp.emacinc.com/LegacyProducts/SBC/pcm … 5/Drivers/Bios/
The VGA BIOS starts at 00010000
I had to reflash one of my Advantech SBCs because it would only display on the LCD. So no good for me, but that's how I learned about the inclusion of these files since they're gone from Advantech's site from what I recall.

You could snip out the VGA BIOS from one of these advantech bioses and put it over the VGA BIOS part of your SBC's BIOS and see if it works?
Take backups before and have a way to restore it without screen if things go wrong. I've used a TL866 eeprom writer since it's easy to recover if the rom can be flashed externally.

I've done something similar with a laptop BIOS, I had 2 AST Bravo-NB laptops, one with colour DSTN screen and one with mono. The mono one had the newer bios that's entirely unavailable now. The colour one had an earlier BIOS that could only use the hard drive geometry it shipped with, the dead 120MB drive. So I took the 1.07 BIOS, put the colour VGA BIOS where the mono VGA BIOS was and reflashed it. Which worked, now both can read hard drives.

It might be a bit more tricky with an award BIOS though, because of checksums maybe.

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As another avenue to consider, yyzkevin has demonstrated using a small program with the IBM PC110 to change the registers on the CT65535 graphics chip, which doesn't require a big file to do so:
https://www.yyzkevin.com/vpatch-success/
That file originated from here I think: https://garakutaen.sakura.ne.jp/pc110/DispImpN.html & https://garakutaen.sakura.ne.jp/pc110/DispImpO.html
If you check the CT65550 datasheet, maybe you could make something similar for the CT65550

Ooh, actually you just use CBROM to extract it and put it back into your own bios: https://www.advantech.com/support/details/faq?id=1+FY+68

Those are instructions for how to extract and modify the VGA BIOS of your own current BIOS. Edit it with the files attached on that page, integrate it back into your BIOS and that should get you working with CRT-only 😀

Thank you, I will try those BIOSes. I do see the chip being used in many systems (including laptops), but I did not know where to look for a bios.
Also, the external dump of the big ROM chip does not contain the VGA BIOS.

In the meantime, I kept reading the manuals, and it seems this simple debug script (type file|debug) is a good workaround:

1o 3d0,1
2o 3d1,1
3q

When I ran debug, "i 3d0" already shows 1, but I just wanted to make sure.
However, this is not robust, as it might screw another card when I don't use the onboard VGA.

Glad I found this post, having a few quirky video issues with a SBC that has a chips F65548 that can be fixed with switching to CRT mode (as tested in w95). So will try solutions above and possible amend the bios.

I want to check if the 65548 will be similar enough to the 65550 and that I can use most of the same tools without bricking the video section of the bios.

Or does anyone know of other tools to amend the video bios ?

UPDATE
This link contains window 3.11 /dos drivers - for 65545 but the package includes additional com files that work in dos 6.22 and behave similar to the window95 driver.

Ct.com - appears to switch mode to crt only.
Off.com- turns off the vga output (screen goes into standby)
On.com- turn on vga output (screen comes out of standby)
Sm.com - simultaneous mode.
Fp.com - internal flat panel only mode.

crt mode corrects the weird/ out of range refresh rates and restores outputs more conventional IBM resolutions which vga lcd monitors like much better. It does not stay resident in memory from what I can see.

It has helped immensely for monitor centering and incompatibility issues on this SBC ive been working on.

https://oemdrivers.com/graphics-chips-f65545

Mirror - in case site becomes unavailable

This should keep me going until I modify the bios.