On page 49 of the manual there is jumper settings for the 2nd version of the motherboard. Try looking on the board if there is a sticker or etching that indicates version 2, or match up the jumper map on page 50 with your motherboard. If it is ver. 2, then try the settings on page 49.

Are you using the write-back or write-through versions of the AMD and Intel DX4 CPUs? Write-back and write-through CPUs (the L1 cache) are jumpered differently on most or all motherboards and is noted on page 49.
That page you linked mentions there is also a BIOS version 2.03 and 2.04. Have you tried these?
What do your CMOS settings look like?
Are all the jumpers set accordingly?

Mine uses version 1.0 mainboard and supports write-back external cache but I'm not sure about internal. On the ver 1 mb there is no jumper setting for wt/wb cache. I have tried BIOS 2.02 but after having the memory issues I gave up on that because it didn't seem to help.
Here is a pic of the AMD cpus I've tried so far. Both were the same.

I think that AMD chip is a wt one

Yeah, I think you need the SV8B version.
http://www.cpu-world.com/CPUs/80486/AMD-S8048 … X4-100SV8B.html

NV8T = Write-Through
http://www.cpu-world.com/CPUs/80486/AMD-A8048 … X4-100NV8T.html

If it were me, I would try BIOS version 2.04. If that failed, I would try an Intel DX4-100 with write-through L1 cache. The Intel DX4-100 came in write-back and write-through versions of 16 KB size. The AMD DX4-100 came in 8 KB, write-back and write-though types. There was also a less common 16 KB AMD DX4-100 in write-back mode. Considering that your motherboard lacks the WB/WT jumper, I would first speculate that the motherboard is expecting a CPU with write-through L1 cache, especially since your MB version is 1.0. The MB may also be anticipating a chip with 16 KB of L1 cache, although I sorta doubt the L1 cache size has bearing on your situation, but worth a shot.

If you do not have alternate CPUs to play with, I would suggest BIOS 2.04 and jumpering your motherboard to that of a DX2-66 and try your AMD DX4 again. The majority of Intel DX2 chips are write-through type (and 8 KB). Your AMD chip is 8 KB, write-through.

Thanks for the suggestions so far guys it's appreciated. I do have a DX2-66 that works fine. The label on the cpu reads 80486DX2-66N. With this cpu the system posts fine and boots to DOS 6.22.
I'll be honest I'm a little hesitant to update the BIOS even more. It's very complicated to do and the instructions are in broken English and not very clear. I think I will try a different cache cpu and see if that doesn't work. I will keep every posted on how it goes and if it works.

I mean, try the AMD DX4 as a DX2 to see if the problem is the CPU itself, or the frequency you are trying to run it at. An AMD DX4 should run as a DX2 without problem.
The Intel DX2-66 came in WB and WT cache versions. You need to list off the S-spec from the ceramic surface to determine which. SX955 are the write-back chips and have an &EW printed on the surface.

I have the EXACT same problem on an umc vlb board (don't know the brand/type, just got it couple of days ago).

It works fine with an Intel dx2 66 and it will not work with an amd dx4-100 (wt chip). Also tried the dx2 settings (but 3,3v instead of 5v) and that also didn't post. The amd runs fine in another vlb (sis) board.
Is bios support really needed?

Just started this 486 journey, won the ''most jumpers i've ever seen'' award and it's fun.

meljor, did you try an Intel DX4-100 (WT) on that same UMC board? If so, did it work, or not?

feipoa wrote:

meljor, did you try an Intel DX4-100 (WT) on that same UMC board? If so, did it work, or not?

unfortunately i don't have one. I'm looking for an intel dx4-100, or cyrix or amd 5x86 133 but for now i only have the Intel dx2-66, amd dx4-100 and amd dx4-120. I didn't try the last one on the UMC board.
The UMC even boots with edo with the intel dx2. I tested with edo, fastpage and even an isa vga card but no go with the dx4-100.

If the sis board is faster i don't care, but if the umc is faster i want that in my vlb build.

Any suggestions?

meljor wrote:

Any suggestions?

I'd think the Intel Overdrive (the DX4 one) would have the best chances to work with older boards (they were specifically made to be somewhat plug in replacements for older chips) and those don't need lower voltages.

I thought that selecting the right jumpers would do it but after some reading i think it probably needs a vrm or something. I'll look into it....

Just a quick update for anyone interested. I thought I had purchased the right cpu and when it arrived today it was the wrong one despite what the ebay listing said it was.
I called the seller today and I will be sending it back and he promised to send the right cpu the next time. Thanks for your help so far guys and if/when I do receive the right cpu I will keep this thread updated.

Which CPU did you order and which one did you receive?

I thought I had ordered a AMDA80486DX4-100NV8T. I got the SV8B instead. Here's a link to the original listing. http://www.ebay.com/itm/111797557219?_trksid= … K%3AMEBIDX%3AIT

I guess these things happen. 😀

I hope that seller is paying for your return shipping expense.

Success! Sorry it took so long for my follow up but with the holidays and the need to return the wrong cpu it just took awhile.

Anyway, it looks like you guys were correct. I installed a DX4-100NV8T cpu tonight and it booted immediately. My system recognized the cpu without any need to change jumpers or BIOS settings and has been rock solid for the past hour.
Thank you guys so much for helping me solve this. Much appreciated.