SCO UNIX and SCSI devices

[Doug Gwyn]

In article <2170 at olsa99.UUCP> mike at olsa99.UUCP (Mike Foo) writes:
>Apparently when one has to install 2 SCSI drives and a SCSI tape, the
>second hard drive has to be installed AFTER the tape. Has anyone had the
>opportunity to try this?
>Can anyone explain the difference between the id no and the lun no?
>Can SCSI devices be placed anywhere on the daisy chain? And do the
>connectors have any privileges? ie those closer to the controller have
>higher priority on the bus?

I don't know about SCO's systems, but in general, a SCSI bus supports
up to 8 devices, with different "SCSI priorities" that are normally
configured either by jumpers on the device's SCSI controller or by a
thumbwheel switch (or similar) mounted on the back of the device case.
The computer in a setup like that normally has the highest SCSI priority
(7) and tapes are normally assigned lower priorities than disks; each
device must have a unique priority.  There should also be two SCSI bus
terminators (normally a couple of resistor packs constitutes a terminator),
one at each end of the SCSI bus.  Probably your computer SCSI interface
has one terminator on-board, leaving just one needed at the end of the
"daisy chain" (really just a 50-wire bus with devices tapped onto it).
Some SCSI peripherals come with terminators installed, some without.
The simplest (but most costly) approach is to remove all terminators
except on the computer, and to use an external terminator at the end of
the chain; that's how Apple SCSI systems are set up.  A cheaper solution
is to remove all internal terminators except for the last device in the
chain; obviously this is more hassle if you add another device to the
chain.

The priority is determined just by the jumpers, not by placement in
the chain.