mattd@Apple.COM (Matt Deatherage) (07/03/89)
There have been enough questions posted on this net and others over the past year about SCSI to make someone's head spin. Most of them, though, can be covered in about three or four paragraphs, which is what this off-the-cuff note will attempt to do. "What do you mean by SCSI?" =========================== SCSI stands for "Small Computer System Interface", and is a national or international standard for peripherals and computers -- they use the protocols specified in the SCSI specifications to know how to talk to each other. This takes on special meaning in the Apple world, since Apple produces SCSI interface cards for the Apple II (the "Apple II SCSI Card") and builds SCSI interfaces into most Macintosh computers. Apple also produces a series of periperhals that connect to computers through SCSI. Currently, this includes a line of hard disk drives, a scanner, one model of LaserWriter, a CD-ROM drive, and a tape backup unit. To use these peripherals on your system, you have to have an Apple II SCSI card. "So where does the confusion start?" ==================================== The confusion starts with the system software, most notably GS/OS for the Apple IIgs. GS/OS supports the concept of "loaded drivers", where code that actually controls peripherals such as disk drives or modems is loaded from the boot disk when the computer is started. Apple provides drivers for SCSI peripherals (SCSI.DRIVER for System Software 4.0, SCSI.MANAGER for System Software 5.0) to provide greatly increased performance. If there is not a driver loaded for a particular peripheral, GS/OS makes one up, or "generates" one, so that the peripherals can still be used. This is what happens for old ProDOS-compatible peripherals for which no loaded driver is supplied - GS/OS knows it's an old ProDOS device and generates a ProDOS driver so it can be used. Here's the key - APPLE'S SCSI DRIVERS OR MANAGERS WORK ONLY WITH APPLE II SCSI CARDS. It is up to each manufacturer to provide a loaded driver for their peripheral if they want the increased performance it gives; Apple does this for hard disks and CD-ROM drives connected to the Apple II SCSI card with the SCSI drivers or managers included with GS/OS and the System Software. Although other manufacturers (such as CMS or Chinook) may make their own interface cards which communicate with their SCSI drives, Apple's SCSI driver has ABSOLUTELY NOTHING to do with those cards. "So will they still work?" ========================== Yes. Any peripheral that works under ProDOS 8 will *always* work under GS/OS. A generated driver will be created that allows GS/OS to work with a ProDOS device if a loaded driver for it is not present. It will not be as fast as a loaded driver, but it will work. So although the speed increases in SCSI noted for System Software 5.0 do not apply to non-Apple SCSI cards, they don't hinder the performance of those cards either. Nor is it *required* to have a driver for a peripheral to make it work with GS/OS - as long as it works with ProDOS, it works with GS/OS, though loaded drivers will speed things up. "What about the 8-bit world?" ============================= The Apple II SCSI card contains firmware on it so that it works with har drives and CD-ROM drives. Because of ProDOS 8 limitations, no matter how many SCSI partitions or devices are present, only two will show up, since ProDOS 8 can only deal with two devices per slot. The one exception is that ProDOS 8 will allow up to four devices on an interface card in slot 5, as long as that card has SmartPort firmware (and the Apple II SCSI card does). The third and fourth devices/volumes are "shadowed" to slot 2, drives 1 and 2, just as extra 3.5" drives or a RAMdisk is handled on the Apple IIgs (in fact, it's the same mechanism). The firmware does not contain support for devices other than hard disks or CD-ROM drives at this time. "What about homebrew SCSI?" =========================== Some people are finding it desirable to build their own SCSI hard drives from scratch, finding the experience valuable or the cost savings worthwhile. While articles have abounded on this technique over the past year, I point to one word of caution: SCSI devices connected to Apple computers are expected by the system software to follow Apple's extensions to SCSI. A drive which does *not* do this may find itself not working with future versions of the System Software. I can't tell you off the top of my head how to get the Apple SCSI extensions document, as I do not know if it is information Apple must legally have a non-disclosure agreement on to disseminate. I will attempt to find out, since there are a lot of people out there making their own hard drives including an increasing number of new retail hard drive manufacturers. Note that for legal reasons, this information may not be distributable, and Apple may not have a choice in the matter. I honestly don't know. (So please don't fill the net with your opinions on the subject.) I will try to find out. That should cover all the major bases. ----------------------------------------------------------------------------- Matt Deatherage, Apple Computer, Inc. | "The opinions expressed in this tome Send PERSONAL mail ONLY (please) to: | should not be construed to imply that AppleLink PE: Matt DTS GEnie: AIIDTS | Apple Computer, Inc., or any of its CompuServe: 76703,3030 | subsidiaries, in whole or in part, Usenet: mattd@apple.com | have any opinion on any subject." UUCP: (other stuff)!ames!apple!mattd | "So there." -----------------------------------------------------------------------------
gwyn@smoke.BRL.MIL (Doug Gwyn) (07/07/89)
In article <32829@apple.Apple.COM> mattd@Apple.COM (Matt Deatherage) writes: >SCSI stands for "Small Computer System Interface", and is a national or >international standard for peripherals and computers ... >SCSI devices connected to Apple computers are expected by >the system software to follow Apple's extensions to SCSI. The real problem is that Apple persists in calling these products "SCSI" even though they have the wrong connectors and follow a different (though related) standard than true SCSI. How about picking another name such as "ASCSI".