Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!apple!oliveb!tymix!tardis!jms
From: jms@tardis.Tymnet.COM (Joe Smith)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Hard Drive/Controller
Summary: 7*8 = 56 disks max
Message-ID: <840@tardis.Tymnet.COM>
Date: 1 Dec 89 04:12:55 GMT
References: <1989Nov21.224423.3829@athena.mit.edu> <18611@watdragon.waterloo.edu> <24531@cup.portal.com> <128499@sun.Eng.Sun.COM> <2037@atanasoff.cs.iastate.edu>
Reply-To: jms@tardis.Tymnet.COM (Joe Smith)
Organization: BT Tymnet, San Jose, CA
Lines: 25

In article <2037@atanasoff.cs.iastate.edu> jwright@atanasoff.cs.iastate.edu (Jim Wright) writes:
>I seem to remember something about 8 devices on the SCSI bus, with up to
>8 units associated with each device.  Anybody know the truth about this?

The specifications for SCSI is in ANSI X3.131.  I have the 1986 edition.

Yes, there is a limit of 8 controllers on the bus.  It has to do with the
way arbitration is done on the 8-bit data bus.  When the bus is idle, any
one of the 8 controllers may request the bus.  If controller #0 wants the
bus, it pulls down on the wire for bit 0, controller #1 uses the wire for
bit 1, etc, up to controller #7 using bit 7.

A single SCSI controller may have several logical units attached to it.
Each SCSI command has a 3 bit field that can be used by the controller to
address a particular unit on that controller.  This means up to 8 disk units
can be attached to a single disk controller.  Since you have to have one
controller attached to the CPU, that means you can have 7 disk controllers
with 8 disks each for a total of 56 disks.

Using 700 megabyte CDC Wren VI disks, that's a total of 39 gigabytes!
-- 
Joe Smith (408)922-6220 | SMTP: JMS@F74.TYMNET.COM or jms@gemini.tymnet.com
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