Older 486 motherboards would require a new crystal oscillator to be upgraded.

If I were you and had an original 1989 486dx25 and wanted to upgrade
I would drop in a dx2 and call it a day.

A DX50 will only work if the chipset and cache are up for that speed, Does the bios support different bus divisors and different memory timing?

Yeah you need a DX2-50. I think you could put in the much more common DX2-66 and just run it underclocked with no problem.
If you haven't read about it already, the DX-50 was kind of a special case, as its 1:1 clock rate means the entire board has to run at 50MHz, and lots of boards weren't up to the task once you add RAM, cache, and VLB cards, depending on the particular combination.

If you can take a high-resolution picture of the full board, someone here may be able to work some magic if it turns out it can actually run at different speeds. It may involve soldering.

Oh interesting, I wasn't aware of that distinction between DX50 and DX2-50!

Given that the motherboard has no jumpers or bios settings related to CPU speed, do you expect a DX2-50 would just be "auto detected" at 50mhz by this board and work?

Try 33MHz or 40MHz using this DX50 CPU by changing crystals.

If not, set board up as 33MHz and use a 5V DX2 66 CPU, either amd or intel.

Cheers,

Inrit wrote on 2020-11-03, 01:28:

Oh interesting, I wasn't aware of that distinction between DX50 and DX2-50!

Given that the motherboard has no jumpers or bios settings related to CPU speed, do you expect a DX2-50 would just be "auto detected" at 50mhz by this board and work?

With these 486 boards it's often possible to get away with using a faster/newer CPU than the BIOS has programmed into it in its table of known CPUs, so I wouldn't worry about that too much. It's not like a modern board where the BIOS has to be updated every time there is a new cpu released for the socket. Especially with this board possibly coming in AT420,25,33,50 variants, they probably use the same BIOS for all of them.
If you'd like, you could probably find some software like AMISETUP to dump the BIOS image, upload it here, and we can look inside it and see what CPUs it knows about.

As pentiumspeed said, speeding it up might require you to put a different crystal on that runs at a higher frequency. Or, maybe it has a chip to produce the different frequencies and the settings are just undocumented.

I'm sure it works like this board of mine : http://www.win3x.org/uh19/motherboard/show/60

See the name ? FU325/FU333/FU340. The 25/33/40 part is just the speed of the crystal installed. The PCB remains the same. Your board has a similar naming scheme.

Looking at a photo here http://www.win3x.org/uh19/motherboard/show/77 you can see that the crystal is simply zip-tied in place. You can simply take it off and attach a faster one in there 😀

By the way can you please make a backup of your bios ? I'll add it to the page.

Good thing is that 486 DX2 and SX2 CPUs double their speed totally internally and autonomous. So it is really just plug and play! Maybe some old biosses do not show the speed correctly, thats about it. Oh beware of AMD 3VOLT DX2 CPUs they need a newer motherboard or an interposer.

Inrit wrote on 2020-11-03, 01:28:

Oh interesting, I wasn't aware of that distinction between DX50 and DX2-50!

Given that the motherboard has no jumpers or bios settings related to CPU speed, do you expect a DX2-50 would just be "auto detected" at 50mhz by this board and work?

"Detection"? This is early 1990s, there's no such thing as detection 😮

The board has a 486 bus, you stick a 486 into it and it's all run at whatever speed the oscillator crystal indicates. If it's clock doubled (SX2/DX2) at twice that speed. If the parts can handle the bus speed, it runs fine, if not, it doesn't run at all, isn't stable. It's up to the user to figure it out.

According to the TH99 info the board could support 50MHz, and in fact as it's ISA only and not VLB there's a decent chance it would actually do that - but only if cache can handle it.

Hi folks,

Thanks again for the great advice, I wanted to give an update for anyone else who has one of these AT420/AT425/AT433/AT450 boards.

In my experience, it works best with a DX2, running at 50mhz. I upgraded it to 32mb of RAM, and got a decent 1MB SVGA card for it.

I tried going faster than this, but only met with failure. Here are the two upgrades I attempted:

• Tried replacing CPU with Am5x86-P75 AMD AM486DX5-133W16BHC (which has built-in 5v to 3.3v converter), but system would not boot
• Tried replacing the 25mhz crystal with a 33mhz crystal. System extremely unstable, sometimes gets through initial boot screen, but always has screen corruption and lockup within 1-2 seconds of DOS and drivers loading. Tried disabling BIOS setting "BYPASS AUTO_BUS Function" and setting ACLK divider to a higher value (was 1/4, tried 1/5 and 1/6) and still get instability, sometimes don't even get initial BIOS screen and need to reset CMOS via jumper. When it did get past BIOS screen and tried to load DOS, I did indeed see it recognized as "66mhz" (with the DX2 CPU), but it can't boot.

