I did a little quick digging and found a mirror of the IBM BBS FTP something or other.

http://john.ccac.rwth-aachen.de:8000/ftp/mirr … dns.org/pccbbs/

Maybe you've already found this...there's a few BIOS updates in the aptiva directory.

Already crawled through that stuff, see my links above.
I might try another model number soon, as the sticker with the model name doesn't have to be correct. Got a C-2 mainboard, though...

Have you given p3flash a try? The North American 2144-6xP looks pretty similar spec wise to the 785...

Disclaimer: I may have no idea what I'm talking about 🤣

Yes, I did. And it didn't want it in the first three tries. Suddenly in a next run it could flash the BIOS, finally. 😀
Now it is done and I have BIOS upgraded from 1994 to 1998 date.

Next problem: The upgrade CPU I want to install, an AMD 5x86-133, is now recognized (with 1994 BIOS it didn't even POST or showed IBM logo on screen), RAM counter is running up and then it hangs. What's the issue with that now?

Got two versions Am486 DX5-133W16BGC and AMD-X5-133ADZ both used with a voltage regulator adaptor, both don't boot, but POST, as far as I can judge that by counting up the whole RAM (and then hangs).

I also cannot get into BIOS anymore to setup anything. So in the end or after the POST process, something must be wrong.
Also can't figure out the correct jumper settings yet, but it looks like it takes anything and boots up even the original CPU 486 DX/2-66 with quite any jumper setting.

Does anyone know if and where is a SL-C Type C-1 BIOS update for the 2144-27P? This board was used in other models as well. If you have the two RAM socket board, chances are good that you have the Type C-1 instead of the four SIMM Type C-2 model. If you have a BIOS newer than 1994, please kindly send me a rom dump as I have a chip programmer and a chip eraser that uses ultraviolet light. If you need a 1994 bios rom, I have backed it up as well.

Edit: Apparently, the Type C-1 motherboard works fine with the Am5x86 provided a voltage regulator adapter is used on the current 1994 BIOS it came with. The 133MHz Am5x86 is detected as a 75MHz 486DX4 but I was able to see it just fine in Windows 95 C (OSR 2.5) via CPU-Z Vintage Edition. 2x which translates to 4x for this CPU was the only setting I needed. The Intel Overdrive (486DX4 100MHz) works fine as well. The Overdrive uses its own voltage regulator so there's no need for an adapter on the overdrives. The Pentium overdrive is not compatible as this is a socket 1 motherboard and you need a socket 3 motherboard for that. The evergreen Technologies 5x86 and similar CPU upgrades can just be installed directly as they have their own voltage regulator onboard. I used default settings here of 4x and WT. WB is untested as of this post. I'm guessing L2 Cache must be installed first as there is none.

Do make sure you set the jumpers to DX instead of SX to enable the FPU. Here's a link to the board's layout and jumper settings:

https://stason.org/TULARC/pc/motherboards/I/I … -APTIVA-21.html

This board has no sockets for cache but has a location for sockets and the BIOS reports 0KB so my guess is L2 can be installed and enabled on this board. I can use 2x32MB Parity FPM gold fingered modules for a total of 64MB. With this setup, Duke Nukem 3d Atomic Edition is just barely playable at 320x240 in Windows 95 C (OSR 2.5).

As for Larger hard drive support, one must use the XT-IDE ISA BIOS on a IDE controller. The reason for this is there's no way to tell the IBM BIOS there's no Hard drive installed to allow the XT-IDE BIOS to take over the onboard controllers to avoid conflicts. If this were possible, the XT-IDE BIOS could be flashed to an EEPROM located on an ISA NIC boot ROM to detect the larger drives properly.

Hope this research helps those out there as there's very little documentation for this board.