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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.periphs,net.micro,net.arch
Subject: simple SASI/SCSI interfaces
Message-ID: <5043@utzoo.UUCP>
Date: Sat, 9-Feb-85 01:08:58 EST
Article-I.D.: utzoo.5043
Posted: Sat Feb  9 01:08:58 1985
Date-Received: Sat, 9-Feb-85 01:08:58 EST
Organization: U of Toronto Zoology
Lines: 44

A group of friends have asked my opinion on several questions of
hardware design, including one that I'm unsure of the answer to.

The context is a desire to make it possible to connect "fast" devices
to a small computer, while not incurring much expense in the basic
system.  The system has no expansion-oriented bus.  For various reasons,
providing DMA for the fast-device connection is out.  The cpu is hefty,
on the order of a 10 MHz 68000, and can copy data as fast as most any DMA
device could.  Of course, it wants to do the copying in one huge burst,
starting at a time of its own choosing, rather than in little dribbles
as the fast device runs.  Fast devices of interest are disks, tapes,
and Ethernet.

An interesting-looking possibility is a very simple host adapter for the
SASI/SCSI bus, not much more than some parallel ports with a few hooks
for interrupts and such.  The question is, how well does this meet the
objectives?  I'm not very familiar with either SCSI, or the currently-
available SCSI controllers, so...

Is it possible to find SCSI disk and tape controllers with reasonable
performance?  (Defined as delivering a useful fraction of the potential
speed of the device, i.e. not requiring 5:1 interleaving on the disk
because the stupid controller can't keep up.)  General impressions and
specific pointers both welcome.

Is anybody making an SCSI Ethernet interface?  Planning to?

Is the idea of a very simple host adapter, with the cpu doing most of the
work except for the bare-bones handshaking, reasonable?  Or is decent
performance impossible without smarts in the adapter?  (Two obvious
issues are whether common SCSI controllers want their data in dribbles
instead of full-speed burst transfers, and whether they demand tight
real-time response from the host at any time.)  Investing some cpu time
in exchange for the simple adapter is acceptable, but having the cpu
spending all of its time managing the SCSI interactions is not.

My understanding is that SCSI is about to be an ANSI standard but isn't
quite there yet.  How can I get the current draft?  (I know how to get
ANSI standards, but don't know who to contact for an SCSI draft.)  To
what extent has ANSI changed/complicated/messed-up SCSI?  Badly enough
to shoot down the idea of a dead-simple interface?  I hear evil rumors.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry