Path: utzoo!attcan!uunet!zephyr.ens.tek.com!tektronix!percy!parsely!agora!billsey
From: billsey@agora.uucp (Bill Seymour)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Multiple SCSI controllers
Message-ID: <1990May27.170747.13732@agora.uucp>
Date: 27 May 90 17:07:47 GMT
References: <90146.174753BRANDON@ncsuvm.ncsu.edu>
Reply-To: billsey@.UUCP (Bill Seymour)
Organization: Betazoid Central
Lines: 55

In article <90146.174753BRANDON@ncsuvm.ncsu.edu> BRANDON@ncsuvm.ncsu.edu writes:
:
:  I was wondering if I could have multiple SCSI host controllers on the
:same cable run.  I have 2 amigas with SCSI controllers and one disk drive.
:Currently, I am using the parallel port network from the Software Distillery
:to have both computers access the same drive.

	You can do this easily, but with some ...features. In most usage,
your SCSI interface will only be looking for data on the SCSI buss for
requests that your interface has initiated. That means you can hook the
two up and mount the drive on both systems. The drive sends data out when
requested and both computers recieve that data, but only the computer that
asked for the data will actually use it. The biggest problem you run into
with this type of setup is that one machine will not know when the other
machine has made a change to a partition. If you have a partition that's
completely empty hooked to computer A and to computer B and you save a 50K
file onto the partition from comuter A, computer B will think the partition
is still empty. If computer B then saves a 20K file onto the partition,
computer A will now find a currupt file system when it tries to read. The
solution (short term) is to always do a diskchange before accessing a
partition, and never cause a file or directory lock that would defeat that
diskchange. The solution (long term) is to change the SCSI ID of one of
the computer interfaces and rewrite the file system to notify one SCSI
host when the other SCSI host makes changes... Or use ParNet to arbitrate
your changes... :-)

:  It would seem to me that there should be nothing wrong with having one
:disk drive and two host controllers on one SCSI "network" from a hardware
:point of view.  From the software end, I realize that I would have to have
:separate partitions on the disk to write to, as there would be no way to
:coordinate write requests from both machines.

	I actually used this type of 'network' on my system here at home
for a couple of years. I had Supra interfaces hooked to both the 1000 and
the 2000. They shared a (read only most of the time) workbench and each
had a partition that was earmarked for their personal use... I have
switched to ParNet now and am much happier... I also have to rebuild
my hard drive setup much less often. :-) ParNet runs about twice the
speed of a floppy, so it's not too bad for me to use.
	Note that I have an expansion card for my 2000 with another
parallel port on it, so printing while both machines are using the
parallel port isn't too much of a hassle...

:  Has anyone attempted this?  As all I need to do this is two header
:connectors and a few feet of ribbon cable, I'm tempted to try it just to test
:the other SCSI controller.
:                                                  BlH
:



     -Bill Seymour             ...tektronix!reed!percival!agora!billsey
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