When you compare each CPU, you can see that JP16 is always set to "closed" or "1-2" for 3x cpus (and the 5x86) while the DX2s have it to "open"

Though it might be something completely different, all we can do here is guessing ...

Deksor wrote:

When you compare each CPU, you can see that JP16 is always set to "closed" or "1-2" for 3x cpus (and the 5x86) while the DX2s have it to "open"

Frankly, I thought it was a voltage selector (3.45v vs 5v) since it's open for all DX2s, but your comment made me recall that some AMD DX2s were 3.45v. However, I tried it in "open", and the benchmark didn't change.

Though it might be something completely different, all we can do here is guessing ...

It's just that I've seen a lot of people mention this board on Vogons, so I think someone might be able to give me some insight.

Another thing that may help would be to update the bios. My Aopen AP43 needed one in order to make my 5x86 to work or else mine would hang at POST (it was working configured as a DX4) though that shouldn't change the frenquency

BIOS R2.30 is the latest version available I'm afraid. I also have that board, but it's the 5V only version (no voltage regulator) so I cannot test my 5x86 there.

This link may be of help however:

http://www.cpu-collection.de/?l0=co&l1=amd&l2=5x86

In particular:

"Since having a clock multiplier of four was not part of the original Socket 3 design, AMD made the 5x86 look for a two times setting from the motherboard and interpret that as four times instead. In other words, to use the 5x86 you want to set the motherboard to the 2x setting. This will actually cause the 5x86 to run at 4x."

Deksor wrote:

Another thing that may help would be to update the bios. My Aopen AP43 needed one in order to make my 5x86 to work or else mine would hang at POST (it was working configured as a DX4) though that shouldn't change the frenquency

I had the exact same thing with the AP43 board. No way it would run the 586 cpu at 2x/4x but it ran fine on the 3x33 and 3x40 setting as an Amd dx4 cpu.
As soon as i set the 2x setting it went all bananas on me and would hang right after post or run extremely slow. A bios update made it all go away and the board is running 4x 40mhz at the moment, fast and stable.

So i don't know what it is with these cpu's but bios support can be very important. This usually is not a problem with the Cyrix 5x86 (as far as i know).

Looks like AOpen is even hosting BIOS files for this board, and 2.30 is really the last version. This version is dated '93, so it's obvious that I'll never get the board to show 5x86 in POST.

Furthermore, it looks like L1 cache is in write-through mode. I can set L2 to write-back in BIOS settings, but L1 options are greyed out, so I assume it should be configured through jumpers? Can this explain why right now the CPU doesn't even reach DX4-100 levels?

So it looks like I need to a) find 2x option b) change cache to write-back.

To find out how to enable write-back, just compare the settings for the amd DX4 write-trough and write-back ^^

jheronimus wrote:

Looks like AOpen is even hosting BIOS files for this board, and 2.30 is really the last version. This version is dated '93, so it's obvious that I'll never get the board to show 5x86 in POST.

Furthermore, it looks like L1 cache is in write-through mode. I can set L2 to write-back in BIOS settings, but L1 options are greyed out, so I assume it should be configured through jumpers? Can this explain why right now the CPU doesn't even reach DX4-100 levels?

So it looks like I need to a) find 2x option b) change cache to write-back.

When options are greyed out it will be slow, happened also on the ap43 board. Your best option is to set it as a dx4-120 (3x40mhz). This way those options are probably not greyed out and you don't need bios support. It will still be fast and with 16kb cache. On the ap43 even 3x50 was a stable option but memory timings had to be set to pretty slow.

hi, friend. I know this is an old thread, but it's the only place I've found to ask.
I have that same Acer/AOpen VI15G motherboard (actually, I have two), the revision with a voltage regulator and support for 3.3 volts. It is part of my two old PCs, stored about 20 years ago and that nostalgia has made me want to put them together again (the other is a pentium).
I remember that I had it with an Am5x86 installed, or so I think, but since I wasn't sure I changed the configuration (I forgot to write down the previous one, a pity, and important!). In the documentation it mentions a "Misc 5x86", but with that the 5x86 did not work. Then I read that it is the one corresponding to Cyrix´s 5x86, and that it was best to use the configuration for the Intel 486dx4 . however with this configuration I measured 5 volts in the socket!! I tried with the configuration for the AMD DX2 and Dx4 and I got 3.45 volts. The bad thing is that this was after testing the 5x86 with the intel configuration, so I'm afraid I fried it (hope not). but I have another 80Mhz Amd 486dx2 (3.3v) and a 33mhz Intel DX. I tried with the DX2 anyway, the boards does not boot, nor give beeps, anything
two questions:
Do you have any idea why with the configuration for the intel dx4 it is giving 5 volts?
Could you share the configuration of the jumpers you are using?

kinetix wrote on 2023-07-27, 03:28:

Do you have any idea why with the configuration for the intel dx4 it is giving 5 volts?
Could you share the configuration of the jumpers you are using?

