486 Overdrive upgrade - Any experience?

[OVERKILL]

Got some time to work on my old computer museum last night and I'm looking at my old IBM Imagination Station 486 SX/25, which has the math copro update socket, which I can apparently put a 486DX4/75 Overdrive CPU into (have to match the bus speed, the DX4/75 has a 25Mhz bus and a 3x multiplier). I located an IDE Plextor CD-R, not sure if it works or not (or whether it will work or not in this computer), and now that I have my two Pentium systems together and operational, both with floppy drives, I can actually make a boot floppy!

I'm still PO'd that I'm missing the RAM for it. It had 32MB IIRC when it was parked at my parent's and now I've only got 8MB in it.

Plan is to put NT4 on it.

Anyway, I'm looking at picking up the aforementioned Intel 486 Overdrive upgrade for it and just wondering if anybody has any experience with these? I wanted to make sure the computer worked before I went any further, which I've now verified it does, so I'm considering this upgrade path for it.

 

[racer12306]

Zero experience, but I don't get to tell this story often.

We had a 486 way back when and Desktop Dyno was so slow, like 30+min to run a dyno simulation. Mathco processor and boom couple minutes per dyno simulation. Made toying around as a kid a lot more fun lol

 

[wander2039]

I did a similar upgrade once but it's been 30 years so it's fuzzy. Don't think it made much of a difference. This was on a Packard Bell 486SX also. Was there a 133Mhz upgrade also? Seems like I tried another it worked better.

 

[OVERKILL]

I did a similar upgrade once but it's been 30 years so it's fuzzy. Don't think it made much of a difference. This was on a Packard Bell 486SX also. Was there a 133Mhz upgrade also? Seems like I tried another it worked better.

I'm going by this list:
https://www.cpushack.com/UpgradeProcessors.html

For the 486 ODP (which uses the math copro socket), the options are:

So I could do:
- ODP486SX-25 (50Mhz, 25Mhz bus)
- SX2ODP50 (50Mhz, 25Mhz bus)
- DX2ODP50 (50Mhz, 25Mhz bus)
- DX4ODP75 (75Mhz, 25Mhz bus)

The others all don't have the right bus speed. The DX4 100 is a 33Mhz bus with a 3x multiplier.

 

[LDM]

I remember trying to put one into my IBM PS/1 486 SX 25, never could get it to even POST. My guess is that the one I had wasn't compatible with the motherboard. Hard to remember the exact info as this was at least 25 years ago at this point. I did end up finding a 25 Mhz DX chip to add to it a few years ago and that worked without any issues. Pretty much for the old 486 SX chips when you put the "coprocessor" in the socket it just disabled the SX chip and everything runs through the new chip. Not sure on the Overdrive chip but the 487 coprocessor is nothing more than a full DX chip that has a working coprocessor. All 486 SX chips have the coprocessor disabled as it was a defect chip that didn't work properly.

 

[MRC01]

That computer is 35 years old?! You could buy a better faster computer for less than it would cost to restore it. If it's a demonstration computer for a museum, all it needs to do is boot up and run. Does it really need the math coprocessor and extra RAM?

IIRC, the 486 had the math coprocessor built-in (integrated FPU). If it doesn't have that, it must be some kind of early 486 that I've never heard of.

 

[Rmay635703]

If you check on vogons it was discovered on most IBM 486sx systems that there are small resistors soldered to the board where the FSB jumpers would normally be.

If you bridge/unbridge certain resistors you get different FSBs, some boards are stable at 40mhz FSB .

There is also an open source interposer project you can can regulate and jumper the 4x multiplier on 5x86 CPUS (AMD/Cyrix)

A few have used a socket extension to install a p24t to the board leaving the extra pins hanging

Good luck

 

[OVERKILL]

I remember trying to put one into my IBM PS/1 486 SX 25, never could get it to even POST. My guess is that the one I had wasn't compatible with the motherboard. Hard to remember the exact info as this was at least 25 years ago at this point. I did end up finding a 25 Mhz DX chip to add to it a few years ago and that worked without any issues. Pretty much for the old 486 SX chips when you put the "coprocessor" in the socket it just disabled the SX chip and everything runs through the new chip. Not sure on the Overdrive chip but the 487 coprocessor is nothing more than a full DX chip that has a working coprocessor. All 486 SX chips have the coprocessor disabled as it was a defect chip that didn't work properly.

Yes, that's my understanding. The "OD" chip has 1 extra pin to disable the onboard CPU and then the new CPU (which in this case is a 486DX4 @ 75Mhz) takes over. You have to match the bus speed, that's the important bit, or it won't work.

 

[OVERKILL]

That computer is 35 years old?! You could buy a better faster computer for less than it would cost to restore it. If it's a demonstration computer for a museum, all it needs to do is boot up and run. Does it really need the math coprocessor and extra RAM?

IIRC, the 486 had the math coprocessor built-in (integrated FPU). If it doesn't have that, it must be some kind of early 486 that I've never heard of.

It's an SX, they had no math copro because they were "value" CPU's, like the later Celeron CPU's.

