Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!udel!burdvax!sdcrdcf!trwrb!trwrc!agnew
From: agnew@trwrc.UUCP
Newsgroups: comp.periphs
Subject: Re: Info requested about SCSI tape drives
Message-ID: <230@trwrc.UUCP>
Date: Thu, 1-Oct-87 21:26:11 EDT
Article-I.D.: trwrc.230
Posted: Thu Oct  1 21:26:11 1987
Date-Received: Sat, 3-Oct-87 10:07:06 EDT
References: <133@bcsfse.UUCP>
Reply-To: agnew@trwrc.UUCP (R.A. Agnew)
Distribution: world
Organization: TRW/MEAD San Diego, Ca.
Lines: 26

In article <133@bcsfse.UUCP> bill@bcsfse.UUCP (Bill Sears) writes:
>
>	2.  I have a set of tape driver devices built into my system.  Is it
>		reasonable to expect them to work with any tape drive connected by
>		a SCSI interface?  In other words, the interface protocol is standard
>		but is the device command syntax standard?
>
Not even all the SCSI interfaces implement the same subset. There is something
called the "command command subset" which some controllers support. However,
you've got a bigger problem! SCSI is only a transport layer protocol; you
have to make sure that the tape controller supports the appropriate tape
"standard" like QIC-24, QIC-32, QIC-02, etc. Make sure your drivers support it.

>	3.  What problems might I run into if I try to connect more than one
>		device to the SCSI "port"?  I would also like top connect a large hard
>		disk, but am I only going to be able to have device connected at a time?
>
To use more than one SCSI device (8 maximum) on the bus, all of the controllers
must support the "attention" bit. The biggest problem with a SCSI streaming tape
is that many SCSI tape controllers cannot or will not disconnect from the bus
during slow operations like tape movement to give the disk a chance to do a 
few reads or writes. This effectively defeats the so-called "streaming" action.
A good example of this is the SUN-2 and SUN-3. Without a large buffer, a SUN-3
can take an hour to search a 1/4" cartridge. To get maximum performance out of
multiple disks on the bus, the SCSI disk controllers must disconnect while
doing a head seek. Also they should support "multi-threading".