Can you take close photos of the components around the CPU and regulator area, including the markings on the regulator chip? I have this board, but without the regulator, so I need to know which components are populated to determine what jumper setting(s) is correct for 3.3V processors. With that info I should be able to provide all the necessary settings for the Am5x86.

TheMobRules wrote on 2023-07-27, 06:49:

kinetix wrote on 2023-07-27, 03:28:

Do you have any idea why with the configuration for the intel dx4 it is giving 5 volts?
Could you share the configuration of the jumpers you are using?

Can you take close photos of the components around the CPU and regulator area, including the markings on the regulator chip? I have this board, but without the regulator, so I need to know which components are populated to determine what jumper setting(s) is correct for 3.3V processors. With that info I should be able to provide all the necessary settings for the Am5x86.

the regulator is: LT1085CT
the transistor is: NDP406BL

I've used this info for the jumper settings:
https://reesesradical.tech/builds/486DX4_1/12 … 360744216_n.jpg
download/file.php?id=94738&mode=view

Thanks for the pictures! Give me some time while I go through the jumpers with my multimeter and I'll get back to you.

TheMobRules wrote on 2023-07-27, 16:42:

Thanks for the pictures! Give me some time while I go through the jumpers with my multimeter and I'll get back to you.

you welcome.
but, if you only have the 5v version, how can you measure the 3.45 settings?
I asked because in your initial post you said you had installed the Am5x86 on you board

kinetix wrote on 2023-07-27, 20:33:

but, if you only have the 5v version, how can you measure the 3.45 settings?
I asked because in your initial post you said you had installed the Am5x86 on you board

No, I don't have a 5x86 installed on this board but the 5V version is pretty much the same as the one with the regulator, just that some components are not populated and a few other minor changes. So all the pads for the regulator circuit are there even if the main components are not installed, and I can trace the configuration back to the jumper blocks.

kinetix wrote on 2023-07-27, 03:28:

In the documentation it mentions a "Misc 5x86", but with that the 5x86 did not work. Then I read that it is the one corresponding to Cyrix´s 5x86, and that it was best to use the configuration for the Intel 486dx4 .

You are correct about the "Misc 5x86" being for Cyrix CPUs, the configuration for that one is completely different to the AMD.

kinetix wrote on 2023-07-27, 03:28:

however with this configuration I measured 5 volts in the socket!!

Hmm, when you measured the voltage, did you do it with the CPU in the socket or was it just a test without CPU? The reason I'm asking is I think the board is using the VOLDET pin of the CPU to auto-detect 3.3V CPUs and it uses the transistor next to the regulator to toggle between 5V or 3.3V depending on that. So, unless you're using a 3.3V CPU that grounds the VOLDET pin (or you ground that pin manually on the empty socket) you're always going to see 5V.

In any case, the Intel DX4 settings should work, but the "AMD DX4-S SV8B" is the one you want to use for an Am5x86. With that option it should run at DX4-100 speed with L1 WB cache, the only thing to do after that would be to set the multiplier to 4x to run at 133MHz but that is easy and we can deal with it later. For now I'd focus on trying to run it as an AMD Enhanced DX4-100 (SV8B) with those settings.

Just for completion as there are parts of the circuit that I'm still trying to understand: can you make out the values on the resistors R40, R75 and R76? These are not populated in my board so I have no way of knowing. I have marked the locations in red in the attached photo.

on the resistor to the left: 150 ohms
on the resistor to the right: 10K

I measured the voltage without the CPU in the socket.
I will test every jumper config and tell you the voltage in each case

I gave the regulator a close look. for the Intel dx4 on the ADJ leg of the voltage regulator there are 3.3 volts!!!., however on the output leg (vout, the middle), the input leg and the VCC pins on the socket , 5v. the FET has 12v on the G leg and 5 on the other two.

also I measure no shorts, not in the MB power lines, nor in the regulators.

I measured Vcc for most of the 3.45v configs. off course no CPU in the socket
Intel DX4 W/T: 5.1 v
Intel DX4 W/B: 5.1 v
AMD DX2: 3.53 v
AMD DX4 V8T: 3.53 v
AMD DX4-S SV8B: 3.53 v
AMD DX4-S SV8T: 5.1v
"Misc 5x86": 5.1v

see?

Anyway, I'll stay working with the configuration AMD DX4-S SV8B for the amd5x86, and AMD DX2 for this one, to check what other problems there may be. or in the worst case, go back to 5V with an Intel DX-33
I will also measure with an oscilloscope to see if there are clock pulses, to be sure in that regard.

