Hi

I'm interested in one more thing since I do not have much experience with this 16/32 bit world.
This motherboard was designed for a 20 MHz processor by using a 40 MHz quartz. Apart from this quartz there is another one of 14.318 MHz ( which seem to be standard on all 286 motherboards ) and the 32.78 KHz one for the RTC.
On this particular motherboard the rest of the system clocks seems to derived from the 14.318 MHz one so the 40 MHz one looks to be used for the processor only so my question would be, if on another motherboard ( featuring a 16 MHz processor ) , I replace the 32MHz quartz with the 40 MHz one and replace the processor also, would that board potentially function stable with a 20 MHz processor or there may be a risk of instability ?
I'm not sure exactly how the processor cycles are synchronized between these different clocks and if all/almost all of the motherboards back then were implementing the same approach of deriving the bus clock from the 14 MHz one or some of them were using the processor one .
Thank you very much.

Some 286 processors will run at 20MHz from 16MHz... but by no means all of them. You'll find that it tends to be the supporting chips, and not the processor, that is the limiting factor when overclocking the 286 - usually the memory controller will barf when run outside its standard settings (it's [always?] a synchronous clock on the 286, with/without wait states); a 4MHz overclock is a pretty big jump for early stuff like that.

Some chipsets will tolerate it, a lot won't.

Interestingly I've also got a 82C311 'Scamp' 286 board; a Biostar branded one, which I sourced from a Russian e-waste company a few years ago. I could never get it to work - the pcb was bent like a banana. It's got to be the latest chipset possible for the 286.

Here's mine: https://www.target-earth.net/wiki/doku.php?id … _vlsi_vl82c311l

unxaix wrote on 2023-01-04, 09:19:

The BIOS file is almost empty (only 13 bytes are written starting with address 65520) but I guess this is how it should be.

Yes, all it does is two OUTs to port 0x80, then HLT - so it fits into 16 bytes. Won't beep. If you don't get anything on the POST card then you do have some sort of chipset issue.
You do get beeps on your original BIOS and that makes me think that perhaps the issue is with address lines - chipset will intercept I/O to motherboard components (like PIT, DMA, INTC, KBC) and pass the rest to ISA. If you don't get any ISA I/O writes perhaps the chipset sees the address wrong and intercepts (and passes is to wrong device).

Test all address lines between CPU and chipset. Preferably not on the CPU socket solder points but CPU pins itself in case it's a bad contact with the socket (this is actually very rare from my experience, but can happen). BTW don't stop testing on A11, test all address lines. While ISA only uses A0-A11, if some higher address line is hanging high weird things can happen.