THE DEVELOPMENT OF THE DH11 MULTIPLEXER
John E. McNamara, former DEC employee and the designer of the DH11 multiplexer,
tells the story of the design of the DH11
This story is reproduced here with John E. McNamara's permission, from an
email correspondence with W. Ashley Carder dated March 12, 2005
Back in about 1972, the PDP11 communications group undertook two
simultaneous 16-line multiplexor projects, the DJ11 and the DH11. We
referred to the DJ11 as the "simpleplexor" and the DH11 as the
"sexyplexor." Both products used two identical line cards utilizing eight
of the recently-developed UART chips. Both products also used a "silo
buffer card" that stored received characters in first-in first-out buffer
chips, also recently developed. The PDP11 was notorious for long interrupt
entry and exit times, so the idea was that software would respond to an
interrupt and suck as many characters (along with their associated line
numbers) from the silo/fifo as possible on each interrupt service. The neat
thing was that the heavier the traffic was, the more characters the
software serviced per interrupt, so the efficiency actually improved with
load! As interrupt service times improved in later processors, this idea
fell into disfavor due to the context switching involved by having to sort
out the traffic from sixteen different lines.
There were four major difference between the DJ and the DH . The first was
that the line speeds
and formats in a DJ were configured by hard-wired jumpers, while they were
fully programmable in the DH. The second was that the DJ was EIA-only or
20ma-only, depending upon the "distribution panel" used. In contrast, the DH
(at least the -AA and -AC models) had a multi-slot distribution panel that
could accommodate a mix of EIA, 20 ma, modems, etc. The third difference
was that the DH had the option of full modem control, using a set of modules
called the DM-11-BB, a remnant of a previous multiplexor called the DM-11.
The fourth and final difference was that the DJ was program-interrupt on
both receive (the silo/fifo) and on transmit. While the DH11 was program
interrupt on receiver (the silo/fifo), the transmission was DMA.
One of the consequences of the DH's many features was that it took a 9-slot
"double system unit," whereas the DJ took only a single 4-slot "system
unit." It also took lots of power and generated lots of heat. Another
problem turned out to be cost. While the marketing folks suggested that I
should design a version with 8 speeds rather than 16, this would have meant
using 8-to-1 mux chips rather than 16-to-1 mux chips, saving maybe $5 in
manufactured costs. A little research determined that the really big
expense was the external distribution panel, which had its own power supply
and could accommodate any mix of level converter boards. By redesigning
some of the boards in the main product to include EIA level converters on
board and thus use the el-cheapo DJ distribution panel, we could cut the
manufactured cost by 40% and still offer all of the features of the DH
except the mix-and-match level converters. The new products were the
DH11-AD and -AE (with and without modem control,
All-in-all, the DH was a very successful product, and over
100,000 were produced. It was on the price list for ten years, as it was
used in PDP10's and some DECSystem20's in addition to PDP11's.
Within the next year or two, progress in semiconductors made the DZ11 possible. As
you know, this was a single board containing everything needed for 8 lines
with modem control. I do not remember whether the DZ11 has programmable
speeds and formats or not. I think it does.
Looking at the pictures of your 11/40, I'm not sure that you would want a DH,
as it would take a lot of space and power, especially the DH11-AA or -AC.
Emulex and Able made newer
technology, program-compatible versions, however. Able's product, the DH/DM
is a single hex board with limited modem control, but claims to have all the
other features. I've not seen one.
As for my other projects, the major one was the DV11, a 16-line synchronous
multiplexor. I also did some work on card readers.
I look forward to hearing of your future adventures!
I (Ashley) told John McNamara that I would try to get a DH11 up and running on one of my
systems since it was part of the original Wofford Witch, and he said,
"I would love to know that one of my "children" was alive someplace ;-)"
Now I must find a DH11 and get it up and running in a multi-terminal timesharing environment.