I asked in two other threads with the same board to see what else I get. one is a DX4 overdrive so does nt apply in regard to the 3.45V, the other seems to be an common AMD AM486DX4 and seem to be using the AMD DX4 V8T config, looking at the only 3 rows of pins visible on the pictures

kinetix wrote on 2023-07-29, 00:16:

I measured Vcc for most of the configs. off course no CPU in the socket Intel DX4 W/T: 5.1 v Intel DX4 W/B: 5.1 v AMD DX2: 3.53 […]
Show full quote

I measured Vcc for most of the configs. off course no CPU in the socket
Intel DX4 W/T: 5.1 v
Intel DX4 W/B: 5.1 v
AMD DX2: 3.53 v
AMD DX4 V8T: 3.53 v
AMD DX4-S SV8B: 3.53 v
AMD DX4-S SV8T: 5.1v
"Misc 5x86": 5.1v

see?

Anyway, I'll stay working with the configuration AMD DX4-S SV8B for the amd5x86, and AMD DX2 for this one, to check what other problems there may be. or in the worst case, go back to 5V with an Intel DX-33
I will also measure with an oscilloscope to see if there are clock pulses, to be sure in that regard.

I asked in two other threads with the same board to see what else I get. one is a DX4 overdrive so does nt apply in regard to the 3.45V, the other seems to be an common AMD AM486DX4 and seem to be using the AMD DX4 V8T config, looking at the only 3 rows of pins visible on the pictures

Interesting... not sure why this motherboard handles the voltage differently for AMD CPUs, but who knows? Anyways, if you get the 5x86 working with the SV8B configuration the only thing remaining would be to set the 4x multiplier for 133MHz. At least on my version of the motherboard you can do that by placing a jumper cap over JP25 pins 7 and 8. Doing this grounds the CLKMUL pin of the CPU (R-17), which on a regular DX4 is used to set 2x multiplier, but on the 5x86 it sets 4x for the full speed.

kinetix wrote on 2023-07-29, 00:16:

I measured Vcc for most of the configs. off course no CPU in the socket Intel DX4 W/T: 5.1 v Intel DX4 W/B: 5.1 v AMD DX2: 3.53 […]
Show full quote

I measured Vcc for most of the configs. off course no CPU in the socket
Intel DX4 W/T: 5.1 v
Intel DX4 W/B: 5.1 v
AMD DX2: 3.53 v
AMD DX4 V8T: 3.53 v
AMD DX4-S SV8B: 3.53 v
AMD DX4-S SV8T: 5.1v
"Misc 5x86": 5.1v

see?

Anyway, I'll stay working with the configuration AMD DX4-S SV8B for the amd5x86, and AMD DX2 for this one, to check what other problems there may be. or in the worst case, go back to 5V with an Intel DX-33
I will also measure with an oscilloscope to see if there are clock pulses, to be sure in that regard.

I asked in two other threads with the same board to see what else I get. one is a DX4 overdrive so does nt apply in regard to the 3.45V, the other seems to be an common AMD AM486DX4 and seem to be using the AMD DX4 V8T config, looking at the only 3 rows of pins visible on the pictures

This is the pitfall of motherboard manuals just telling you "for CPU A use configuration x, y, z" instead of actually telling you what each jumper setting actually does- particularly if quite a few of the settings seem downright wrong...

I agree with the suggestion that the AMD DX4-S SV8B should give you correct settings for the Am5x86 (apart from perhaps multiplier), but for the rest it would make sense to reverse-engineer the actual settings. I'd suggest the difference between "AMD DX4 V8T" "AMD DX4-S SV8T" would be the 3.45V/5V voltage setting. Similarly the difference between "AMD DX4-S SV8B" and "AMD DX4 V8T" should be the selector for WT/WB cache.

TheMobRules wrote on 2023-07-29, 03:00:

Interesting... not sure why this motherboard handles the voltage differently for AMD CPUs, but who knows? Anyways, if you get the 5x86 working with the SV8B configuration the only thing remaining would be to set the 4x multiplier for 133MHz. At least on my version of the motherboard you can do that by placing a jumper cap over JP25 pins 7 and 8. Doing this grounds the CLKMUL pin of the CPU (R-17), which on a regular DX4 is used to set 2x multiplier, but on the 5x86 it sets 4x for the full speed.

in my opinion, something is wrong in the jumpers settings documentation for 3.45v. I´ve been trying to find the correspondences to setting as voltage, cache type and multiplier, on both boards, the same. Unfortunately, I did not write down the configuration the board (both) had before changing it. I'm pretty sure I had it 20 years ago with that Am5x86. It would have been nice to have a "known" functional reference. and that is what I have been trying to find, the jumper configuration of those, anyone, who have one of those 3.45v processors installed on this board.

PS: Today, by chance and happily, I found the Pentium MMX-200 of the other PC, which I had kept very carefully in a small box since 20 years ago, but I couldn't find this box and was afraid I had thrown it away by mistake, with the CPU inside. I have another MMX but I never tested it and due to discoloration on the bottom I have always assumed it was fried. although I have several more pentiums, 75, 100 and two of 133 MHz (even one of the original 60 or 66MHz) the maximum interest of course is the MMX. i´ll work latter on this other PC.