🚀 go-pugleaf

In article <4455@rosie.NeXT.COM> amm@NeXT.COM (Alan M. Marcum) writes:
>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).
>

This is bunk.  The SCSI bus is driven by open-collector devices.  A 
properly behaved SCSI peripheral may be left in the powered down state without
any ill effects.


>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.
>

SCSI signals are not passed through a SCSI device.  Rather the bus is
tapped by each device on the bus.  This is why it is called a "bus".
Quite often the terminator power comes from the host, not the drive.  This
is true of *both* the near and far terminators.


>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....
>

This is good advice if for no other reason than you may choose to power 
up a device at the same time then NeXT decides to swap or something and
who knows how well behaved a device going through power up is.


>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

Nothing personal, Alan, but is this the kind of answer I can expect from
NeXT tech support at $225/Question ?


-Brian


-- 
Brian Cuthie
Systemix Software, Inc.
brian@systemix.com