> I have been thinking of maximizing one of my
> 3100 boxes to 5x2gig scsi drives, internally,
> but am running into the problem of the internal
> scsi A bus cable only has a max of 3 connectors
> for scsi devices.  Was there ever a 5 connector
> A bus cable?  If not, what is a practical way to
> expand the cable to 5 connectors?

If you have a VAXstation 3100 M40 or M48, then I think
it is just a flat ribbon cable and you can make up 
your own. I've seen some of these boxes (while working
for DEC) with two disks "above" and at least
two devices below, so I assume that there is a
suitable cable available. (I cannot be sure
because all the ones I have are the smaller M3x
enclosure).

If you have a VAXstation 3100 M30 or M38, then 5 devices
will be a bit of a squeeze!

If you have a MicroVAX 3100 M10[e] (smaller enclosure)
or M20[e] (larger enclosue), I think the considerations
are similar.

If OTOH you have a VAXstation 3100 M76, then the internal
cable is "special" and is actually two cables one of which
servers the internal devices and the other connects
to the external connection. I've never needed to look
too closely at this, so I don't know how tricky it would
be to replace with some home-brew arrangement.

> The only way I can see easily to do it is to cut
> the cable, and, using 50 pin dip headers, patch
> in a section of ribbon cable into the loop that
> is sufficient to reach all 5 internal devices.

I'd be tempted to leave the existing cable untouched 
and just make your own ribbon cable plus connectors.
I've made them up in the past (without any test equipment
to tell me if they were within "spec" or not) and
they've worked perfectly well for me (on PCs rather than
VAXstations, but for this era I expect it's the same).

I assume that the correct ribbon cable and connectors
are available still (otherwise you cannot even splice
anything in!)

> My goal is to maximize the A bus internally, and
> maximize the B bus externally.  Why DEC did not
> do that, originally, is perplexing....

If you have one disk shelf inside, then you have the
smaller box (and DEC would not have managed more
than three internal devices ... usually two disks and 
one TZ30). If you have the taller one, I would have
expected the cable to accommodate the maximum 
number of devices. OTOH, maybe the 5 device cable
was something you had to buy and the 3 device one was
the default that shipped with all orders that did
not need any better.

Antonio

 
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Antonio Carlini             arcarlini@iee.org