[SunRescue] sun3/80 scsi

[Greg A. Woods]

[ On Monday, December 25, 2000 at 14:57:53 (-0500), Sam Creasey wrote: ]
> Subject: [SunRescue] sun3/80 scsu
>
> Are there any bus oddities related to the *two* internal scsi ports?

Nope.

> Where does this thing want terminators attached to the scsi bus?

Only on the outside, and normally only if there's a cable and another
device plugged into it too.

The internal connectors are what's known as "stub" connectors and so
long as the internal cables are 10cm or less in length then no
terminator is needed.

The host end of the bus is terminated on the motherboard, so you need
only terminate the external connector if it is extended by a cable to
some other device.  As far as I know Sun only ever used external
terminators on gear of this vintage, so there are always two connectors
on external devices, the second either to daisy-chain to the next box,
or to install a terminator on.

You can sometimes get away with an interanlly terminated drive on the
end of the internal chain of an external box if the stub is less than
10cm long, but it's definitely not advisable to try.

Total bus length limits are very important to observe too.  Six meters
is the maximum for a single-ended bus, and that adds up a lot quicker
than you'd think because it includes everthing but stubs.  Using the
correct cable, i.e. with the correct impedance (132 ohms is ideal, but
100 ohms +/- 10% for ribbon, and 90+ ohms for shielded is OK), is
important too.  They say that cables of different impedance should not
be used on the same bus, but that's difficult to do in some situations,
esp. if you've got a ribbon arrangement inside a separate chassis.

Also if you're making up your own external components, make sure you're
supplying termination power to the bus from the most distant device too
(and of course make sure it's the correct polarity, or else you'll find
that one of the SCSI cable wires lets the smoke out real quick!).

There may be issues with termination power on some models of 3/80,
though IIRC those issues only affect 3/x60's and older.  Someone else on
the list will no doubt be able to confirm/deny this with more authority
than I.

>  (and is
> it proper for the scsi interface to freak out if no terminator is attached
> to either an internal port or the external port?)

Yup.  The signal will be pretty crappy if termination is not correct and
data packets will get corrupted....

> Is it supported/proper to dangle two disks from a single port
> internally?

Nope, not at all, never.  (though it might "work" once in a blue moon)

> I'm seeing a lot of odd behavior which changes as I move disks/cables
> around, which is making the driver *extremely* difficult to debug.

You may have one of the drives terminated and shouldn't, or you may have
a bad cable or connector somewhere.  Try correcting any termination
issues, make sure everything is connected properly and securely, inspect
all cables and connectors for visible damage (replace if necessary), and
then remove one device at a time until things "work", and replace/repair
or re-configure the offending device and/or its connecting cable.

-- 
							Greg A. Woods

+1 416 218-0098      VE3TCP      <gwoods at acm.org>      <robohack!woods>
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