So this machine will be a nice 50mhz DX2 it seems. I can run almost everything I wanted to on this, was just hoping to eek out that 66mhz for a bit smoother experience in a few games, but I'm still delighted with the machine overall.

Someone had asked for a BIOS dump so they could archive it, but I don't have AMI setup. If anyone knows where I can get it (or another util to backup the BIOS) let me know, and I'm happy to share the BIOS for others with this board.

Thanks all.

Good on you. Thanks for the update.

Inrit wrote on 2020-12-22, 16:38:

Hi folks, […]
Show full quote

Hi folks,

Thanks again for the great advice, I wanted to give an update for anyone else who has one of these AT420/AT425/AT433/AT450 boards.

In my experience, it works best with a DX2, running at 50mhz. I upgraded it to 32mb of RAM, and got a decent 1MB SVGA card for it.

I tried going faster than this, but only met with failure. Here are the two upgrades I attempted:

• Tried replacing CPU with Am5x86-P75 AMD AM486DX5-133W16BHC (which has built-in 5v to 3.3v converter), but system would not boot
• Tried replacing the 25mhz crystal with a 33mhz crystal. System extremely unstable, sometimes gets through initial boot screen, but always has screen corruption and lockup within 1-2 seconds of DOS and drivers loading. Tried disabling BIOS setting "BYPASS AUTO_BUS Function" and setting ACLK divider to a higher value (was 1/4, tried 1/5 and 1/6) and still get instability, sometimes don't even get initial BIOS screen and need to reset CMOS via jumper. When it did get past BIOS screen and tried to load DOS, I did indeed see it recognized as "66mhz" (with the DX2 CPU), but it can't boot.

So this machine will be a nice 50mhz DX2 it seems. I can run almost everything I wanted to on this, was just hoping to eek out that 66mhz for a bit smoother experience in a few games, but I'm still delighted with the machine overall.

Someone had asked for a BIOS dump so they could archive it, but I don't have AMI setup. If anyone knows where I can get it (or another util to backup the BIOS) let me know, and I'm happy to share the BIOS for others with this board.

Thanks all.

That's me 😀
You can dump it simply using that piece of software http://cd.textfiles.com/microhaus/mhblackbox3 … MORY/GETROM.ZIP.

As for your conclusion, that's a useful piece of information although kinda confusing. I mean, that board should support 33MHz FSB as it's perfectly standard (the photo we have on UH19 shows a 486 DX-33 being used). Furthermore there are other boards using that chipset and they support 33MHz FSB http://www.win3x.org/uh19/motherboard/result/?chipsetId=412
What's the speed of the RAM you've installed ? Maybe look for undocumented jumpers settings ? There might be a waitstate that have to be included or something.

Deksor wrote on 2020-12-22, 17:43:

That's me 😀
You can dump it simply using that piece of software http://cd.textfiles.com/microhaus/mhblackbox3 … MORY/GETROM.ZIP.

OK, will do later and share here. Thanks!

Deksor wrote on 2020-12-22, 17:43:

As for your conclusion, that's a useful piece of information although kinda confusing. I mean, that board should support 33MHz FSB as it's perfectly standard (the photo we have on UH19 shows a 486 DX-33 being used). Furthermore there are other boards using that chipset and they support 33MHz FSB http://www.win3x.org/uh19/motherboard/result/?chipsetId=412
What's the speed of the RAM you've installed ? Maybe look for undocumented jumpers settings ? There might be a waitstate that have to be included or something.

Yes I expected 33mhz would work as well because AT433 is one of the configurations of this board, I admit I could very well be missing something to make this work. The only jumper setting I can't account for is the mysterious JP8 which says "Factory configured - do not alter" in the references I can find (including the one you linked).

The 2 settings I tinkered with in BIOS were "BYPASS AUTO_BUS Function" (which I tried both enabled and disabled) and "Bus Clock Frequency Selecte" which I tried 1/4 ACLK, 1/5 ACLK, and 1/6 ACLK. There is nothing about waitstates in the BIOS settings.

The RAM I'm using is 60ns.

Thanks.