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Path: utzoo!utgpu!attcan!uunet!peregrine!elroy!ames!husc6!purdue!decwrl!vixie
From: vixie@decwrl.dec.com (Paul Vixie)
Newsgroups: comp.periphs,comp.sys.atari.st
Subject: Re: ESDI questions
Message-ID: <35@volition.dec.com>
Date: 13 Aug 88 01:39:03 GMT
References: <168@cstw01.UUCP> <1988Aug10.165519.20257@utzoo.uucp>
Organization: DEC Western Research Lab
Lines: 46

henry@utzoo.uucp (Henry Spencer) writes:
# ESDI is sort-of-kind-of an improved ST506/412.  It was designed to be a
# better and faster drive-to-controller interface.  Its relationship to
# ST506/412 is as a (superior) competitor.  It is unrelated to SCSI, which
# is a controller-to-host interface.

This is a visual and semantic problem that causes a great deal of confusion.

In ST506 and ESDI, there is a board on the system IO bus (or some chips and a
port on the motherboard) to which one attaches a cable; the other end of this
cable is attached to a disk drive.

In SCSI, there is STILL a board on the system IO bus (or some chips and a
port on the motherboard) to which one attaches a cable; the other end of this
cable is attached to a disk drive, which has an on-drive disk controller.

It's just that in ST506 and ESDI,
	we call the board-on-the-bus or chips-on-the-motherboard a
	"controller" and the cable running to the drive(s) is called a
	"drive-to-controller interface",
while in SCSI,
	we call the board-on-the-bus or chips-on-the-motherboard a
	"SCSI interface" and the cable running to the on-drive disk
	controller is called a "controller-to-host interface".

To most people, there's still just a board in the machine and a cable going
to a disk drive.  The fact that the board is called an "interface" in SCSI
because the controller is in the drive doesn't change the common terminology
of calling the board in the CPU cabinet a "controller".

It is more accurate to call it an "interface", of course, because the 
controller (the thing that deals with the MFM or whatever modulation the
drive uses) is in fact elsewhere.

It all hangs on calling the board a "controller" when in SCSI, it's an
"interface".  I can't repeat that too many times -- us board-swappers
that don't learn the details of things are easily confused by details
like this one.

(I don't mean that _you_ are confused, Henry -- it's just that your answer
didn't really ease the confusion that had prompted the question.)
-- 
Paul Vixie
Digital Equipment Corporation	Work:  vixie@dec.com	Play:  paul@vixie.UUCP
Western Research Laboratory	 uunet!decwrl!vixie	   uunet!vixie!paul
Palo Alto, California, USA	  +1 415 853 6600	   +1 415 864 7013