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From: oster@dewey.soe.berkeley.edu (David Phillip Oster)
Newsgroups: comp.sys.mac
Subject: Re: Making *own* SCSI Hard Drive
Message-ID: <19683@ucbvax.BERKELEY.EDU>
Date: Mon, 13-Jul-87 02:18:47 EDT
Article-I.D.: ucbvax.19683
Posted: Mon Jul 13 02:18:47 1987
Date-Received: Tue, 14-Jul-87 00:44:29 EDT
References: <380@umbc3.UMD.EDU> <458@osupyr.UUCP>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: oster@dewey.soe.berkeley.edu.UUCP (David Phillip Oster)
Organization: School of Education, UC-Berkeley
Lines: 69
Keywords: mondo cheap  ibm haters

In article <458@osupyr.UUCP> czei@osupyr.UUCP (Michael S Czeiszperger) writes:
>I've never actually seen a SCSI controller, so I don't have any concrete
>examples of what they cost, but one thing is certain: they're alot more
>expensive than an IBM standard controller.

Nonsense. On an IBM you start with a drive that already speaks the
SCSI interface standard. I'll repeat that, because it is important.
Drives for the IBM PC market already speak the SCSI interface
standard. 

Now, this isn't absolutely true, some of the older ones only
speak SASI, the precursor of SCSI. SASI differs from SCSI in that SCSI
has improved daisy-chaining, but if you don't need to hang multiple
hard disks off your mac SASI works just as well.

If IBM drives already speak SCSI, then why do you need a "Controller
board" to attach one to a PC? Because the PC doesn't speak it, the
"Controller board" just translates from SCSI to the IBM PC bus. When
you put a hard disk on a Mac Plus, since the Mac Plus directly speaks
SCSI, you don't need a controller board.

Now, what is on that IBM "Controller board"? To answer this, I have to
explain a little bit about the SCSI protocol.  SCSI is basically a
byte-oriented parallel protocol. A SCSI message has a header, data,
and is sent a byte at a time. Today, designers use special purpose
chips that decode the header. In the old days we just used to use a
Parallel Interface Adapter (PIA), the same chip that was used to
implement the "IBM" (really Centronics, remember them?) parallel
printer interface. And the header was decoded in software rather than
on the fancy NCR chip. I myself have written hard disk drivers for
SASI drives using this technique. Apple used to sell a single parallel
interface adapter card for the Lisa that would take either a printer
or a disk. 

The printer was a parallel
interface imagewriter (Called a C. Itoh ProPrinter back then, but
identical to an Imagewriter 1). The drive was a Profile hard disk. The
Profile was slow, but I am convinced that that was not the fault of
the Lisa interface card, after all the interfaces I wrote ran at
reasonable speed.

All you need to connect such a drive to a mac is an appropriate cable.
(The mac has a 25 pin connector, the drive a 50. Half of the drive's
pins are tied to ground.)  A bare bones SCSI driver is trivial. The
ROMS use the SCSI standard to read the first few sectors into memory,
these should contain your MAC disk driver. 
For those sites on arpanet, A generic SCSI driver kit
by Vishniac is archived on [SUMEX.STANFORD.EDU]  (see your
local ftp documentation.)

The extra cost of the packaged Macintosh disk over those bare bargain
drives goes for:

1.) a drive power supply
2.) a case
3.) software
4.) service (handling DOA drives, software upgrades
5.) cables
6.) profit.

If you want to do it yourself, then you save the cost of all that.

If you don't have a MacPlus or later, you'll need to add hardware
inside your Mac to recieve the signals from the drive. The newer Macs
have this built in. Thanks Apple!

--- David Phillip Oster            --My Good News: "I'm a perfectionist."
Arpa: oster@dewey.soe.berkeley.edu --My Bad News: "I don't charge by the hour."
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