Vectronix 2/VECTRONIX2.iso/FILES_08/EXDOS24A.LZH/EXTENDOS.24A/TEXT/SCSI.TXT

Text File | 1996-06-08 | 10KB | 198 lines

    SCSI - A Game With Many Rules and no Rulebook?    Version 1.11

        Copyright Gary Field, 1994 - All rights reserved.
    Special Internet edition - Freely distributable for non-commercial use.
Author:    Gary Field ([e-mail]) - SCSI hacker since 1985
    With a little help from my friends.
Last Updated: September 9, 1994

Trademarks:
MSDOS and Windows are trademarks of Microsoft Corp.
Unix and Netware are trademarks of Novell Corp.
OS/2 is a trademark of IBM Corp.

Enough with the legal mumbo-jumbo already.

A little scuzzy humor:
User#1: I just bought myself a new CDROM drive.
User#2: Is is SCSI?
User#1: Of course not, it's a really nice one!

How do I connect all this stuff to my PC?

So you want to play SCSI eh? Be warned, the road can be rough, but the rewards
are great.

The rules of the SCSI game:
    o Each SCSI device needs to have a unique identifier (ID) from 0 to 7.
      The host adapter is usually given ID 7. Each device must have a
      different ID.
    o It makes no difference what order devices are connected to the
      bus in. Any ID can be in any position on the bus.
    o There must be exactly two terminators for the SCSI bus.
      If you are playing the FAST SCSI game (meaning that you have a
      host adapter that handles the FAST protocol and at least one device
      that uses FAST mode), then you need to use active terminators.
      See discussions about terminator types in the SCSI FAQ list.
    o The total length of all SCSI cabling must be less than 20 feet (6 m)
      (75 feet (25 m) in the differential version of the game).
    o The cables must flow continuously from one device to the next.
      No stubs more than 3" (.1 m) long are permitted.
    o All cables used must have an AC impedance of between 90 and 130 Ohms.
      This is a tough one! Short of connecting your cables to a RF
      impedance meter, you need to have faith on this one. Get your cables
      from a vendor that knows SCSI well. Tip: If you keep the cables
      really short (like < 2 feet(.6 m) total), you can get away with
      murder.
    o For best reliability, all cables should consist of twisted pairs
      for each SCSI signal. This is not optional in the FAST version.
    o A player may mix flat and round cable as long as the previous four
      rules are not violated. In the FAST version you really need to keep
      such transitions to an absolute minimum.
    o Devices may be located internal or external to the system's case.
      The simplest situation being all internal or all external, but
      mixing is allowed IF all above rules are followed to the letter.
    o At least one device MUST supply power to the TERMPWR line on
      the SCSI bus. (see local rules for important details)


Local rules:
    o Supplying TERMPWR is usually considered the responsibility of the
      host adapter. This TERMPWR voltage must be between 4.25V and 5.25V
      with both terminators attached to the bus. If your bus seems to
      work most of the time, but is intermittent, get out the scope or
      voltmeter and check that TERMPWR is within the above range. In
      addition, the presence of any noise pulses on TERMPWR will really
      screw things up. Some external terminators include a capacitor to
      help reduce noise.
    o It is assumed that all devices are connected to power supplies
      that can fully satisfy all the device's power requirements (even
      during the device's peak load). Also, avoid ground loops by plugging
      the system and all external devices into the same outlet if possible.
      These two rules are often overlooked and can result in severe hair
      loss.
    o Some vendors may require that any hard disks to be supported
      by the PC BIOS (Int 13h) interface, be given ID 0 and ID 1
      respectively. Check your manual for this one.
    o Because of the nature of the PC BIOS INT 13h interface, users
      with drives larger than 1024Kbytes (one Gig), should prepare
      for extra fun. (see the comp.periphs.scsi FAQ for details)
    o Some software will allow devices to be powered on only when
      needed. Other software, may require all devices to be powered on
      when the system is started. It is recommended that you start out
      with all devices on until you get everything working.
    o Some devices power up/down cleanly and will not disturb a running
      SCSI bus, others do not. If you tell the doctor "It hurts when I
      do this", he may well reply "Then don't do that".
    o Most vendors properly key the 50 pin connector on the device,
      however, it shall be the responsibility of the player to make
      sure that pin 1 is really connected to pin 1 everywhere.
    o Smoking is a bad idea, even for SCSI devices. Some vendors don't
      provide a fuse in the TERMPWR line. Forwarned is forearmed.
    o SCSI vendors should provide connectors that are called for in the
      ANSI SCSI spec's. Some will try to pass off 25 pin connectors on
      unsuspecting players. These vendors need a good slap. Feel free
      to harrass and ridicule them at every opportunity.
    o Connecting and disconnecting devices to/from a system while it is
      powered on is seriously discouraged. It is strictly verboten on any
      system that caches disk operations (like Unix) since you don't know
      when bus activity might occur. A word to the wise should be sufficient
      on this one.
    o A SCSI device mounted in an external case needs two connectors so
      that proper bus chaining can be maintained. In an apparent attempt
      at sick humor, A small number of vendors have played a joke on their
      customers by supplying only one connector. If you are a victim of
      this joke, you need to put this abomination at one end of your bus.
      "Flow-through" terminators are available to help solve the
      termination problem.
      Hopefully, you haven't fallen for this prank more than once!
      (Fool me once - shame on you. Fool me twice - shame on me!)

A word about fair play:
    o Please don't use unshielded cables for SCSI connections outside
      your computer's case. Many of us use the radio spectrum for
      something other than a trash heap. Keep your SCSI signals to
      yourself, no-one else wants them.

The object of the game:
    o A player must construct a working SCSI bus using the devices and
      adapters supplied to him by multiple vendors.
    o This SCSI bus should be no longer than necessary to attach all
      the given devices.



Wive's tales:
    o Some players will tell you that they needed to connect a third
      terminator or only use one instead of the required two. These
      players do not realize that these symptoms are the result of having
      broken one or more of the other rules. Sometimes, plugging in
      terminator resistor packs backwards or using cables of too low
      impedance can fool these players into thinking these things are
      necessary. Forgive them, for they know not what they're doing.

Judging:
    o The computer will be the final judge of adherence to all of the
      above rules.
    o The SCSI game is unusual in that everyone can win!


The above rules only cover the hardware aspects of SCSI. Without software,
you'll undoubtedly find that your newly constructed SCSI bus is rather boring.
Software is a whole 'nother game!

                ----------

SCSI software game: (Also called DRIVERS)
Completion of the above hardware game is a pre-requisite.

A little history of the game:
Whereas the SCSI hardware game has strict rules, the SCSI drivers game has been
pretty much a free-for-all. Once, there was total chaos in the land of SCSI.
Each vendor provided driver software for the specific devices it decided to
support. If a player later decided he wanted to attach a device that was
not deemed valuable to their chosen vendor, tough! And to make sure that
a player would not write his own drivers these vendors would not provide
interface spec's for their host adapters. All vendors supported hard disks,
but attaching tapes, or CDROMs was not for the faint of heart.

One wise vendor called Adaptec, eventually heard the wailing cries
of it's customers and decreed that henceforth SCSI drivers would talk to their
host adapters via a protocol to be known as ASPI (Adaptec SCSI Programming
Interface). Since ASPI was deemed by many to be too simple for serious players,
the ANSI committee came to the rescue with their CAM (Common Access Method).
These driver interface definitions changed the SCSI software game forever!
Modern players in SCSI software are frequently heard reverently speaking
these acronyms. The ASPI definition covers MSDOS, Windows, OS/2, and Netware.
CAM covers Unix in addition to these.

Still, selection of one of these standards is something of a religious act
of faith. Future Domain has wisely chosen to support both by implementing
CAM as their native interface and creating an ASPI interface that goes
down through CAM to the adapter. CAM cannot be implemented on top of ASPI
since CAM is a super-set of ASPI.
These days, in the MSDOS/Windows world, selection of SCSI software is pretty
much a matter of choosing ASPI or CAM and remaining true to your selection.



And so the disks, the tapes and the CDROM all ran happily ever after
and DAT's the end of the story...

If you think you're ready for real fun, be sure to pick up a copy of
the comp.periphs.scsi FAQ from these fine sources:

(latest version posted during the first week of each month)
comp.answers:
comp.periphs.scsi:
ftp.wang.com:garyf/scsi

Parting shot:
Individually we're scuzzy, together we're scum!

Don't forget -  The early write eats up the WORM!

No sir, the terminator will not make your SCSI bus stop running.