From: bgrubb@dante.nmsu.edu (GRUBB)
Subject: Re: IDE vs SCSI
Organization: New Mexico State University, Las Cruces, NM
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Distribution: world
NNTP-Posting-Host: dante.nmsu.edu

david@c-cat.UUCP (Dave) write:
>are we talking about a SCSI-1 device (e.g. HD) on a SCSI-2 Controller
>or          
>are we talking about a SCSI-1 Controller that had a chip upgrade
>using the same chip that is on a SCSI-2 controller board.
we are talking about a SCSI-1 device (e.g. HD) on a SCSI-2 Controller.

Here is a rewrite of my Mac & Info sheet SCSI section:
SCSI: only external device expansion interface common to both Mac and IBM.
 Allows the use of any device: hard drive, printer, scanner, Nubus card 
 expansion {Mac Plus only}, some monitors, and CD-ROM.  Normal {asynchronous}
 SCSI is 5 Mhz;  fast {synchronous} SCSI is 10 Mhz.  Difference between these
 modes is mainly in the software drivers.
 Main problem:  there are a lot of external devices which are internal 
 terminated which causes problems for more then two devises off the SCSI port 
 {A SCSI chain is supposed to be terminated ONLY at the begining and end. 
 Any other set up causes problems for either Mac or IBM}. 
SCSI-1:  8-bit; 7 devices per SCSI controller.  asynchronous {~1.5MB/s ave}
 and synchronous {5MB/s max} transfers.  8-bit SCSI-2 is often mistaken for
 a fast version of SCSI-1 {see SCSI-2 for details}.
SCSI-2: fully SCSI-1 compliant. SCSI-2 mode - 10 devices per SCSI controller.
 8-bit SCSI-2 is implimented as a very fast SCSI-1 since it can run using
 SCSI-1 hardware and software drivers which limits it to 7 devices, which
 results in it sometimes being mistakenly consitered part of SCSI-1.  16-bit
 and 32-bit SCSI-2 require different ports, electronics, and SCSI software
 drivers from SCSI-1 {Which makes them more expensive than other SCSI
 interfaces}.  32-bit SCSI seems to run only in synchronous mode, hence term
 'wide and fast SCSI' Transfer speeds are 4-6MB/s with 10MB/s burst {8-bit},
 8-12MB/s with 20MB/s burst {16-bit}, and 15-20MB/s with 40MB/s burst
 {32-bit}.

8-bit SCSI-2 is the BIGGEST headache due to the fact that it is the ONLY SCSI-2 
that I know of that can pump SCSI-2 speeds through SCSI-1 devices and software
controlers.  Its MAIN implimentation is on the Mac though you do see adds
for SCSI-2 at 10MB/s maximum throughput for PCs.  Since 8-bit SCSI-2 does NOT
NEED all the hardware {including electonics} and software of 16-bit and 32-bit
SCSI-2 it is the CHEEPEST of the SCSI-2 interfaces.  Just pop out the SCSI-1
electornics and pop in 8-bit SCSI-2 electronics.  Some people consider
16-bit where SCSI-2 REALLY starts {I USED to be one of them} but
4-6MB/s average though put with a theoretical burst of 10MB/s is NOT SCSI-1
but is 8-bit SCSI-2.
Rule of thumb: if it is SCSIn and OVER 5MB/s then it is SOME type of SCSI-2.

8-bit SCSI CAN use part of SCSI-1 electronics which further reduces its cost
compared to 16 and 32-bit SCSI-2.