IBM PC/AT Coprocessor

[lyonadmiral]

I'm assuming that the coprocessor has to match the speed of the processor? If that is the case, without ripping the case apart, how can I tell, I'm assuming software to tell what type of processor is in the system speed wise.

Thanks,
Dan

 

[Agent Orange]

I'm assuming that the coprocessor has to match the speed of the processor? If that is the case, without ripping the case apart, how can I tell, I'm assuming software to tell what type of processor is in the system speed wise.

Thanks,
Dan

Depends on your CPU - what's your current configuration.

 

[per]

It doesn't really matter what the FPU is rated for, as long as it's more than the system clock. IBM's AT systems were 6 or 8 MHz, so you should therefore be safe with an 8MHz part.

The execution unit of the 80287 could also be clocked at a different speed than the main CPU through a separate pin.

 

[lyonadmiral]

It was a later model, so I'm assuming the 8 MHz model. So in that regard, I should look for a 8 MHz FPU.

 

[Jorg]

I'm not even sure if the FPU in an AT runs at the same speed as the CPU, might be lower?
But then you won't easily find any that are rated 6 mhz anymore I think.
Higher is always ok.

Which model AT is it?

 

[Agent Orange]

It was a later model, so I'm assuming the 8 MHz model. So in that regard, I should look for a 8 MHz FPU.

FWIW: Hope this helps. I currently have a 80287 in a Tandy 1000SX along side a V-20 running at 10 MHz. It also was in there with the original 8088 which ran at 4.7 MHz.

 

[lyonadmiral]

Which model AT is it?

It has a type 3 motherboard.

 

[modem7]

It has a type 3 motherboard.

So, the 8 MHz motherboard. On the 8 MHz 5170 motherboard, the coprocessor is fed a 16 MHz clock, and the CKM pin (pin 39) of the coprocessor is tied low, thus dividing the 16 MHz clock by three. Therefore the coprocessor is running at about 5.3 MHz. And so a 6 MHz coprocessor will be adequate.

 

[Jorg]

So, the 8 MHz motherboard. On the 8 MHz 5170 motherboard, the coprocessor is fed a 16 MHz clock, and the CKM pin (pin 39) of the coprocessor is tied low, thus dividing the 16 MHz clock by three. Therefore the coprocessor is running at about 5.3 MHz. And so a 6 MHz coprocessor will be adequate.

Aha - I remembered that that was a point, but not why/how. Thanks for explaining!

 

[modem7]

I've added a 'Math coprocessor' section to the IBM 5170 entry on our VCWiki:
http://wiki.vintage-computer.com/index.php/IBM_PC_AT_(5170)

 

[Shadow Lord]

Or you could avoid all the hassle and run a 80287XL which could run from 6 to 20 MHz and had a broader instruction set (don't quote me on the last part) then the 80287. But they are rare and tend to run in the $50 range.

 

[Agent Orange]

Its all academic at this point when your talking about a FPU on an old AT, XT, or what ever. Basically its 'show & tell' unless you have the software packages that are designed to take advantage of it.