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From: pgn@osupyr.mast.ohio-state.edu (Paul G. Nevai)
Newsgroups: comp.sys.mac,comp.sys.mac.programmer
Subject: SCSI Cables and Terminators
Message-ID: <883@osupyr.mast.ohio-state.edu>
Date: 17 Sep 88 14:32:44 GMT
Reply-To: pgn@osupyr (Paul G. Nevai)
Organization: Mathematical Sciences Computer Lab, Columbus, OH
Lines: 79


		This is about SCSI's and Terminators!!!

Oh boy, I thought I knew everyting about them, but when I opened (the first
time in my life) the little booklet "Apple SCSI Cable System" which comes 
with the SCSI cable it turned out that I was DEAD wrong.

(In what follows, all Hard Disks are assumed to be non-terminated,  System
Cables are DB 25 on one end and Centronics 50 on the other, Cable Extenders
have Centronics 50 (M-F) on both ends.)

For starters, did you know for instance, that

   Mac -- System Cable -- Hard Disk -- Teminator

is wrong? The right configuration is

   Mac -- System Cable -- Teminator -- Hard Disk

OK? Now, for advanced intermediates, did you know that

   Mac -- System Cable -- Cable Extender -- Teminator -- Hard Disk

is wrong? The right configuration is

   Mac -- System Cable -- Terminator -- Cable Extender -- Teminator --
   -- Hard Disk

OK? (See page 7, fig. 5 in quoted booklet.) Isn't this surprising? 

Now, for pros, do you know where the terminator goes in

  Mac -- System Cable -- Cable Extender -- Hard Disk -- Cable Extender --
  -- System Cable -- Hard Disk ?

(In case you wonder how I can have this configuration of cables, that is DB
25 away from the Mac, then I tell you that my last Hard Disk has two DB 25
outlets)

Is it 

  Mac -- System Cable -- Teminator -- Cable Extender -- Hard Disk -- 
  -- Cable Extender  -- System Cable -- Teminator -- Hard Disk

or   

  Mac -- System Cable -- Cable Extender -- Teminator -- Hard Disk -- 
  -- Cable Extender  -- System Cable --Teminator -- Hard Disk

or what else?

For pros, here is another toughie: if the Hard Disk has two DB 25 outlets,
then is it all right to use any DB-25 cable to connect it with the Mac, 
or are there special DB 25 SCSI cables?

One more questions: why are

  ... -- Teminator -- Hard Disk (The End)

and

  ... -- Hard Disk -- Teminator (The End)

different? 

I am anxiously looking forward for prompt replies from The Experts i. e. 
the APPLE, SuperMac, CMS etc. guys!!!

Sincerely, 

Paul Nevai                                pgn@osupyr.mast.ohio-state.edu 
Department of Mathematics                 TS1171@OHSTVMA.bitnet
The Ohio State University                 73057,172.Compu$erve
231 West Eighteenth Avenue                1-614-292-5310.office
Columbus, OH 43210, U.S.A.                1-614-292-4975.dept

P.S. Though one can have up to 20 feet of SCSI cable in a system, do not
ever dare to test the limits or else you will have spectacular crashes.
Trust me.