asynchronuous 80287 co-processor board?

[Martin Hepperle]

The 80286/80287 processor pair can be clocked with separate clocks. In many cases, the same system clock is used for both CPUs.
In my 8 MHz HP Vectra the system clock is 16 MHz which is divided in the 80286 by 2 = 8 MHz and in the 80287 by 3 = 5.33 MHz.

Back in the day, some companies produced small 80287 plugin boards with their own crystal and clock generator, so that the 80287 ran at a higher frequency than the standard system clock. The boards went into the regular co-processor socket. I believe Microway was one such manufacturer.

I got a 80287-10 co-processor and am thinking of making such a board.
The 80287-10 should be able to run at a true speed of 10 MHz, so that a 30 MHz clock (divided by 3 inside the 80287) should work.

Is there a open source PCB design available for such a board? I could not find one.

Can someone provide photographs of such a board?

Martin

 

[Timo W.]

All you need to do is to connect a separate clock crystal to the FPU's pin 32. That's all what is done on mainboards supporting a 2nd crystal for the FPU, too.

Afaik, the 287 does not divide the clock by 3. The FPU is fed with the CPU clock (8 MHz in your case) and uses 2/3 of it (8 : 3 * 2 = 5.33). The XL can use either 1/1 or 1/2, depending on the state of CKM.