Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rutgers!sri-spam!ames!ucbcad!ucbvax!dewey.soe.berkeley.edu!oster 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." Uucp: {seismo,decvax,...}!ucbvax!oster%dewey.soe.berkeley.edu