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286 micro compatibility

Joined
Nov 18, 2011
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Buenos Aires, Argentina
I currently have a 286 motherboard with a 12,5/6 mhz micro. Can I upgrade it with a faster micro?

Let’s suppose the opposite situation: that I have a 286 motherboard with a 20 mhz micro. Could I install the 12,5 mhz micro in it? Or should I change the crystal oscillator too?


Thanks for answering this stupid question :blush:
 
The MHz value in the microprocessors is only a rating, so it guarantees that the micro will run at that speed. If you put a 20 MHz rated 286 in your 12,5/6 MHz motherboard, it will run at 12,5/6 Mhz like the original. If you do the opposite, you will be overclocking the 12,5 Mhz to 20 Mhz, and surely it will not work correctly.

You can try to replace a crystal oscillator, but the results can vary as there are more clock signals involved. Many ICs need a clock source, and they may be generated from the same source, so changing the base oscillator can affect all this clock signals. Another problem is the RAM, it is asynchronous and doesn't use a clock source but a faster micro processor will try to access it faster, maybe over its specs.

The simple and short answer is: replace both the motherboard and the microprocessor.
 
AFAIK, 286 motherboards designed for >8MHz operation have a separate clock for the ISA bus, to run it at 8MHz, regardless of the CPU clock speed. So in boards like these you can replace the CPU clock and the CPU itself without causing instabilities with ISA cards.
However, in boards from that generation the CPU clock still runs the mainboard too (no multiplier stuff etc), so as nestor said, mainboard components / RAM could still be adversely affected (unless certified for higher speeds).
 
I remember when the 5170 was introduced, the art of overclocking got started. One of the first changes was to bump the CPU speed to 8MHz from the original 6. But a lot of people didn't stop there--if you looked in the back of Computerweek, you could find vendors to sell you sets of crystals to see how fast you could run. There were also a couple of vendors who marketed PLL kits to allow one to vary the CPU speed with a knob.

So, sure, go ahead and goose the CPU speed by changing out the crystal (or oscillator) Just be careful to watch the heat and don't overdo it. If you can find a 80C286/12, the CMOS design will generate less heat. Eventually, your memory speed will catch up with you and you'll have to back off a bit.

Have fun!
 
So, sure, go ahead and goose the CPU speed by changing out the crystal (or oscillator) Just be careful to watch the heat and don't overdo it. If you can find a 80C286/12, the CMOS design will generate less heat.
I know from experience that a 286-10 runs hot enough to burn your finger! Some manufacturers even put a little heatsink on top, which is strange to see in a vintage PC, because most CPUs didn't usually come with a heatsink until we got well into the 486 era.

I've heard of a 25 MHz 286 chip, but have never seen one in real life (or in photographs) -- nor the "last hurrah" of XT clones which came with a 12 or even 16 MHz NEC V20H or V30H chip.
 
I've an AMD 16mhz 286 chip... it's a chip variety (I get confused with the name - PLCC?) similar to this design.

It came with a motherboard I got off of ebay for $10. The real reason for the purchase was the 80287-10 that was on the board. Interesting to me if that has essentially overclocked the copro. The seller was an old vintage computer guy who was getting rid of his spare gear and narrowing down to one machine to keep for nostalgia as he was mostly into his HAM these days.

FWIW, there are three crystal oscillators on this board: a 32.000mhz, a 12.000mhz, and a 14.31818mhz. Also an interesting feature about this board - there are two 8-bit ISA slots, with through-holes in the MB to make them 16-bit. In itself that's not so interesting. But the fact that these holes were not filled over with solder at the factory and were intentionally left open - to me, that's interesting. It's also got a spot to put a coin-cell battery (again, empty through-holes), but no button holder. All in all, an extremely nice board, and I'm glad to have it.
 
Yep Plastic Leadless Chip Carrier (PLCC). 32.000mhz is the CPU clock. The 286 had 50% duty cycle if memory serves me. The 12.000 divded by 2 might be for a 80287-6? Not sure on this one.
 
The real reason for the purchase was the 80287-10 that was on the board. Interesting to me if that has essentially overclocked the copro.
Later 286 boards would usually run the coprocessor at a lower speed than the main CPU. I believe Intel never made a 287 faster than 12 MHz; the 387-based 287XL was rated for 12.5 MHz maximum.
 
Seems like a lot of systems in the days before integrated FPUs would run them at lower clock speeds; I've seen Amiga accelerators with 33MHz CPUs and 25MHz FPUs, I believe my 486 does the same. Was it just to save money by using less expensive parts?
 
...32.000mhz is the CPU clock. The 286 had 50% duty cycle if memory serves me. The 12.000 divded by 2 might be for a 80287-6? Not sure on this one.

With the 286-16, this motherboard would most likely run the FPU at two-thirds the CPU clock (one pin tied high or low on the 287 shifts between 2/3rds or full CPU clock): 10.66MHz...
 
Here's how it's done in the 8 Mhz version of the IBM 5170:

5170_8mhz_clock.jpg
 
If the 286 needs a 50% clock duty and the frequency is halved inside the 286, what is the function of the 82248 clock chip? Could it be replaced with a TTL Oscillator?
 
If the 286 needs a 50% clock duty and the frequency is halved inside the 286, what is the function of the 82248 clock chip?
In this use of the 82248, I expect to:
* Provide the additional circuitry required to get the crystal to oscillate, and
* Square up the signal from the crystal, and
* Provide sufficient drive to the 80286, 80287 and other devices.

Could it be replaced with a TTL Oscillator?
If the particular TTL Oscillator has the required frequency, duty cycle, driving power, then I can't see why not.
 
Here's how it's done in the 8 Mhz version of the IBM 5170...

My notes say that if pin 39 of the 287 is tied to ground, it runs at two-thirds of the CPU clock (i.e. IBM AT, XT 286, Model 25 286, Model 30 286), tied high, it runs at CPU clock (Model 50, Model 50Z, Model 60)...
 
My notes say that if pin 39 of the 287 is tied to ground, it runs at two-thirds of the CPU clock (i.e. IBM AT, XT 286, Model 25 286, Model 30 286)
Yes, and so in the case of the 5170s (where pin 39 [CKM] tied low, causing a divide by 3):

8 MHz 5170: 5.333 / 8 = 0.666 (two thirds)
6 MHz 5170: 4 / 6 = 0.666 (two thirds)
 
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