Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!bbn.com!sher
From: sher@bbn.com (Lawrence D. Sher)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Can you speed up an old 6Mhz IBM-AT to 12Mhz?
Message-ID: <58011@bbn.BBN.COM>
Date: 6 Jul 90 12:14:12 GMT
References: <6692@vax1.acs.udel.EDU> <57993@bbn.BBN.COM> <3816@rodan.acs.syr.edu>
Sender: news@bbn.com
Reply-To: sher@labs-n.bbn.com (Lawrence D. Sher)
Organization: Bolt Beranek and Newman Inc., Cambridge MA
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>In <3816@rodan.acs.syr.edu> amichiel@rodan.acs.syr.edu (Allen J Michielsen) writes:
>>In article <57993@bbn.BBN.COM> sher@labs-n.bbn.com (Lawrence D. Sher) writes:
>>>In article <6692@vax1.acs.udel.EDU> bach@vax1.udel.edu (Baskaran Subramaniam) 

>>>>Is it possible to speed up an old 6Mhz IBM-AT (not a compatible) to 12Mhz
>>>>by changing the crystal in it?

>>>The ISA bus has a practical upper limit of just about 8 MHz.

>>  If the AT bus has a practical upper limit of just about 8 Mhz, then all the
>>clone 10 & 12 Mhz bus computers must not work.

Amazingly, the timing specs for the ISA bus have never been published!  Look in
with a logic analyzer and you will find that above 8 MHz, conventional circuits
cannot reliably respond to *some* bus signals in the time available, like <15
nanoseconds.  (This time is *not* inversely proportional to the bus speed.  It
is actually the difference between two times, the difference becoming
negative at speeds in the vicinity of 10-12 MHz.) Consequently, if the computer
has only one clock cycle time for both processor and bus, then 8 MHz is as high
as one should reasonably dare.  At 10 MHz, some 3rd party cards will definitely
fail to work properly.  The solution that has been widely adopted is to run the
bus at 8 MHz and the processor at some higher speed.  I believe that most
(all?) 386-based ISA-bus machines do this.

Since the basic 6 MHz IBM PC/AT Model 239 has only one cycle time for both bus
and processor, simply changing the crystal is constrained by the timing needs
of the bus.  That, in turn, is constrained by the cards using the bus.  You may
be lucky enough to have cards that can work at 10 MHz, but don't count on it.
Once you opt for a solution that uses multiple clocks, more upgrade paths
become possible, but none of them can reliably run the bus faster than 8 MHz.