Arcane modem configuration

[Henry Spencer]

> Curiously enough, the DZ11 is the newer device, not the older.  The DH11 (or
> actually, its associated DM11) provided modem control interrupts.  I'm told
> that RSX-11 was better at polling than responding to interrupts, so I suspect
> that the DZ was designed around its limitations.

My understanding is slightly different, based on some DEC documentation
of the time.  The problem was not that DHs were too fancy for RSX, but
that DHs were too expensive and too bulky for a lot of people who wanted
terminal multiplexors.  A Dec DH was a 9-slot backplane full of boards,
and cost accordingly.  The point of the DZ was to capture the minimum
necessary features in a much cheaper implementation.  Well, they sort
of succeeded.  No DMA, for cost reasons, and if you read the documents
(at least the early ones) they cautioned you that the total expected
throughput of a DZ was about 9600 baud -- that's *total*, not per line.
No split speeds, because virtually none of the DH customers ever used
the DH split-speed capability.  Minimal modem control, because almost
none of the DH customers used the extra modem-control lines.  Only 8
lines, because that's what fit on a board.  I don't think they justified
the polled modem control explicitly, I think it was just a question of
not being able to fit a scanner onto the board or into the (cramped)
Unibus address space they gave themselves.  The Dec DZ board was pretty
full, and the Unibus registers were already doubled up most annoyingly
(i.e. a read-only register and a write-only register with different
roles at the same address).
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,decvax,pyramid}!utzoo!henry