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IBM Model 30 Coprocessor

Brian Todd

Brian Todd

Experienced Member
Joined
Nov 21, 2022
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Location
The Netherlands
As I reported in an other thread, I recently picked up a low mileage PS/2 Model 30. There is currently a D8087 coprocessor installed (see pic below). I have checked it with 87test and it passes all stress tests with flying colours. Nevertheless, unless I am mistaken the D8087 runs at 5MHz versus the 8MHz of the 8086. Does that mean the coprocessor is now "overclocked" or is the 8086 now running at 5MHz. If this is no big deal then I will just leave as is, and otherwise I will source a D8087-1 which will always be ok.

IMG_0991.jpg



Would much appreciate your thoughts on the coprocessor please!
 
Brian Todd said:
Does that mean the coprocessor is now "overclocked" or is the 8086 now running at 5MHz.
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The system has no idea about the speed the FPU is rated for. So the FPU runs at whatever speed the 8086 runs. If that is 8 MHz, the FPU is being overclocked.
 
Where?

If you are worried about a 8087 becoming hot, I had the same feeling. I have several 8087s running in various machines. In some cases I added a little fan directly at the 8087 and it stays cool.
 
I found it at HeinPragt.nl ... he seems to have a shed load of chips in all shapes and forms! For that money, I couldnt even get a decent fan for the 8087 🤣
 
The Intel coprocessors usually run at a different clock speed than the main CPU.
The same main clock of the main board is divided by different ratios for the main CPU and the FPU.
Typically the FPU ran at 2/3 of the main CPU clock. 8 MHZ -> 5.33 MHZ.

At the time, you could buy PCBs with their own clock generator to speed up the FPU to any desired frequency to speed up math intensive applications.
Maybe the IBM model 30 is different - the technical manual should tell you.
 
That is wrong. On an XT, the FPU runs at the same clock speed as the CPU. On a 286, it runs at 2/3 speed (exception: the 287XL, which undoes the 2/3 clocking). On a 386, it runs at full clock again. Additionally, on most 286 and 386 boards, an optional second crystal can be used to run the FPU at any other speed.
 
The difference lies in the interface between the various NDP chips and their host CPUs. The coupling between the 8086/88 and the 8087 is very tight--they both live on the same bus and the 8087 "watches" the activity of the 8086/88. With the 287 and subsequent, the interface is more of an I/O port nature, which allows for different CPU/NDP speeds.
 
Brian Todd said:
I found it at HeinPragt.nl ... he seems to have a shed load of chips in all shapes and forms! For that money, I couldnt even get a decent fan for the 8087 🤣
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Sold out :( OK, that's life. But indeed a very interesting site!
 
Ruud said:
Sold out :( OK, that's life. But indeed a very interesting site!
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Not to worry Ruud .. it was NOS and dead as a doornail ... when inserted the PS/2 would not boot and when removed I had to use the disk to set time and date again. Unfortunately the last one and the seller has no idea whether he well get them back in stock. For now, I will continue to use the 8087 which seems quite happy to run at a higher frequency.
 
Brian Todd said:
Sorry about snatching the last one Ruud! You are looking for an 8087-1 too?
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Yes. I have some 5 and 8 MHz but I could use a 10 MHz one. I already underclocked one machines so I could use an 8 MHz 8087. Speed is not important but if I can run it at the original speed, the better.
 
Ruud said:
Yes. I have some 5 and 8 MHz but I could use a 10 MHz one. I already underclocked one machines so I could use an 8 MHz 8087. Speed is not important but if I can run it at the original speed, the better.
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Ruud, if you like I can send the 8087-1 to you and you can see if you have better luck than me with it. If you want it just send me a pm! Ideally I would like to source an 8 MHz chip but guess I will have to have some patience .. unless I want to take a chance on AliExpress lol.
 
I sent aPM.

Aliexpress is not bad at all. My Philips and Commodore PCs run at 9.77 MHz so I ordered five 10 MHz 8087s but got 8 MHz ones instead. I asked for a partial refund but the seller asked me not to give a bad review and would send five new ones. Received three 8 MHz and two 5 MHz ones. I asked and got a complete refund. I can use these 8087s at least in my original IBM 5150 and 5160 :)
 
Hello Brian, just received it! Certainly should have some free time the day after tomorrow, Hemelvaartsdag, to test it.
 
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