Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!AppleLink.Apple.COM!Smyers.S
From: Smyers.S@AppleLink.Apple.COM (Scott Smyers)
Newsgroups: comp.periphs.scsi
Subject: Re: Diff. between SCSI I & II
Message-ID: <10581@goofy.Apple.COM>
Date: 5 Oct 90 20:17:36 GMT
References: <1990Oct4.003204.20738@idt.unit.no>
Sender: usenet@Apple.COM
Organization: Apple Computer
Lines: 36

In article <1990Oct4.003204.20738@idt.unit.no> bernhard@mime.idt.unit.no 
(Kjetil Bernhard Thomassen) writes:
> I would like to know the difference between SCSI I and SCSI II.

I'm sure you're going to get a lot of replies to this general question,
but I thought I'd start with what I know.

One of the major differences between SCSI I and II is the addition of
command queueing.  This allows a mass storage device to receive multiple
commands from the same or different initiators and execute them later. 
For example, during a seek operation, if disconnected, the drive may
receive several additional commands and add them to its queue.  There are
a lot of details to this feature, but the benefits in speed and overall
system throughput should be immediately clear.

> Is SCSI II 32 bit, or what?

SCSI II has a 16 bit and a 32 bit option, but it can still operate in 8
bit mode.  There is also a higher speed timing option (called fast SCSI)
which permits up to 10 megatransfers/sec.  This gives SCSI II the
capability to go up to 10 megabytes/sec on an 8 bit cable, or up to 40
megabytes/sec on a 32 bit cable.

> How many peripherals is it possible to connect to the same controller
> without using any kind of "undocumented features"?

Still 8 devices total.

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Scott Smyers @ Apple Computer
Cupertino CA
Apple doesn't necessarily agree with this stuff.


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The ideas presented here are my own, not Apple's.