Path: utzoo!attcan!uunet!tektronix!teklds!amadeus.LA.TEK.COM!jamesa
From: jamesa@amadeus.LA.TEK.COM (James Akiyama)
Newsgroups: comp.sys.ibm.pc
Subject: Re: IBM PS/2 70 vs MAC II
Message-ID: <3955@teklds.TEK.COM>
Date: 12 Sep 88 22:26:14 GMT
Sender: nobody@teklds.TEK.COM
Reply-To: jamesa@amadeus.LA.TEK.COM (James Akiyama)
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 42

ESDI is not an enhanced and extended SCSI; it is, rather, an enhanced and
extended ST506/412 bus.  Note that both, ESDI and ST506/412, are low-level
disk interfaces.  Both specifications actually influence the method information
is recorded onto the disk surfaces.

SCSI is a high-level interface which is designed to be relatively device
independent (although it is clearly designed for devices which access data in
blocks rather than byte-by-byte).

Initially, a properly designed ESDI interface had considerable performance
benefits over its SCSI counterpart.  This was due to the SCSI protocol
overhead, and the fact that many SCSI drives were (and still are) basically
ESDI encoded drives with a ESDI-to-SCSI translator.  These factors (along with
others) often caused about a 20% performance penalty.

Currently, the margin is lower and less well defined.  SCSI overhead has been
reduced considerably and newer translators are much more tightly coupled to
the physical disk drive.  On-board track buffering and optimal seeking have
made many SCSI drives equal to ESDI counterparts (sometimes outperforming
them).  While ESDI drives are limited to fixed transfer rate increments (and
only up to 20 Mbits/secs), SCSI drives can transfer data at any speed (up to 40
MBits/sec asynchronously; faster synchronously).  

Also note that SCSI possesses much greater future capability since higher
level protocols allow for much more "SMARTS" being placed in the disk drive
(disk data caching, optimized request queuing, optimized seeking based on
logical disk blocks, etc).  Also, since SCSI is an 8-bit interface it should
be possible to design a disk drive which simulataneously reads all eight data
bits from parallel heads.  Finally, SCSI is easily extended to 16-bit and
32-bit formats.

All in all, I believe SCSI will take over ESDI in the long-haul.  It is my
belief that ESDI was offered more as an interim solution while the SCSI
standard was (and still is) being ironed out.  SCSI allows intrinsic drive
optimization to be placed where it belongs, in the disk drive.

James E. Akiyama
jamesa@amadeus.LA.TEK.COM
UUCP: ....!tektronix!amadeus!jamesa
ARPA: jamesa%amadeus.LA.TEK.COM@RELAY.CS.NET

DISCLAMER:  These opinion are mine, not Tek's.