Probably won’t be stable, that is almost a double overclock. What use is an fpu on a typical 286 system anyways, if it is stable overclocked it is probably because it is unused 😃

You confirm that must be same freq as the cpu. i like to have all my collection with no socket empty

i am using a 8mhz co-processor on a 10 mhz system. works fine.

That is definitely the case for 386 copro’s, I read that the 287 may operate asynchronously, but at a different speed would depend on the motherboard. The 30MHz osc would make the 286 go at 10, if the same one feeds the 287 you should also use a 10 part. (Or maybe an 8 ) Overclocking a 6 to 10 is like clocking a 2400 to 4000 in modern language, it will typically not work well.

st31276a wrote on 2023-10-25, 19:50:

I read that the 287 may operate asynchronously, but at a different speed would depend on the motherboard.

AMD's 287 is not capable of running asynchronously, it will always run at 2/3rd of the CPU clock. Any attempt to use an oscillator to control the frequency will cause instabilities, even when the FPU isn't being used. Running the wolf_286 benchmark will always result in a desync.

The Intel i287XL is fully capable at running on an asynchronous clock, however - I have been using mine at 27Mhz with the CPU at 24Mhz and I've had no side effects when it comes to using Doom map editors on my 286+287XL setup, which are very CAD-like in their interface. Another benefit I've seen with using the AMD 287 against the Intel i287XL is that certain features in Doom editors (like creating circular rooms) will result in severe issues with using the 287 where lines are thrown all over the place, while the i287XL is perfectly fine.

DEAT wrote on 2023-10-26, 05:17:

st31276a wrote on 2023-10-25, 19:50:

I read that the 287 may operate asynchronously, but at a different speed would depend on the motherboard.

AMD's 287 is not capable of running asynchronously, it will always run at 2/3rd of the CPU clock. Any attempt to use an oscillator to control the frequency will cause instabilities, even when the FPU isn't being used. Running the wolf_286 benchmark will always result in a desync.

The Intel i287XL is fully capable at running on an asynchronous clock, however - I have been using mine at 27Mhz with the CPU at 24Mhz and I've had no side effects when it comes to using Doom map editors on my 286+287XL setup, which are very CAD-like in their interface. Another benefit I've seen with using the AMD 287 against the Intel i287XL is that certain features in Doom editors (like creating circular rooms) will result in severe issues with using the 287 where lines are thrown all over the place, while the i287XL is perfectly fine.

You say thata i need to find an intel 287xl? Does what you wrote also apply to me?

It is obvious from my writing that my 286 never had a copro…

I remember the 2/3 speed now, that is correct. If you have a 10MHz 286, the 287 will run at 6,7MHz. A 6MHz part might just work fine. Try yours out and let us know if it works.

The 287xl is also a much later design than typical 287 fpus, since it is based on the 387 core with the same design and process improvements that has over the original 287 chips.

It will almost certainly run in any 286 system at any clock speed you set it at.

If there's only one crystal on your board - 20 MHz - it gets divided by 2 for the CPU (10 MHz), but the FPU divides it by 3 (6.666 MHz), so the 6 MHz variant should run fine.
If there's a separate crystal for the FPU, everything depends on that crystal...

this is my board:

However, I tried to install the coprocessor and the configuration utility on the PS/2 floppy disk detects it, but I don't know if it works or not and at what frequency it works.

OK, the 20 MHz crystal is obvious.
The 48 MHz one, however, is a mystery - what uses such a high frequency on a 286 board? Can't be the FPU...

Anyway, if you have already installed it, then run some diagnostics/benchmark software.
Many such programs attempt to detect the clock frequency of CPU and FPU, try eg. Landmark, Checkit 3.0, Checkit Pro...

The ultimate solution is always using a logic probe, and frequency meter (eg. oscilloscope) on the appropriate pins of the FPU socket.

so, i have a chance that my copro work correctly?

The 80287 usually are set on all motherboards to run at much lower clock frequencies than 80286 CPUs. Please note that there are even 386 motherboards that can utilize 80287 FPUs. Use Intel's MCPDIAG DOS utility to test if your 287 is running properly on your motherboard. The tests last about 5 minutes.

ok i will try soon as i can and i report ere... thanks for now.

Grzyb wrote on 2023-10-26, 07:16:

287.jpg
If there's only one crystal on your board - 20 MHz - it gets divided by 2 for the CPU (10 MHz), but the FPU divides it by 3 (6.666 MHz), so the 6 MHz variant should run fine.
If there's a separate crystal for the FPU, everything depends on that crystal...

Now i even have an explaination why my 8mhz intel fpu works in my 10mhz 286.
Neat. Thanks.