bobl@pro-graphics.cts.com (System Administrator) (08/27/89)
CS-ID: #1161.amiga/net.general@pro-graphics 8662 chars
Date: Sat, 15 Jul 89 20:02:19 EST
From: thad@cup.portal.com (Thad P Floryan)
Subject: Re: Supra Controllers and SCSI ID's
Perhaps now is the time to briefly describe a SCSI bus. Consider the
following (rough) illustration of a fully-configured (and loaded!) bus:
Interface Devices whose LUNs are:
+----------++-----------+
| Your || SCSI host | SCSI +-----+ +---+ +---+
| Computer || Adaptor |==========::=|SID=6|-----| 0 |--- ... ---| 7 |
| (Amiga!) ||SID=7,LUN=0| BUS || +-----+ +---+ +---+
+----------++-----------+ ||
|| +-----+ +---+ +---+
::=|SID=5|-----| 0 |--- ... ---| 7 |
|| +-----+ +---+ +---+
where: ..
..
SID = SCSI ID [0..7] ..
LUN = Local Unit Number [0..7] || +-----+ +---+ +---+
::=|SID=0|-----| 0 |--- ... ---| 7 |
|| +-----+ +---+ +---+
||
-/\/\/\-
Bus Termination
Given the proper interface at each SCSI ID, one could place up to, say, 8
disk drives on it, and with 7 interfaces, one could have, say, 56 disk
drives on one's bus. This is what the SCSI bus and its specs permit.
In reality, however, a interface typically will be designed to operate only
one or two devices. For example:
a) An embedded-SCSI disk drive (such as Seagate ST138N, Maxtor XT3380S,
Quantum 80S, etc.) This is an integrated SCSI interface with disk
drive controller. The SID should be set to a unique value on the
bus, and the LUN is hard-coded to be zero (0).
b) A (multi) disk-drive adaptor/translator card (such as Adaptec 4000
and 5000 series, Omti, etc.). This is a stand-alone SCSI interface
card which is usually bolted by the end-user or system-integrator to
the back of a disk drive (for convenience) and will adapt a non-SCSI
disk drive (such as ST251, Maxtor XT1140 or XT2190, Miniscribe 6085,
etc.) to the SCSI bus; such cards usually have provision to handle
up to 2 drives whose LUNs will be 0 and 1 as determined by the
connector on the interface to which one plugs the drive's cabling.
c) Other embedded-SCSI devices, such as tape drives, laser printers,
modems, page scanners, etc. Such devices permit setting a unique
SCSI ID and will have a hardcoded LUN=0.
d) Other adapter cards (such as NCR, Fujitsu, etc.) designed to adapt
specific devices (such as floppy and/or tape drives, scanners, etc.)
to the SCSI bus; these cards permit setting a unique SCSI ID and
"may" provide multiple devices with LUNs ranging from 0 to 7.
e) "Host adapter" cards. Such cards are the interface between a
computer and the SCSI bus. One CAN have more than one "computer"
on the bus, each with its own SCSI ID. Typically the LUN is
hard-coded at zero.
In a full-SCSI, arbitrating [de]-select setup (which I have NOT seen yet for
the Amiga), the choice of a SCSI ID takes on additional meaning because it
will determine the device's PRIORITY. The highest SCSI ID, 7, will have the
highest priority on the bus. With such a setup, one would want to have the
host adapter at 7, the primary disk drive at 6, other devices at 5 thru 1,
and your tape backup system at SCSI ID 0. It is possible for devices whose
IDs are, say, 5 and 4 to be "talking" to one another and exchanging data
while the host is talking to, say, a tape drive at ID 0. Note that only ONE
Initiator-Target pair has its data on the bus at any given instant, though.
SCSI setups for the Amiga are (usually) non-arbitrating systems, and all
bus control is under direction of the host adaptor (and its software), so
the choice of SCSI ID isn't as important (except that all interfaces must
still have a unique ID! :-).
What all this is leading up to is that the address of a specfic device on
a SCSI bus is determined by the combined SCSI ID and LUN. A device will
only be selected and activated when it sees its "address" on the bus.
If, for some reason, one is attempting to address the typical embedded-SCSI
hard disk with a non-zero LUN, the drive will simply NOT respond. This is
the situation I saw with one person's setup last year, and it "may" be the
problem Doug is having.
3) Another set of potential problems concerns an improperly terminated SCSI
bus, miswired cabling, and/or mis- or non-use of the SCSI terminator power
line on the bus. SCSI bus wire 26 has been perverted by many manufacturers
and users, and others have "done" non-standard things with other "undefined"
SCSI bus wires.
In a previous posting, I've detailed the SCSI bus lines that one should be
SURE are not connected (improperly). I think this was also reprinted in one
of the Amiga-related magazines, but I haven't seen it.
In any event, among the mis-wiring offenders I've seen are included Supra,
Seagate, Apple, etc. And a lot of "Apple" 3rd party disk subsystem
assembly houses really screw the bus by having too long a "stub" off the bus
(spec is 10cm MAX, and I've seen up to 40cm; sheesh!).
4) And a BIG problem with using Maxtor drives is the use of an inadequate
power supply. These mongo drives require tremendous startup-surge
capability AND very clean power during operation. Doug should look at both
the +5 and +12 VDC lines on a 'scope while powering-up and running the
drive.
All the above should provide some clues as to where to look for problems.
When everything is assembled correctly, operation will be faultless. The SCSI
busses on my Amiga systems extend to the full 20 feet (6 m) permitted by the
SCSI spec, "talk" to 4 or 5 interfaces on each bus (which includes several
Maxtors), and are rock-solid and operated 24 hours/day, 7 days/week.
The interfaces I've personally verified as operating fine on the Supra-based
SCSI busses on my systems include:
1) ALL embedded-SCSI drives I could get my hands on. This includes
Fujitsu, Seagate, Maxtor, Imprimis, Connor, Quantum, CAST, etc.
2) Adaptec interfaces/translators. This includes the 4000A and 4070 used
to operate Seagates, Maxtors, LaPines, Miniscribes, Microscience,
Micropolis, Priam, etc.
3) Fujitsu M10008A (for driving ESDI tape drives)
Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]
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