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486 to 386 problem

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Frankie

Experienced Member
Joined
Oct 26, 2003
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55
I have a FX-3000 Rev: 1.0 motherboard with a i486SX-25 CPU installed and no cache memory (I’ll list more details below). I’m trying to downgrade it to a AM386DX-40 CPU. The problem is that the computer detected it as a 16 MHz processor instead of a 40 MHz. I checked all the jumper settings and BIOS and can’t find why its running at a low clock speed.

Can anyone suggest anything about this problem?

4 MB
BIOS type: 40-0100-001131-00101111-121291-UMCAUTO-F
Diagram/Jumpers settings: http://www.peteweb.com/ftp/user-submitted/us3486.zip
 
It is possible that when the manufacturer states, "Supports 80386DX,80487SX/80486SX/DX/DX2 at 25/33/40/50/66MHz', they don't mean all of those CPUS at all those speeds, just particular combinations. I think that's unlikely.

Could be one of those motherboards where setting certain RAM speeds limits available CPU clock speeds (and vice-versa).
 
iirc, not many of those machines of that era detected much of anything, and you have to set it bios. Can you get to the bios? If you can, look for a setting that you can change. Also, I didn't even know that there was a 40mhz 80386. Look at the actual chip and see what it says. It might say 16mhz.

Nathan
 
Modem7 may be on the right track with the memory thing. Check the settings and try some tweaking. Also, are you using a 387 on the board too? Be sure it is the same speed as the 386. Some boards support asynchronous operation, and some do not. On a board that supports only synchronous, it might kick the CPU speed back to match the NPU.
On my '386 board is a jumper to select between 33MHz & 40, but it's always been flakey when run at 40, so I just keep it set back to 33, although both chips are rated for 40.

--T
 
compu_85 said:
I've got some Karlnet wireless Brouters that are based on a 40mHz AMD 386.

-Jason
Click to expand...
Ah, I stand corrected. I looked in my little box o' processors and found a 40mhz 386. I guess I said that cause I never had one of those actually inside of one of my machines. It's in the box, though.
 
I should have spotted that earlier. Intel only went to 33MHz on their 386 class processors. AMD had a 40Mhz.

My workhorse vintage system for programming has one of these with a 128K L2 cache and it is quite competitive with low-end 486s except for the lack of floating point.
 
NathanAllan said:
iirc, not many of those machines of that era detected much of anything, and you have to set it bios. Can you get to the bios? If you can, look for a setting that you can change. Also, I didn't even know that there was a 40mhz 80386. Look at the actual chip and see what it says. It might say 16mhz.

Nathan
Click to expand...

It won't have a bios/cmos setup, it will have only the jumpers which you have to set the correct bus speed by.
 
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I don’t have a coprocessor installed. This computer does have a BIOS. The information in the zip folder shows that there are no jumper settings to change the clock speed. As for the BIOS, there are no settings to change the clock speed either. I tried changing the memory speed but that had no effect.

CPU type: A80386DX/DXL-40
 
There is probably an undocumented jumper for selecing the speed for a 386 CPU. Are there any on the motherboard that don't appear in the gif image, or that are not documented in the text file?

I found a few "similar" motherbords where J4 would select a 20MHz clock or 16MHz clock for the 386.
 
I think I found the problem. After doing a through look of the motherboard I found a crystal oscillator. I didn't think much of it at first, but when I tested a i486DX2-66 processor yesterday, the computer detected a clock speed of 50 MHz, the original was a i486SX-25. When I checked the crystal, its was rated at 25 MHz. Also it's socket, not soldered.

If I replace it with a 40 Mhz crystal, will it fix the problem?
 
Frankie said:
If I replace it with a 40 Mhz crystal, will it fix the problem?
Click to expand...

Might work, as long as all the other components can keep up with the extra speed. Oscillators are cheap, so it's probably worth a try (at your own risk, of course, keeping in mind that it might fry sum'n). I would imagine that the crystal is socketed for a reason...

