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From: "Rodney W. Grimes" <rgrimes@gndrsh.aac.dev.com>
Message-Id: <199507181110.EAA03940@gndrsh.aac.dev.com>
Subject: Re: scsi problems
To: paul@FreeBSD.org
Date: Tue, 18 Jul 1995 04:10:41 -0700 (PDT)
Cc: dufault@hda.com, FreeBSD-current@FreeBSD.org
In-Reply-To: <199507181035.LAA02599@server.netcraft.co.uk> from "Paul Richards" at Jul 18, 95 11:35:53 am
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> 
> In reply to Rodney W. Grimes who said
> > 
> > > Everything is external.
> > 
> > I know you've heard this a hundred times, but, do you have the terminators
> > installed and enabled on the 1542?  And are your scsi cables double shielded
> > SCSI-ii rated (meaning there should be about 1/2" in diameter, not 3/8")?
> 
> Yes, everthying's terminated correctly :-)

:-), well, you know how it goes.. I just have to ask it...  :-)

> Umm not all my cables are scsi-ii though. I'm only running the scsi
> bus as 5Mb/s anyway though since the ISA bus on this motherboard
> won't go any faster.

There is no reason to throttle a scsi bus to 5MB/sec just becuase the
ISA bus is only capabile of something less.  The aha1542 has a buffer on
it so that the scsi bus can run wide open minimizing transfer from the
drive to the controller.

If you mean you set the host DMA transfer rate to 5.0MB/sec, this did
not slow the scsi bus down at all, that is in the per device setup
menu (if this is a 1542C or CF).  If it is a 1542B forget it, that
controller won't do fast scsi-ii, it maxes out at scsi-ii 5MB/sec sync.

> Just for general info (and it may be the cause after all) what's the
> difference in the specs between a scsi-i and scsi-ii cable and when is
> it safe to use scsi-i and when not.

The biggest difference in the spec is the allowed impendence of the
cable.  I can't recall the numbers off the top of my head, but scsi-ii
narrows the range around the 110 ohm ideal value.  My rule pretty much
anymore is use scsi-ii cables for _anything_ doing sync scsi, along
with active terminations.

> I have had this setup working in the past though, since I installed
> off the DAT originally.

It is a rather simple thing to get good high quality cables and active
termination, even if it was not the problem, at least you know that
never well be the problem in the future, something I find conforting.


-- 
Rod Grimes                                      rgrimes@gndrsh.aac.dev.com
Accurate Automation Company                 Reliable computers for FreeBSD