2940 and 75MHz CPU clock

[Doug Ledford]

--------
> Hi there,
> 
> Has anyone on this list been able to get the
> 2940 SCSI controller to work with a 75MHz CPU clock?
> 
> The controller always hangs when there is heavy
> disk access. I tried many things and is still
> unable to resolve the problem.
> 
> I tried that abort/reset patch, which reset the
> controller alright, but this is very annonying 
> as it resets very, very often.

I've heard (although I haven't tried it myself) that the Adaptec cards are 
very bad about locking up when the PCI clock is above 33Mhz.  This problem 
almost sounds like there could be a clock problem other than what you 
mention.  There are three relevant clock speeds on a Pentium machine.  The 
CPU clock, the memory bus clock, and the PCI bus clock.  Normally, for a 
Pentium 75, the clocks and multipliers would be set as follows:

Memory clock, 50Mhz
CPU clock, 1.5xmemory clock = 75Mhz
PCI clock, 1/2 memory clock = 25Mhz
           or
PCI clock, 3/5 memory clock = 30Mhz

Given that setup, things should work fairly reliably as far as the card is 
concerned.  There are two other things I can think of that could be a factor 
here.  Around the time that the Pent75 was being actively sold, the 2940 
cards were somewhat buggy and unreliable.  The indicator to tell if your 
2940 card is one of the buggy models is to check and see if the oscillator 
on the card is 42Mhz instead of 40Mhz.  The third and final problem I can 
think of is your hard drive.  Depending on the brand/model of drive in use, 
and the firmware on the drive, SCSI bus lockups can be *very* common 
occurences.  For example, knowing what load pattern to use, a person can 
cause the Quantum Fireball Tempest drives with firmware revision 300X to 
lockup the entire SCSI bus as often as once a minute or more (as a matter of 
fact, I've made *extensive* use of this "feature" during my abort/reset code 
testing).

-- 
*****************************************************************************
* Doug Ledford                      *   Unix, Novell, Dos, Windows 3.x,     *
* dledford at dialnet.net    873-DIAL  *     WfW, Windows 95 & NT Technician   *
*   PPP access $14.95/month         *****************************************
*   Springfield, MO and surrounding * Usenet news, e-mail and shell account.*
*   communities.  Sign-up online at * Web page creation and hosting, other  *
*   873-9000 V.34                   * services available, call for info.    *
*****************************************************************************