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From: news@wrgate.WR.TEK.COM (-Network News daemon)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Does 80287-8 will work with 80286-12
Keywords: In article <2044@bruce.OZ> alanf@bruce.OZ (Alan Grant Finlay) writes:
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Message-ID: <2259@wrgate.WR.TEK.COM>
Date: 14 Apr 90 22:24:25 GMT
References: <5975@shamu.WV.TEK.COM> <2044@bruce.OZ>
Reply-To: michaelk@copper.WR.TEK.COM.UUCP (Michael D. Kersenbrock
Organization: Tektronix Inc., Beaverton, Or.
Lines: 23

Path: copper!michaelk

That "rule" is NOT the pushing of speed specifications.  The "normal"
way of connecting a 287 to a 286 is to have the same clock source tied
to BOTH, however:

	1. A 286 uses a clock source twice what the chip runs at.

	2. A 287 uses a clock source three times what the chip runs at.

So, with a 12-Mhz 286 system, the clock source is 24 Mhz.  The 286 divides
by two internally, yielding the 12 Mhz that it runs at.  The 287 divides
internally by three, yielding the 8 Mhz it runs at.  Thus "the rule".

I personally run a 5 Mhz 287 in my 12-Mhz 286 machine, but I doctored the
circuitry so that the 287 actually runs at 5 Mhz.  And it works just fine
thankyou.

--
Mike Kersenbrock
Tektronix Microprocessor Development Products
michaelk@copper.WR.TEK.COM
Aloha, Oregon