The Am5x86 requires settings with write-back L1.
Try to use the settings "Cx486S + Cx487S", that should be OK with the AMD.

Since you are using an interposer, I would rather jumper the board for 486DX/DX2. The interposer should take care of the rest, as that is what it was made for. On the interposer, you may need to change the "Intel" jumper.

What's with the jumper for the Cyrix 1X/2X mode select. I've never seen a board with that jumper before. Looks pretty interesting.

Also, Am5x86 doesn't require write back L1. It's optional. However, with it enabled it will be faster.

Anonymous Coward wrote:

What's with the jumper for the Cyrix 1X/2X mode select. I've never seen a board with that jumper before.

Curious. Perhaps the Cyrix DX2 can be run in 1x mode? Or perhaps the BIOS changes some timings for clock doubled CPUs and this jumper is merely to assist the BIOS in its detection of chips.

The AMD 5x86 chip may not be working because of L1 write-back mode. Many of these older 486 boards didn't work with the Am5x86 in write-back mode. It is possible that some of the jumpers on the interposer are for setting L1 in either write-back or write-through mode. This should be fairly easy to verify with a multi-meter.

Check the WB/WT input pin on the CPU to see which header pin corresponds to WB/WT. If WB/WT is pulled high (3-5 V) at reset, the Am5x86 will run in write-back mode. If WB/WT is pulled to GND, the Am5x86 will run in write-through mode. WB/WT is pin B13. Even if forced to WT mode, there is no guarantee it will work in this motherboard, but on most newer PCI 486 motherboards, the Intel DX4-WB shares the same CPU configuration jumpers as the Am5x86, except for CLKMUL.

The other possibility is that jumper just enables/disables an oscillator divider. I think the actual function of this jumper should be investigated.

Hm i use the same Interposer on a older Industrial SBC which had a DX33 installed, just set 2x Multipler and WT would suffice.

This early '94 board will not boot with Am5x86 if you don't have the latest bios installed (version 3276GN1)

Thanks all.

I got it working.

First I found the manual for the interposer.

For future reference:
J1: Multiplier for CPUs
1-2 for 2x and 4x 2-3 for 2.5X, open for 3x
J2: 2x Multiplier for AMD DX4-80/100 only
J3: Voltage select
1-2 for 3.45V, 2-3 for 3.30V

So I put the missing J3 jumper to select 3.45 which is the correct voltage for my 133W16BGC version of 5x86

Then I configured the board to 486DX2 and left the resistor array on top of the board empty. Then the mb booted fine.

However, the cache seems to be running in write-through mode no matter what I do. So I guess this particular board is not the best base for this CPU.

The trick was to configure as 486 DX2 and remove the resistor array.