VOGONS


First post, by TimWolf

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Okay, finally getting a motherboard that can take a 25mhz 286. Now I'm looking to get either a Harris or Intersil. What is the best, any downside to either brand? Pairing this up with a 287 math co-processor, what is the best/fastest model out there. Was looking at perhaps the Intel C80287XL.

Your thoughts, suggestions, and recommendations are welcome!

Reply 1 of 16, by kixs

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Fastest would be Cyrix 287XL+. But it's pretty rare. Intel 287XL is also very good. They usually run at 2/3 of the cpu speed. I think the latest with 387 core (like cyrix and intel XL) run at full speed (1:1 to cpu).

Requests are also possible... /msg kixs

Reply 2 of 16, by TimWolf

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kixs wrote:

Fastest would be Cyrix 287XL+. But it's pretty rare. Intel 287XL is also very good. They usually run at 2/3 of the cpu speed. I think the latest with 387 core (like cyrix and intel XL) run at full speed (1:1 to cpu).

Have any model numbers on those 387 core units? Would like to put some search terms set up to alert if anything ever comes up.

Thanks

Reply 4 of 16, by BlueSkullKey

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TimWolf wrote:

Now I'm looking to get either a Harris or Intersil. What is the best,

Intersil was a part of Harris that made semiconductors before it was separated in 1999 and then was bought by Renesas in 2017, if I understand it all right. They still made 80286 CPUs through 200x years at least. No idea if there were any changes to the designs though 😕

Reply 5 of 16, by Anonymous Coward

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I could be mistaken, but I believe that all AMD, IIT and Cyrix 287 FPUs use a 387SX core. I think of the Intel parts, only the 287XL has it.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 6 of 16, by Jo22

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kixs wrote:

Fastest would be Cyrix 287XL+. But it's pretty rare. Intel 287XL is also very good.
They usually run at 2/3 of the cpu speed. I think the latest with 387 core (like cyrix and intel XL) run at full speed (1:1 to cpu).

Really ? I'm confused now 😕 . I thought only the real i80287 from intel used that 2/3 thing, because of technological limits of the day
and that those chips based on the i387 design (287XL) or from competitors ran at "full speed" (3/2 of 286 clock rate for compensation).

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

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Reply 7 of 16, by Anonymous Coward

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The original 80287 can also ben run at the same speed as the CPU. There is a pin that controls this.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 8 of 16, by Jo22

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Thanks for the info - I wonder, is that the same pin that can be used to run the 287 async/to use an external xtal oscillator ?
if memory serves, I've seen some ads for 287 daughter cards (or interposers) that had their own circuitry.
This must have been in older paper magazines, maybe CHIP, PC World or Byte Magazine, not sure anymore. 🙁

"Time, it seems, doesn't flow. For some it's fast, for some it's slow.
In what to one race is no time at all, another race can rise and fall..." - The Minstrel

//My video channel//

Reply 9 of 16, by kixs

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Anonymous Coward wrote:

I could be mistaken, but I believe that all AMD, IIT and Cyrix 287 FPUs use a 387SX core. I think of the Intel parts, only the 287XL has it.

I think AMD 287 doesn't use 387 core. It performs the worst of them all (maybe the same as Intel 287 but I don't have it).

Requests are also possible... /msg kixs

Reply 10 of 16, by Anonymous Coward

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According to the datasheet it's compliant with the IEEE 754 standards, therefore it should support the extra features of the i387.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 11 of 16, by TimWolf

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This is the motherboard I'm upgrading:
https://stason.org/TULARC/pc/motherboards/L/L … 286-5500AT.html
Looking to max it out with 25mhz/287/5mb

Right now I'm leaning towards Intersil /Intel 287XL due to availability.

Reply 12 of 16, by Anonymous Coward

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I've always been curious how far the 287XL can be pushed. Officially it's only rated for up to 12.5MHz, mostly because that's the fastest 286 intel made. However, intel made 387SX chips that ran up to 33MHz, so if the core is the same there's a good chance it could work. Otherwise, you can just run the FPU at half speed.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 13 of 16, by Wireless

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Ok, just re-read this thread, my current 286 Motherboard has a Harris 286-16, and a 287-6, there's also a large 32MHz Crystal which I'm assuming is to clock rate the 286-16, plus there's two other small Crystals, a 16MHz, and a 14.31818MHz, which I've no real idea what they control, but presumably at least one clocks the 287-6.

Now I've bought a Harris 286-25, and a replacement large 50MHz Crystal, which I can only assume will control the faster CPU.

However, I've just bought an IIT 2C87-12, how do I get the right crystals to run that?

My assumption here is the 16MHz Crystal currently steps down the clock rate of the existing 286-16, although that doesn't make any sense, so possibly its the crystal selected to supply a harmonic frequency combined with the 14.31818MHz crystal, and must somehow deliver a 6MHz clockrate to the existing 287-6, but I'm also aware that 286 ISA ran at either 6MHz or 8MHz on these boards, with the original speed on 8086/88 boards at 4.77MHz, tell me if I'm barking up the wrong tree here.

Ah, 3x 4.77MHz is 14.31, so this is an ISA reference crystal, so I can't touch this one (14.31818 MHz).

To get to 6MHz requires a leap of faith, but see what you think

4.77 MHz X 16 MHz = 76.32 MHz

then
76.32 MHz / 14.31818 MHz = 5.33 MHz

or 6 MHz near as damn it supplied to the 287-6

So I'm thinking a 32 MHz Crystal to replace the 16 MHz

giving me 10.66 MHz for the 287-12, which is at least twice as quick as the old 287-6 IF my assumptions are correct

Although I still have no idea how the ISA standard is supplied either 6 MHz or 8 MHz, since the NTSC normally use a reference of 14.31818 MHz for ISA, perhaps this is an older motherboard and ISA still runs at the old 4.77 MHz Bus.

Does this sound anything like on the right track?

Any ideas gentlemen/ladies?

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Reply 14 of 16, by Anonymous Coward

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Every 286 board does things differently, so it's pretty difficult to know for certain what speed your ISA and FPU are actually running at. Some 286 boards run ISA pretty far out of spec. The later models let you adjust the ISA speed either in the BIOS or with jumpers.

If I had to guess, I would say that the 16MHz crystal is the one that sets the speed for your FPU, and it's actually being overclocked to 8MHz.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 15 of 16, by BloodyCactus

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IIT made a 25mhz 287. Good luck finding one tho.

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Reply 16 of 16, by TimWolf

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0I know this one has been a while but I've started working on it again. I did get a i287XL for it. Right now I'm waiting on a PLCC68 SMD socket. Wondering if anyone has any specs or knowledge on the motherboard I am working with right now? It has a 24mhz crystal for the processor, and I am going to put a socket in there with a 50mhz crystal. However, there is a section with resistors that looks allot like clock multipliers or something like that.

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