Advice on VAX peripherals wanted, note re HSC50

[Bill Shannon]

No, there is no 730 in the HSC50.  I believe there are several PDP-11's
and some special purpose processors just to move data.  One of the big
differences between the UDA and the HSC is that the UDA is attached
directly to a local bus (the Unibus), whereas the HSC is really
attached to a network (the CI).  Talking to the HSC is more like
writing a network protocol driver.  You'd like to use that nice, fast
CI bus to do other things besides just talk to your disk subsystem,
like use it as a very fast, very local network (which it is).

When I wrote the UDA driver I thought about trying to share some code
with a (future) HSC driver and decided that the two beasts were
different enough that it was not worth making the UDA suffer to fit
into the HSC scheme of things.  DEC actually has an "architecture" for
the interface between "class drivers" (e.g. a general DSA disk driver)
and "port drivers" (that actually handle getting the MSCP commands to
and from the device). At the time it seemed like a lot of excess
baggage (besides not fitting well into the UNIX kernel structure) but
as more MSCP devices come along it may be worthwhile.

Also, unlike something I read on the net, the shadowing feature of the
HSC is to allow you to maintain two disks with the same data invisibly
to the user, NOT write to tape.  I don't remember but there may be a
journally facility to log all transactions to tape.

All this is based on information I had 1.5 years ago.  Armando, correct
me if I remember wrong or things have changed.