Yes, I'm aware of how old it is, I got it when I was about 16, and it was a few years old then (it's from '93). I have a bit of an old computer hobby going on in my basement (some people call it "retro computing", but it's not really retro, it's more like classic or antique) and while I haven't been able to justify spending the money on some of the systems I want (SGI and Sun Microsystems, along with a DEC box), I'm OK with spending a bit on some of these for the experience.

Does it NEED the math copro? No, but doing the upgrade would be part of the experience. It also needs a new power button, or the existing one repaired. I will be looking around for RAM, I'm not willing to spend a ton on it, but two 16MB sticks shouldn't be unobtanium.

 

[Bottom_Feeder]

I put a Pentium OverDrive 63 in the family's 486SX/25-powered IBM PS/ValuePoint back 30 years ago. It was my first real experience doing any kind of PC hardware installing/upgrading/troubleshooting. It didn't work at first and needed a BIOS update downloaded from IBM's BBS or FTP or somewhere.

It made a big difference at the time but it wasn't good value for the money since 'real' Pentium systems had some out and were much faster. $449 in 1995 is almost a grand nowadays.

Looking at some retro tech YouTube videos lately, the 486 OverDrive CPUs were almost as fast and sometimes faster than the Pentium versions and they were cheaper. If I only knew then...

 

[Rmay635703]

I put a Pentium OverDrive 63 in the family's 486SX/25-powered IBM PS/ValuePoint back 30 years ago. It was my first real experience doing any kind of PC hardware installing/upgrading/troubleshooting. It didn't work at first and needed a BIOS update downloaded from IBM's BBS or FTP or somewhere.

It made a big difference at the time but it wasn't good value for the money since 'real' Pentium systems had some out and were much faster. $449 in 1995 is almost a grand nowadays.

Looking at some retro tech YouTube videos lately, the 486 OverDrive CPUs were almost as fast and sometimes faster than the Pentium versions and they were cheaper. If I only knew then...

Except for Quake,

There is now a regulator mod that allows 100% of POD83’s to overclock to 100mhz which is a game changer.

https://www.vogons.org/viewtopic.php?t=45365&start=60

At that speed even a 5x86-180 is having trouble cleanly outperforming the p24t.

https://www.vogons.org/viewtopic.php?f=61&t=88514

https://www.vogons.org/viewtopic.php?f=46&t=87175

Changing FSB by soldering

https://www.vogons.org/viewtopic.php?p=1194510&hilit=ibm 486 fsb resistor solder#p1194510

Yes, that's my understanding. The "OD" chip has 1 extra pin to disable the onboard CPU and then the new CPU (which in this case is a 486DX4 @ 75Mhz) takes over. You have to match the bus speed, that's the important bit, or it won't work.

An extra pin and typically voltage regulation, a pin mod/wire mod can allow a standard 486dx2 to work in an overdrive socket (as an example)

 

[PandaBear]

Got some time to work on my old computer museum last night and I'm looking at my old IBM Imagination Station 486 SX/25, which has the math copro update socket, which I can apparently put a 486DX4/75 Overdrive CPU into (have to match the bus speed, the DX4/75 has a 25Mhz bus and a 3x multiplier). I located an IDE Plextor CD-R, not sure if it works or not (or whether it will work or not in this computer), and now that I have my two Pentium systems together and operational, both with floppy drives, I can actually make a boot floppy!

I'm still PO'd that I'm missing the RAM for it. It had 32MB IIRC when it was parked at my parent's and now I've only got 8MB in it.

Plan is to put NT4 on it.

Anyway, I'm looking at picking up the aforementioned Intel 486 Overdrive upgrade for it and just wondering if anybody has any experience with these? I wanted to make sure the computer worked before I went any further, which I've now verified it does, so I'm considering this upgrade path for it.

View attachment 258964

Ah, my memory lane.

I bought one of those overdrive chips back in 93 or 94 and it "works", but not as fast as having a completely new build including the motherboard. Basically the concept of overdrive is a chip that keeps the external IO and memory the same as the older SX platform while the processing part of the CPU replaced. In 2025 it doesn't matter anymore since you are doing a retro build, but just keep that in mind it is going to be slower than a real 486 DX of the same frequency.

Math coprocessor were really not a big deal back then because nobody were using that many instructions with "math" on it. If you are running retro stuff it should be fine without it, and you have other PCs to use for those needing them anyways.

What is your plan for this PC in 2025?

 

[OVERKILL]

Ah, my memory lane.

I bought one of those overdrive chips back in 93 or 94 and it "works", but not as fast as having a completely new build including the motherboard. Basically the concept of overdrive is a chip that keeps the external IO and memory the same as the older SX platform while the processing part of the CPU replaced. In 2025 it doesn't matter anymore since you are doing a retro build, but just keep that in mind it is going to be slower than a real 486 DX of the same frequency.

Math coprocessor were really not a big deal back then because nobody were using that many instructions with "math" on it. If you are running retro stuff it should be fine without it, and you have other PCs to use for those needing them anyways.

What is your plan for this PC in 2025?

