Just insert it in it's socket - oriented toward the PSU connector. It should work without any special setup.

I think he's just worried about the relative nakedness of the board near it, and whether any of those chips supported it.... but yeah, they usually drop in. Some boards had a crystal socket for FPU, but that's usually for running them asynch to the CPU speed.

The question is: what exactly model is the FPU?
For synchronous operation, 286@18 MHz requires the FPU to be rated for at least 12 MHz (if based on the original 287), or at least 18 MHz (if based on 287XL).

Grzyb wrote on 2021-09-13, 22:02:

The question is: what exactly model is the FPU?
For synchronous operation, 286@18 MHz requires the FPU to be rated for at least 12 MHz (if based on the original 287), or at least 18 MHz (if based on 287XL).

18MHz is an odd speed.

Cyrix and IIT are good choices too if you can find them. The 80287XL is way easier to find and probably cheaper too.

The original i80287 is only made up to 10MHZ and NMOS (very hot), so avoid those. AMD is probably not worth bothering with either as they are normally only found as 10MHz parts.

Just get an Intel 80287XL and it should work fine.

First of all should have been taking out that barrel battery.

Second, for a real cool mod, you could try to solder the simm memmory connctors (from other donor board)...

D80287-10 (dip40) are $11
C80287XL -12.5 (dip40) are $25

both variants with shipment included

I rather go the cheapest way (because i have 5 MoBo-s to equip)

The question is how to use the 10mhz 287 in my system (i can go back to 12 Mhz), do i need to install the OSC3 [probably 20 Mhz] and jumper headers JP5 and JP6, what about R46 and c60?

nuno14272 - Of course the goal is to see 8-10 mb (why not even more 😀 286 system (up to 16mb of course)

Try cpu world. You might get a price lower than 25. In my opinion 287xl is worth your trouble if you want an fpu.

holoprox wrote on 2021-09-14, 12:19:

The question is how to use the 10mhz 287 in my system (i can go back to 12 Mhz), do i need to install the OSC3 [probably 20 Mhz] and jumper headers JP5 and JP6, what about R46 and c60?

First and foremost, check the state of the CKM pin, and measure the frequency in the CLK pin of the coprocessor socket.

Most likely the CKM is LOW, and the CLK is equal to OSC1.
With a 36 MHz OSC1, it would run the 287 at 12 MHz, and the 287XL at 18 MHz... TOO FAST!

Which leaves two options:
- go back to the 25 MHz OSC1
- install an appropriate OSC3, figure out how to route to OSC3-derived frequency to the CLK input of the FPU, and set the CKM to HIGH - detailed board schematics would come in handy...

18MHz is just fine for the 287xl. They work perfectly up to like 20 or so

holoprox wrote on 2021-09-14, 12:19:

D80287-10 (dip40) are $11 C80287XL -12.5 (dip40) are $25 […]
Show full quote

D80287-10 (dip40) are $11
C80287XL -12.5 (dip40) are $25

both variants with shipment included

I rather go the cheapest way (because i have 5 MoBo-s to equip)

The question is how to use the 10mhz 287 in my system (i can go back to 12 Mhz), do i need to install the OSC3 [probably 20 Mhz] and jumper headers JP5 and JP6, what about R46 and c60?

nuno14272 - Of course the goal is to see 8-10 mb (why not even more 😀 286 system (up to 16mb of course)

Its not about the amount of memory. Of course 8MB is already overkill for a 286, but its easy to add memory using simms that using chips.
Even more, because the ebay and ali are full of fake memory ic's. I bought miself some in the past for spectruns and other 8-bit computers and half where fake or defective.

Do you have any software in mind that you want to run on a 286 that will even use a FPU?

nuno14272 wrote on 2021-09-15, 16:21:

Even more, because the ebay and ali are full of fake memory ic's. I bought miself some in the past for spectruns and other 8-bit computers and half where fake or defective.

Half defective is original for Speccy 48k RAM 🤣

I've attached datasheets for both the 80287 and 287XL to this post. As you can see, when pin 39 (CKM) is HIGH the clock is NOT divided. When CKM is low, the 287XL does a 1/2 and the 80287 does a 1/3 divider

maxtherabbit wrote on 2021-09-15, 18:31:

I've attached datasheets for both the 80287 and 287XL to this post. As you can see, when pin 39 (CKM) is HIGH the clock is NOT divided. When CKM is low, the 287XL does a 1/2 and the 80287 does a 1/3 divider

Thank you. It is good to have accurate documentation/data. An old Staff Sgt of mine use to say"Assumption is the mother of all fuck ups" And that still applies.

cyclone3d wrote on 2021-09-15, 16:33:

Do you have any software in mind that you want to run on a 286 that will even use a FPU?

Early versions of MS Flight Sim maybe? If I could acquire an FPU locally at reasonable cost for my Zenith 268 I would just to fill the socket...😉

maxtherabbit wrote on 2021-09-15, 14:30:

18MHz is just fine for the 287xl. They work perfectly up to like 20 or so

OK, but that's overclocking.
287XL is rated for up to 12.5 MHz.

Grzyb wrote on 2021-09-15, 22:07:

maxtherabbit wrote on 2021-09-15, 14:30:

18MHz is just fine for the 287xl. They work perfectly up to like 20 or so

OK, but that's overclocking.
287XL is rated for up to 12.5 MHz.

Technically correct, but my understanding is the 287xl is a 387 core. I've never heard of one not working at 20MHz