🚀 go-pugleaf

In <3keemr$eg9@munnari.OZ.AU> muller@munnari.OZ.AU (Paul Muller) writes:

>My question is what are the rules for terminating the SCSI bus for the internal
>connectors (one and/or two disks) and the external bus?

Ok, SCSI bus termination is simple - you have one set of terminators at
each physical end of the cable. Normally this is one set inside the
computer that has the SCSI bus host device, and one set on the device
that's the furthest along your SCSI cable. Any intermediate devices on the
cable should have terminating resistor packs removed.

I have a 3/60 and a 3/80 here and they both have terminating resistor
packs soldering to the main boards near their respective SCSI controller
chips.

It generally will not matter if you forget to terminate the last device on
the cable as long as the cable isn't longer than a metre or so, but if the
cable is longer than this then you can have problems since SCSI is
essentially a series of parallel transmission lines and to have long
signal lines without proper termination can cause ringing, etc. on these
lines which results in a very real slowdown in SCSI i/o throughput.

The other alternative is that you might forget to remove terminating packs
from one or more intermediate devices on the SCSI cable. This is once
again not usually a problem, but it is best avoided.

An interesting aside to this is that my 3/80 has a SCSI-2 bus controller
in it whereas the 3/60 has a SCSI-1 controller. I read a lot of things
that suggest that for SCSI-2, active termination is best for large device
arrays, but to date no-one I've spoken to has said that active termination
is actually better in practise than regular passive termination if
attention is paid to making sure that the passive terminators are properly
installed in the first place.

>What are the side effects if this is incorrectly done? I have noted there are
>no SCSI/Ethernet/Mouse fuses as with earlier Sun models, does this mean I can
>blow up the drive and or SCSI controller on the motherboard?

This fuse was (as far as I know) connected in series with the SCSI
TERMPOWER pin (pin #26 I think?), which is meant to supply power to
terminators at the far end of the cable. No modern drives that I have seen
use this line so I guess that's why later Sun models don't have them.

Regards,

Craig.
--
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