Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!xstor!iverson
From: iverson@xstor.UUCP (Tim Iverson)
Newsgroups: comp.periphs.scsi
Subject: Re: PC SCSI interface
Keywords: PC, SCSI, MSDOS
Message-ID: <203@xstor.UUCP>
Date: 25 Sep 90 17:51:09 GMT
References: <1990Sep20.175400.18626@maths.tcd.ie>
Reply-To: iverson@xstor.UUCP (Tim Iverson)
Organization: Storage Dimensions, Inc.
Lines: 35

In article <1990Sep20.175400.18626@maths.tcd.ie>
tim@maths.tcd.ie (Timothy Murphy) writes:
> What board would you advise for using a SCSI drive (a) on a PC/XT, (b) on
> an AT?  I see some really cheap ($30-40) boards advertised, while others
> are much more expensive.  What is the difference?  Any info on actual
> boards, and experience using them, gratefully received.
>
>Timothy Murphy  
>e-mail: tim@maths.tcd.ie

It really depends more on what OS you're going to be running, not on the
machine.  If you're going to run UNIX or some other multiprocessing OS, then
the more expensive boards (e.g. the Apaptec 154x series) are nicer to have.
This kind of board does 1st party DMA - moves bytes from disk to memory
all by itself - which frees up the CPU for other tasks.

Unfortunately, this type of offloading has a rather high overhead on each
command.  The cheap boards require the CPU to move the bytes around, but
generally have much lower overhead.  So, if you use DOS, the expensive
bus-mastering boards are actually slower than their cheap counterparts.

Of course, when buying a cheap board it's often hard to know which firm is
reputable: we had one manufacturer that claimed a 10x jump in transfer rate
for any drive.  Well, we tested it and sure enough, the benchmark showed
that this little poky 200k/s seagate was doing 2M/s.  Since this was
physically impossible, we investigated and found that interrupts were
deliberately being disabled in the BIOS, presumably to prevent the timer
from ticking, thus giving a skewed reading on the benchmark.

I realize that you would like names rather than general advice, but other
than the Adaptec 154x (good for UNIX), there are no other boards that I can
give a positive recomendation for.  Hope this helps.

- Tim Iverson
  uunet!xstor!iverson