DH11 history

[Dave Martindale]

In article <7155 at utzoo.UUCP> henry at utzoo.UUCP (Henry Spencer) writes:
>
>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.

Actually, a *modern* DH11 is just a 9-slot backplane, plus a
distribution panel with 16 DB25 connectors.  The *original* DH11 didn't
have any of the RS232 interface logic included in its backplane.
Instead, there were a couple of cables that ran TTL signals to another
backplane, of about 24 slots, with its *own* rack-mounted power
supply.  This backplane had 16 slots into which you plugged individual
level-converter cards, either current loop, RS232 data only, or RS232
with modem control.  The ultimate in flexibility (and expense).

But by the time the DZ11 came out, it must have been possible to bring
out a newly-redesigned DH11 that was much more competitive in cost.
When did the Able and Emulex DH emulators appear?  (True, they don't
handle *all* the modem control a real DH does, but they're sure better
than a DZ).