VAX 11/750 rescued, alas...

J Blaser oldcpu2 at
Sun Oct 14 19:53:01 CDT 2007

[apologies for the long delay in responding.  A spontaneous road trip 
without internet connectivity came up...]

 Ethan Dicks wrote:
> I know I went over bits of this before, but perhaps a different
> approach might be clearer... Systems Industries made a variety of
> disk/tape systems for various DEC machines.  I have personally seen
> Qbus and 11/750 host cards, and I think there were others (Unibus, at
> least).  To order an SI system, you'd tell the salesman what box you
> had, and what devices you wanted to attach.  They would spec out the
> right host controller, and the right cards for the SI9900 external box
> to do what you wanted to do.  I think there was also support for
> multiple hosts to access multiple disks - not like a true cluster, but
> more like multi-port access between a set of CPUs and a set of disks
> and/or tape.

Okay, thanks for the further clarification, and sorry for being somewhat 
dense on this topic.  I've had no success at finding _any_ SI 9700/9900 
documentation, and since I have no prior VAX (or even DEC, except for 
playing with a few qbus -11s this past year) experience, I'm glad for 
the extra education.

So, I guess that means that without the companion gear this 9700 of mine 
is pretty much useless to me, at present anyway.

> In your case, it sounds like the SI 9900 box you would run across (if
> it's still on-site to be found), would have one board with a pair of
> 40-pin connectors that would hang off of your CPU, then one or more
> tape interfaces, with, presumably, Pertec-style interfaces.

Sadly, I'm pretty sure that this box is long gone...

> One of the benefits of the 11/750 over, say, the 11/730, is that the
> CMI bus (I think that's what it was called) had really high bandwidth
> path to memory.  You could hang a low-speed tape drive or two off of
> the Unibus, but for the free-standing 125 ips vacuum-column monsters,
> I just don't think the Unibus could keep the tape streaming.  On our
> 11/750, we had that SI 9700 for system disk and one data disk, a
> UDA-50/RA81 for user directories, and an RH750 and TU78 for backups.
> It was hell to keep working, but when the TU78 was up, it *screamed*
> through backups.

Ah, more education.  I don't know any of the specs of the CMI bus, but 
what you suggest makes a lot of sense, and would explain the presence of 
the SI 9700 board.

> For your system, since it's going to be easier to find Unibus disk
> than an SI 9900, I'd recommend de-installing the SI 9700 (which
> involves twiddling the backplane jumpers on that slot to pass grant
> across the slot), and picking up a UDA-50.

Yes, this is just what I intend to do...unless, of course, the other 
chunk of the SI 9700/9900 shows itself underneath that enormous pile 
that the donor still has to sort through.

All I need after getting the Unibus interfaces are a couple of gorillas 
to help me lift and mount those Super Eagles and the RA81 into some kind 
of rack (they're just stacked on the floor right now).  :-)

> That's one of the things _I_ would like to do with my 11/750 - mount
> an RA70 in the space to the left of the Unibus - where the
> battery-backup normally goes... stick a VT102 on top and have a
> single-cabinet 11/750 "workstation".  :-)
> I do already have an 3-bay 8300 set up, so I'd also probably run a
> cable over to my RA81 port B, and my MDA 2.3GB ESDI-SDI box, but I'd
> like to be able to boot the 11/750 in-cabinet, without spinning up
> several amps of external disk.

This would be a very cool configuration.  Space and power are sometimes 
in short supply, so these kinds of 'non-standard' solutions serve the 
purpose of keeping these oldies alive.  I might end up doing a similar 
thing.  Depends on how dim the lights get when I turn on the 11/750 
_and_ the Super Eagles and RA81!  ;-)

- Jared

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