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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.periphs,net.micro,net.arch
Subject: Re: simple SASI/SCSI interfaces
Message-ID: <5110@utzoo.UUCP>
Date: Mon, 25-Feb-85 20:42:28 EST
Article-I.D.: utzoo.5110
Posted: Mon Feb 25 20:42:28 1985
Date-Received: Mon, 25-Feb-85 20:42:28 EST
References: <5043@utzoo.UUCP>
Organization: U of Toronto Zoology
Lines: 54

Thanks to everybody who replied, both by followup and by mail, to my
query about SCSI controllers and host adapters.  The following is a
very quick summary of what I found out.

> Is it possible to find SCSI disk and tape controllers with reasonable
> performance?  ...

Yes.  The host adapter is more likely to be the bottleneck, in fact.
A number of companies make decent (although imperfect) controllers.

> Is anybody making an SCSI Ethernet interface?  Planning to?

Nothing announced yet, although there is interest.

> Is the idea of a very simple host adapter, with the cpu doing most of the
> work except for the bare-bones handshaking, reasonable? ...

Yes, if done carefully.  Some small amount of hardware help might be
useful in three specific areas:  (1) stretching the cpu memory access
until data is ready, to avoid having to poll; (2) combining more than
one data byte into a single word for cpu data transfer; and (3) watching
to see whether the type of data-transfer request has changed underfoot.
But it's certainly possible, and has been done successfully, without
any of these extras.  Moreover, performance can be quite good.

Several people mentioned the NCR 5385 chip that does a lot of the work.
I already had a datasheet on it, in fact.  Unfortunately, I don't believe
it's second-sourced yet.  Otherwise it looks good.

> 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?  ...

The official source, which charges money (circa $30), is

	Ms. Martha Prinsen
	X3 Secretariat/CBEMA
	311 First Street NW
	Suite 500
	Washington, DC 20001

The magic word to indicate what you want is "X3T9.2 rev 14A".

> ... To what extent has ANSI changed/complicated/messed-up SCSI? ...

Not very much.  In particular, even old host adapters should be able
to run new peripherals.  The additions mostly concern multi-controller
systems where parallel activities are desired; the old versions basically
insist on doing one thing (with one controller) at a time.  This is a
wart with things like magtapes because operations like rewinds can tie
up the bus for a long time.  An alternate approach to this is to have
an SASI/SCSI bus per controller.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry