Unhooking scsi devices while machine is running

[Steve Simmons]

harrison at csl.dl.nec.com (Mark Harrison) writes:

>There has been some concern over a regular workstation (Sparc 1+)
>that has an additional external scsi hard disk.  For some reason,
>we are under the impression that to hook the tape drive to that
>machine, we have to bring the machine down, unhook the disk drive,
>plug the tape drive to the machine, and then daisy-chain the disk
>drive to the tape drive.  I think whoever came out and installed
>the thing had it set up that way, and nobody (including me :-)
>wants to take the blame for screwing things by trying it differently.

>My questions:

>1.  Is there *any* significance to the order of daisy chaining?

Yes.  A SCSI bus requires that the *end* of the chain be terminated,
ie, have some special resistors on it.  If those resistors are missing,
you may get bus errors.

There are two ways to terminate.  Most scsi devices have a plug that a
resistor pack can be placed into.  This terminates the SCSI bus in the
device.  It's usually referred to as an internal terminator.  More and
more you'll see external SCSI terminators -- a device that plugs into
one of the SCSI sockets outside the device.  These cost more ($40?) but
are much more flexible.

Note that if you have an internal terminator, that's the end of the
bus.  If you add devices further down the chain, they won't be seen.

>2.  Is it safe to plug and unplug the tape drive while the system
>    is up?

Suns seem to be fairly insensitive to removing the terminator for
short periods.  We regularly remove/add tape drives on 3/50s, 3/60s,
and IPCs *if* they are the last thing on the bus.  Disconnect a disk
and your system will crash almost immediately.  We've also moved
DEC SCSI TK-50s from system to system without problem.

>3.  (this is the point of the whole posting)  Will I screw things
>    up if I simply plug and unplug the tape drive into the disk
>    drive while the system is running?

It depends.  As mentioned above, I've had good luck on specific
systems.  But it has been claimed on some systems if a device is not
on the bus at boot time, it cannot be added later.  Sorry, can't
confirm or deny.

>4.  What FM should I be reading to know about this stuff?

If you find one, let us *all* know.
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