🚀 go-pugleaf

Bob Peirce #305 writes
> I have an HSD scanner at the end of my SCSI chain.  Since I don't
> use it all the time ... I like to turn it off...
> Something I haven't tried is to actually disconnect the scanner
> from the CD drive and plug the SCSI terminator in.  That doesn't
> seem like a good thing to do on a running system...

The SCSI bus is actually an active bus.  To function correctly,
all devices on the bus should be powered on, the cables should
have the proper impedence, and the bus should be terminated at
both ends.  A *very* short bus can be run with termination only
at one end (e.g., just the on-board disk, which has a terminator
on it).

If a device doesn't receive power, it might not pass the bus
signals through correctly.  If it's at the end of the chain, the
terminator probably requires power to operate correctly.

For the same reason you shouldn't remove a device from the SCSI
bus while the system is running, your shouldn't power off a
SCSI device while the system is running.  Yes, these things
MIGHT work.  But, do you want to risk your data?  We've seen
instances where an improperly terminated SCSI bus can lead
to data corruption on the internal hard disk....

What to do?  Talk with the scanner vendors and ask them to
provide a "standby" mode, where the SCSI bus receives the
proper power, but the scanning light is shut off. (Caveat:
there might be scanners which do this already, and there
might be SCSI devices which handle bus signals and termination
when powered off.  Check with the device manufacturer to
be certain.)
--
Alan M. Marcum
NeXT Tech Support
amm@NeXT.COM