I am going to do the same but with an AMD 386sx-33 motherboard.
By disabling CPU and Motherboard cache I should be able to slow it dow to a 286-10mhz at least.

How are you going to cope with the ISA bus. Every 12/12.5MHz motherboard i have used just runs the bus at that speed. I guess it may add some waitstates. The 16MHz 286's AFAIK halve the ISA bus speed. But correct me if i am wrong.

Well, I just ordered a board with a Harris 20Mhz 80286 that has SIMM slots and the BIOS has an option to run the ISA bus synchronously or asynchronously.

The boards I already have are almost for sure not good candidates for faster CPUs and neither has SIMM slots.

See here for a 286 setup that is running at 27.2Mhz:
http://www.vcfed.org/forum/showthread.php?546 … imate-16-bit-PC

I wonder if I can get any faster than that.

cyclone3d wrote on 2020-06-24, 21:42:

Well, I just ordered a board with a Harris 20Mhz 80286 that has SIMM slots and the BIOS has an option to run the ISA bus synchro […]
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Well, I just ordered a board with a Harris 20Mhz 80286 that has SIMM slots and the BIOS has an option to run the ISA bus synchronously or asynchronously.

The boards I already have are almost for sure not good candidates for faster CPUs and neither has SIMM slots.

See here for a 286 setup that is running at 27.2Mhz:
http://www.vcfed.org/forum/showthread.php?546 … imate-16-bit-PC

I wonder if I can get any faster than that.

The chipset itself is usually the limiting factor with super fast 286 builds. Your 20MHz board might not be able to even hit 25 because of it

mR_Slug wrote on 2020-06-24, 21:30:

How are you going to cope with the ISA bus. Every 12/12.5MHz motherboard i have used just runs the bus at that speed. I guess it may add some waitstates. The 16MHz 286's AFAIK halve the ISA bus speed. But correct me if i am wrong.

There were definitely 16MHz 286 motherboards that ran the ISA clock at 16MHz. Some, like you said, divided the system clock by two. There's really no rule for determining how a 286 motherboard behaves based on clock speed. It was up to the board designer to determine the feature set, so you're just going to have to play around and read some old magazines/interenet posts.

In my opinion, if you want a 25MHz 286 you should get one of those half length AT boards that use a 386SX chipset. I have seen full length boards that take 20MHz chips, but I wouldn't just swap the CPU and crystal in a 10 or 12MHz board and expect everything to work. You need to get the chipset datasheet to see what they can handle, and you should check the date on the ICs as well. I would guess almost anything with production code after 1991 should be able to handle fast chips unless it's using a very old design.

Well, the board I am getting is a pretty late board.
It is a baby-AT board and it apparently had a version that included onboard I/O.

The chipset is a Suntac ST62C303-A.. the other part of the chipset has a sticker over the model number so I can't tell what it is.

I looked on statson.org and it isn't there and I haven't really been able to find out much of anything about this particular chipset except I think the model older than this works at 20Mhz although it isn't officially supported.

Edit: looking at what I suspect to be date codes, the latest one I see is on the 40Mhz Crystal and it looks like it was made the 34th week of 1991.

Other date codes are in 1991 as well.

Edit 2: Found pics of a 386sx motherboard with the same chipset so this makes me feel even better about this board working at 25Mhz or higher.

Edit 3: Found a list of Suntac chipsets that lists both part numbers for the chipsets.

http://www.yjfy.com/hardware/mainboardchipset/Suntac.htm

I would like to find the datasheets for these but my search has come up empty so far.