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20 MHz 286 with 4 MB of RAM

Anonymous Coward said:
because the original AT boards used a 2/3 divider.
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Clarification, because the wording might suggest to some that the IBM 5170 (AT) motherboard has a clock divider chip that is dividing the clock by 2/3 for the 80287. Let's look at the second 5170 motherboard as an example. The second 5170 motherboard is considered to run at 8 MHz.

The board has a 16 MHz crystal. It feeds the 82284 clock chip. The CLK output of that chip, which is 16 MHz, is supplied to the 80286 and the 80287 (and other chips).

Within the 80286, that gets divided by 2 to create an internal clock of 8 MHz.

As for the 80287, what it does with the clock fed to it depends on whether the CKM pin (pin 39) is tied high or low. In the IBM 5170, it's tied low, which results in the 80287 dividing the clock by 3, creating an internal clock of about 5.3 MHz.

And so it is said that in the IBM 5170, the 80287 runs at two thirds (5.3 MHz / 8 MHz ) the speed of the 80286.
 
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Sorry for the faulty information. My memory must be fading. Disregard what I said about accidentally overclocking your 287.

Have a look at this neat interposer I found:

IIT-2C87-20_1a.jpg


It looks like it's designed to run the FPU at a fixed frequency regardless of CPU speed.
 
Wow, very impressive. Would the interposer take into account the FPU chip's speed as well? Such as running my 287 chip at 8mhz, for example.
Otherwise, I would need to get a 20mhz 287, which the interposer runs at.
Thanks for the eye opening info, Anonymous Coward.
 
This interposer will run the FPU at 20MHz no matter what speed the CPU runs at. If you wanted to run your FPU at a different speed, I suppose you'd have to replace the IC. Though I suppose since your motherboard already has a place for a second oscillator, I'd just order one of these:

http://cgi.ebay.com/Clock-Oscillato...emQQptZLH_DefaultDomain_0?hash=item2300bb4e9e

then pop in an oscillator chip that corresponds to your FPU, and set the jumper.
 
Thank you for laying out the different possibility. So, for the 16mhz oscillator which you linked to, the FPU would need to be able to run also at 16mhz, correct? In other words, my 8mhz 287 would be overclocked another 8mhz, which would sound unfeasible to me. I would need to get another FPU that can cope with either 16 or 20mhz speeds, most likely. If I'm understanding this wrong, please point that out, as I am just an amateur in this field.

I have tried 3 different sources on the net for getting a 20mhz 287 FPU, but they are extremely pricy; one source wanted a $400 minimum, another $250 minimum, and the cheapest one (which didn't even have a 20mhz chip, but the 12mhz version) is $130.

So, my options seem quite limited, but I will wait and see what more I can learn from all this.

I appreciate your info. very much
 
I would definitely not overclock a 287 unless you have a good heatsink clamped onto it. The Intel 287 in my PS/2 Model 30-286, running at its full rated speed of 10 MHz, runs too hot to touch for more than a second, even when the system is sitting idle. It actually runs hotter than the main 80286 CPU. Maybe the later 287 variants (287XL, Cyrix FasMath, etc.) run cooler, but the original Intel 287 is definitely an egg-fryer.
 
I thought that there also was a Cyrix 20 mhz 287, but I must be mixing things up.

There was a time that I did not want anything dearer than a 20 mhz 286 .. but they kept being too expensive and I skipped the 286.
 
I guess you won't know what speed oscillator is required unless you experiment. It'll be a multiple of 8 for sure. The CPU oscillators are usually rated at twice the speed of the actual CPU, though that depends on the design of the board. I'm not sure about FPUs, but it probably also depends on board design. The 16MHz oscillator could very well be the one you need. In any case, oscillators are cheap. Get a few, start with the slower ones and work your way up. You can use software like "Landmark Speed Test" to see what speed your FPU is running at.
 
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Great, I will do that. I've also been looking at the battery setup for CMOS. Could I use 3 AA alkaline external batteries for the motherboard? I got an encasing for it, with 2 wires (plus and minus) to hook up to the connector.
 
BIOS Dump

BIOS Dump

pianoman72 said:
I bought the motherboard. Thanks for pointing to the link on Ebay, Mike, I just couldn't resist getting a really fast 286 setup, after having wanted one for a long time. Now, which coprocessor would you guys recommend for this motherboard? I only have a 80287-8, so I doubt that would work at 2/3 the speed of this 20mhz CPU.

About the web server, I'm afraid I do not have any ISA network cards with which to set it up. Even if I did, I wouldn't know how to set that up in DOS, as I've never done it before. However, I would be willing to learn, if I could get a hold of the needed hardware.

I'm excited about installing the motherboard in my setup.
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I have a board like this here in Brazil, but it does not work because the BIOS is broken. Would not it be possible for you or someone with this card to make a copy of it and email me? (DUMP)
 
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