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"Overclocking" an original IBM PC/AT 5170

Anonymous Coward

Anonymous Coward

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I am interested in switching out the CPU and crystal in my IBM AT Type 2 motherboard. However, I am worried about the effects of running my board at frequencies higher than it's rated for (8MHz). I have heard of people running their AT and XT/286 motherboards as high as 33MHz?!?!??! Though when I last looked on the internets I couldn't find any information available regarding people tinkering with their ATs. Here is what I know so far:

a) If you wish to run your AT beyond the rated frequency, you will need to replace the ROM BIOS chips as they have built-in capabilities to detect CPU speed and will not allow the system to boot in this situation. In order to address this problem I have replaced my IBM ROM BIOS chips with some nice Phoenix ones.

b) The ISA frequency is fixed by a separate clock crystal, so altering the CPU speed will not cause any problems with installed adapters.

c) Changing the CPU crystal WILL affect operating frequencies of main system memory and all other components on the motherboard. i.e. the big mess of TTL chips that make up the "chipset"

Here is my desired configuation:

I am intending to upgrade my system with a 40MHz clock crystal and a Harris 80286 CPU rated at 20MHz. If everything works well, I may run with my Intel 80287XL. Since the FPU runs at only 2/3 the CPU, I am guessing it should work fine. Intel claims the 287XL operates at upto 12MHz, but it can probably go higher since the rating has more to do with the fact that Intel only produced 286s up to 12MHz than anything. I have also replaced the 150ns chips on the motherboard with 120ns chips. I may have some 100ns chips around, but I'll have to check again.

My question:

Are there any people in here that have tried to accomplish something similar with their AT type2 (319 or 339) systems? Will my components allow me to do what I wish to accomplish, or am I making a really terrible mistake? I don't want to break anything.

Additionally, I have already tried installing a Make-It-486 onto this computer, and have had limited success. The system seems to work fine, except it doesn't seem to want to reboot. Does anyone know what the reason for this might be?
 
Don't do it.

The Type 1 AT was readily overclockable from 6Mhz to 8Mhz. The Type 2 AT is essentially the same hardware, except that it has an 8Mhz clock and a slightly different BIOS.

The TTL chips on the motherboard (it's all TTL) have a particular speed rating. That includes the memory chips, address decode logic, CPU, etc. If you go above the rated speed there is no guarantee that it works. It might be flakiness, overheating, or just outright refusal to work.

You might be able to get to 10Mhz, but what's the point? If you want a 20Mhz AT, buy one of the old clone motherboards with the large ASICs. Those came in faster speeds, and are designed for it.
 
I'll second what Mike said and add that the BIOS in the type 2 AT specifically prohibited overclocking.

In a past life I was able to get my 6MHz AT up to about 9.5 without issue, but that's not really getting you much relative to the other options you have out there.
 
"what's the point?"

It will bring some joy into my otherwise boring life. I guess we really won't know if it will work until we try it. I've seen all TTL 386 and 486 systems that operate at 25MHz, so that reason alone wouldn't rule out the possibility. However, I currently have no way of knowing what the chips on the AT motherboard are rated for. Knowing IBM and their ultra-conservativeness it wouldn't surprise me if they use components that are of much better quality than what is actually needed to do the job.
I already have a real 20MHz 286 board with one of those fancy new chipsets, but I would rather take the big risk of blowing up one of my many AT motherboards. It just seems more entertaining that way.
 
I have a moral objection to people taking unnecessary risks just because they are bored.

Let's look at this from a business perspective. If IBM only needed TTL parts that ran at 6Mhz, they might buy slightly faster parts just to give them a safety margin. But do you really think that parts rated for 6 or 8Mhz are going to run reliably (or run at all) at 12 or more Mhz?

Let's talk about the system design. IBM designed the motherboard using early 1980s technology. If they had designed a system that could run at those speeds, do you really think they'd sandbag that much and not score the extra premium that they could charge?

Long story short. It's not designed for it, the parts aren't rated for it, and you might do damage. If you are bored, go to the library and get a book on electronics.
 
My brief foray into overclocking didn't last long. I got very tired of the "blue screen of death" very quickly. As we all know, you can't change the multiplier in the cpus anymore so you have to overdrive the motherboard. I ended up with stable systems with about 5% overclocking. Wasn't worth the hassle. I had several times where it would seem stable only to freak out doing a network copy. Starting over with a 250MB file transfer wasn't worth that 5%. Plus everything got hotter than normal. That turned into a big negative here in desert *dust* land, even with more fans.
 
I'm going to step up the frequency in 2MHz increments. Who knows, I might not even make it to 10MHz. I doubt anything will explode.
There may be other reasons that IBM didn't want people running their ATs faster than 8MHz. Perhaps they would rather have everyone buy one of their brand new PS/2 units. I know that IBM was planning to release an AT based on the type 1 board that ran at 10MHz. That system wass known as "Skyrocket", but it was scrapped because of the PS/2.
And perhaps the TTL chips were not the limiting factor, but rather the availability of the 80286 CPUs at the time. I'm guessing that the 16 and 20MHz parts weren't around until the late 80s or early 90s. I have read that the TTL chips on the AT boards should be rated for 20MHz operation, but there might be some crosstalk interference. I still haven't found any good information from people who have actually tried it.
Anyway, I'm going to try it...and if it fails people can laugh at me and not repeat the mistake again in the future.

Finally, To Erik:
I am curious about your AT overclock. When you did that, did you replace the CPU and memory with faster parts?
 
Well, give it a shot - you are informed of the risks now.

My original 6Mhz AT was bumped to 8Mhz just by changing the clock crystal. Everything else in the system stayed the same. Despite my dire warnings, the AT design was *very* conservative as a good business machine should be, and nearly all of the 6Mhz units were capable of running at 8Mhz. Which is why on the 8Mhz versions of the machines IBM changed the BIOS to try to prevent overclocking.

Before you start, take a look at the memory chips and try to figure out their speed rating.
 
Anonymous Coward said:
Finally, To Erik:
I am curious about your AT overclock. When you did that, did you replace the CPU and memory with faster parts?
Click to expand...

Nope. I left everything intact except for the crystal. I bought a number of them at Radio Shack with varying clock rates up to about 20 or 21 and swapped them in and out to see how things went.

The crystal in the high 18s turned out to be the most stable (none of the crystals were integers - the one I ended up with was something like 18.9xxx.)

The 6 MHz AT was deliberately underclocked by IBM and all of the parts were capable of at least 8 MHz. Going a bit over that wasn't too much of a risk, but in retrospect the reward was almost meaningless.
 
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