Path: utzoo!attcan!uunet!sjsca4!poffen
From: poffen@sj.ate.slb.com (Russ Poffenberger)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Can you speed up an old 6Mhz IBM-AT to 12Mhz?
Message-ID: <1990Jul6.160750.7688@sj.ate.slb.com>
Date: 6 Jul 90 16:07:50 GMT
References: <6692@vax1.acs.udel.EDU> <57993@bbn.BBN.COM> <3816@rodan.acs.syr.edu>
Reply-To: poffen@sj.ate.slb.com (Russ Poffenberger)
Organization: Schlumberger Technologies, San Jose, CA.
Lines: 40

In article <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?
>>>We have an 80287/6Mhz and 512K RAM+1128K AST Rampage card in it. I think, th
>>>RAM chips have a speed of 120ns.  Would changing the crystal create any probl
>>>with using these?  Do I have to change the '287 and the RAM chips also?  Do I
>>>have to change the '286 chip itself?
>>Putting in a higher speed crystal will work, with increasing flakiness the
>>higher the speed.
>>The Isa bus has
>>practical upper limit of just about 8 MHz.  Even at 8.5 MHz, you may already
>>find some intermittent trouble, like in the disk controller.  I wouldn't try
>>for more than 8.0 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.  PC magazine had a article on
>doing this.  Of course, the 286 & 287 will have to be scrapped.  I too
>would be weary of taking a old war-horse 6 mhz at above 8 Mhz.  But hey
>buy a 12 Mhz 286 (If you can find 1 with the right config--try harris)
>and try it.  After getting a good max speed you'll be happy wiith, get
>a 287 to match it.  When you get all done, if you pay yourslef $ 2 an hour
>ypou just spend more than it costs to get a new 12 mhz motherboard.  But 
>boy what a sense of self accomplishment you'll have.
>al

Clones that run above 8Mhz attempt to stay compatible with the 8Mhz (most of
them) by using a fraction of the CPU speed as the bus speed. As a matter of
fact, my 25Mhz 386 system with C&T neat chipset has the ability to be
programmed to set the bus speed to either AT clock speed (8Mhz), CPU speed / 3,
or CPU speed / 2. I happen to run mine at CPU speed / 2 which equates to 12.5
Mhz and it works for me.

A lot depends on the cards you have plugged in.


Russ Poffenberger               DOMAIN: poffen@sj.ate.slb.com
Schlumberger Technologies       UUCP:   {uunet,decwrl,amdahl}!sjsca4!poffen
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