--T
 
If you do web search, you'll see lots of code showing how to determine the make/model of your 386/486 (e.g. "AMD 486 DX/4").
This is done principally by the use of the CPUID instruction, but on some CPU's (that don't support the CPUID instruction), the presence/absence of certain bugs/features is used.

With the 386/486, the only way you could tell what the rated speed of the CPU was, was to look at what was stamped on the CPU.
As far as speed was concerned, utilities of the time simply gave you benchmark figures (e.g. your CPU is running at the same speed as a 386-20).

And so with the BIOS/POST on 386/486 motherboards not being able to determine the rated speed of the CPU, it is up to the user to configure the motherboard to deliver the correct/desired clock speed to the CPU. That has to be done via jumpers, switches or via the BIOS setup.

Yes, it's odd that the crystal is socketed, but I think it's even more odd that a motherboard manufacturer would produce a motherboard where a PC builder would need to change crystals in order to accommodate all of the supported CPU's ("Supports 80386DX,80487SX/80486SX/DX/DX2 at 25/33/40/50/66MHz").

Could this be one of those motherboards with a hidden BIOS setup screen (i.e. once in the BIOS setup, a special key sequence is then used to access the 'advanced' BIOS settings)? I came across one of these many moons ago but I can't remember the details.
 
http://www.uktsupport.co.uk/acer/faq/acer.htm shows that on some Acer models, the 'advanced' section is hidden.

Also from the Net: "I have Award BIOS ... On my system, Ctrl-F1 while on the main page of the CMOS setup program will un-hide some hidden menus, but it was in the manual."

And so hidden menus in the setup is still a possibility. You mentioned that you tried changing the RAM speed. As the diagram/text you supplied shows no jumpers to do that, you must have done that via the setup. If so, it doesn't make sense to me that a manufacturer (/BIOS programmer) would put RAM speed in the 'standard' section of the setup and then put CPU clock speed (something of similiar complexity to a user) in the 'hidden' section. Still worth looking into. You have an AMI/Award BIOS - try "Ctrl-F1 while on the main page of the CMOS setup".

By the way. You were unsure of the motherboard manufacturer. There's a website that identifies AMI BIOS "40-0100-001131-00101111-121291-UMCAUTO-F" as being used by the 'Elitegroup FX3000' motherboard.
That ties in with the diagram/text for a US3486 motherboard that you supplied - also made by Elitegroup.
 
I know FX-3000 is my motherboard, it's just when I did a web search, it brought me here: http://www.peteweb.com/index.php?showtopic=3984 (look at "Specific Models"). The jumpers matched perfectly with the text and diagram. The only thing different is the local bus, there are holes where it should be.

As for the hidden BIOS options, I could not find any. However, I did find this program http://fresh.t-systems-sfr.com/pc/src/misc/old/amis2990.zip/ which did allow me to access hidden options, but none of them were useful to my problem..
 
Elitegroup obviously released the FX-300 and US3486 about the same time.

I think you're at the point where you need to locate the user manual, either for the FX-3000 or for the US3486.
 
I changed the crystal with one rated at 40 MHz. The computer started up with no problems but its still not detecting the right speed. It did detect a different speed; right now it sees it as a 20 MHz processor instead of 16. The computer does feel faster. I used a program that detects CPU IDs but it didn't work (“N/A”).

Like I said, it did detect a new speed and does feel faster. Could it be that the computer needed a 20 MHz crystal instead? If someone on this forum has a motherboard with an AM386DX-40 CPU, it would help if you can give me the rating on your crystal so I can verify this (look below if you don't know what it looks like)..

modem7 said:
As far as speed was concerned, utilities of the time simply gave you benchmark figures (e.g. your CPU is running at the same speed as a 386-20).
Click to expand...
I would like to use a utility like that to test my CPU (just in case its a BIOS error).
 
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