To tinker with, lol. I've put a Sound Blaster AWE32 in it now, I have a 3COM 509B (visible in the OP) and a new floppy drive in it. Replaced the CMOS battery last night, so just waiting for a deal on an Overdrive. I'll probably put WFWG 3.11 on it (was originally going to do NT4) and maybe see how Doom runs. I unfortunately disposed of all my old CRT's, including my 17" Trinitron, ages ago, so that might be something I go looking for at some point.

 

[PandaBear]

To tinker with, lol. I've put a Sound Blaster AWE32 in it now, I have a 3COM 509B (visible in the OP) and a new floppy drive in it. Replaced the CMOS battery last night, so just waiting for a deal on an Overdrive. I'll probably put WFWG 3.11 on it (was originally going to do NT4) and maybe see how Doom runs. I unfortunately disposed of all my old CRT's, including my 17" Trinitron, ages ago, so that might be something I go looking for at some point.

If you are waiting on a "deal" for the overdrive CPU, you might end up finding a better "deal" with an entire functional 486DX setup instead...

The only real use case I can think of for this overdrive chip vs a complete DX setup is to make a legacy industrial system faster. Some legacy system have limitation and cannot be upgraded to a newer PC so they can only be upgraded using these oddball setups, and the owners may pay an arm and a leg to upgrade them vs a whole PC.

 

[OVERKILL]

If you are waiting on a "deal" for the overdrive CPU, you might end up finding a better "deal" with an entire functional 486DX setup instead...

The only real use case I can think of for this overdrive chip vs a complete DX setup is to make a legacy industrial system faster. Some legacy system have limitation and cannot be upgraded to a newer PC so they can only be upgraded using these oddball setups, and the owners may pay an arm and a leg to upgrade them vs a whole PC.

Well, this was my 2nd computer, so there's sentimental value. I'm also looking to find a 486DX4/100 tower system.

 

[Rmay635703]

Well, this was my 2nd computer, so there's sentimental value. I'm also looking to find a 486DX4/100 tower system.

Your system is upgradable stock to a 5x86-100mhz if you get an interposer (a few just overvolt the 5x86 and run as is)

Desoldering 2x resistors to the clock crystal block and a 5x86-133 is possible

Your on chip video chip livens up at 33mhz FSB but it’s not the fastest.

 

[OVERKILL]

Your system is upgradable stock to a 5x86-100mhz if you get an interposer (a few just overvolt the 5x86 and run as is)

Desoldering 2x resistors to the clock crystal block and a 5x86-133 is possible

Your on chip video chip livens up at 33mhz FSB but it’s not the fastest.

Well, the reason for the Overdrive is I've always wanted one. They were insanely expensive when I was 15-16, which is when I was using this system, so I never bought one. I'm not necessarily trying to get "peak performance" out of this, more doing stuff I wanted to do to it back when it was my main computer.

So yes, I appreciate I could really go to town and do some more in-depth mods (and looks like I'll need to get a BIOS programmer, since the factory BIOS has a stroke with the Overdrive CPU's apparently and disables the onboard cache, throwing an error, but there's a modded BIOS available), including cranking up the FSB, but I'm more than happy with just trying to make it reasonably period-correct with "bolt ons" so to speak, if that makes sense?

 

[Rmay635703]

Well, the reason for the Overdrive is I've always wanted one. They were insanely expensive when I was 15-16, which is when I was using this system, so I never bought one. I'm not necessarily trying to get "peak performance" out of this, more doing stuff I wanted to do to it back when it was my main computer.

So yes, I appreciate I could really go to town and do some more in-depth mods (and looks like I'll need to get a BIOS programmer, since the factory BIOS has a stroke with the Overdrive CPU's apparently and disables the onboard cache, throwing an error, but there's a modded BIOS available), including cranking up the FSB, but I'm more than happy with just trying to make it reasonably period-correct with "bolt ons" so to speak, if that makes sense?

Yes,

Buy an Intel ODP DX4-100 overdrive if you can find one, it will run fine at 75mhz, if you change course later shooting for 100mhz it’s ready.

Intel made some perplexing decisions on its DX4 which caused some instructional timing differences which causes some early systems to have a stroke.

The dx2 though should be fine, p24t (POD) may or may not be ok.

 

[OVERKILL]

Yes,

Buy an Intel ODP DX4-100 overdrive if you can find one, it will run fine at 75mhz, if you change course later shooting for 100mhz it’s ready.

Intel made some perplexing decisions on its DX4 which caused some instructional timing differences which causes some early systems to have a stroke.

The dx2 though should be fine, p24t (POD) may or may not be ok.

OK, I've got an ODP DX4-100 coming from Australia. Will post an update once it's in-hand.

Now, onto RAM!

I've ordered the ZIP40 video RAM upgrade and also 64MB of FPM from Ebay, so hopefully those upgrades go smoothly.

 

[OVERKILL]

Before the parts start arriving, baseline benchmark results!

Also found Duke Nukem 3D and Hexen on CD, so going to install those. I've currently got WIndows 95C on it (yes, I've changed OS direction several times now), as that's what was on the hard drive.