DH11 asynchronous serial line interface

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The DH11 asynchronous serial line interface is a UNIBUS device which supports up to 16 asynchronous serial lines; as a UNIBUS device, it was usable on the PDP-11 and VAX computers. Although it first appeared very early on in the lifetime of the PDP-11, it was extremely powerful and flexible.

The parameters for each line:

  • separate input and output baud rates (50 to 9600)
  • character length (5-8 bits)
  • stop bits (1, 1-1/2 and 2)
  • parity (odd, even, none)

could be set dynamically (unlike, for example, the DJ11, which set parameters for groups of lines using jumpers). Output used DMA (with each line having its own buffer pointer and count); on input a 64-character FIFO buffer made over-runs unlikely. A 'break' condition on the line (i.e. continuous assertion) could also be generated and detected.

In the initial version, modular 'line conditioning' units allowed support of either 20mA or EIA RS-232 serial lines.

For modem control, a single DM11-BB per DH11, mounted in the main DH11 backplane along with the rest of the DH11 cards, was required. (The DM11-BB is logically a separate device, albeit one housed in, and only used with, the DH11.)

Initial implementation

The original implementation took an entire double system unit, containing multiple cards (see list below).

It came in 'earlier' and 'later' versions. In the 'early' versions, the line conditioning units (to adapt from TTL to EIA, 20mA, etc) were installed in a separate rack-mounted dual-height backplane (which required its own independent power supply). In the 'later' units, line conditioning was performed by boards mounted in the main DH11 backplane, and a rack-mounted passive distribution panel provided connectors for individual lines.

For the 'early' versions, the available line conditioning units included the DM11-DA (20mA line conditioning) and the DM11-DA (EIA, no modem conrol). Each supported four lines; thus, line conditioning could be mixed in groups of four. The 'early' version of the DM11-BB connected to four DM11-DC's.

The 'later' versions of the DH11 only supported EIA-type line conditioning (with or without modem control); to add modem control, a different implementation of the DM11-BB was required (one which did the EIA level conversion internally).


The 'early' versions of the DH11 were:

  • DH11-AA - Basic unit (110VAC), requires DM11-D's in addition
  • DH11-AB - Telegraph line version (see below)
  • DH11-AC - Basic unit (220VAC), requires DM11-D's in addition

The 'later' versions were:

  • DH11-AD - EIA level conversion, modem control
  • DH11-AE - EIA level conversion, no modem control

For the telegraph line option, the line conditioning backplane and its power supply were omitted, and up to two DH11's were connected to a DC08CS Telegraph Converter Panel.

Main Boards

The 'basic' DH11 included the following boards:

  • M7277 - Transmit scanner and NPR
  • M7278 - Registers and byte count
  • M7279 - FIFO buffer
  • M7280 (2) - Multiple UARTs
  • M7288 - Line parameters control
  • M7289 - Receiver scanner
  • M4540 - Crystal clock
  • M796 - Unibus master control
  • M7821 (2) - Interrupt control (transmit and receive)

The 'early' DM11-BB used:

  • M7246 - Control Scan
  • M7247 (2) - Control Mux
  • M105 - Address selector
  • M7821 - Interrupt control

The 'later' versions of the DH11 both included:

  • M5906 EIA conversion module

which occupied a dual slot (used by an M7360 Priority Selector card and an M971 Cable Connector card, used to connect the DH11 to the line conditioning units, in the 'early' versions).

The -AD version, which included a 'later' DM11-BB for modem control, also used:

  • M7807 - Mux and Bus Control
  • M7808 - Mux and Modem Scan Control

These boards fitted into the same slots occupied by the DM11-BB boards in the 'early' versions.

Finally, an optional M794 maintenance card could be plugged into the slot normally used by the M971, for testing.

Line conditioning boards

The line conditioning units, mounted in the separate backplane used in the 'early' versions, included, in the DM11-DA:

  • M596 - TTL to 20mA Level Converter (4 channels each)
  • M973 - Mate-N-Lok header

and in the DM11-DB:

  • M594 - TTL to EIA Level Converter (4 channels each)
  • W404 - DTR Jumper card

The DM11-DC used only the M594.

Distribution panels

The rack-mounted passive distribution panel initially used in the 'later' versions was the H317-B distribution panel, which mounted directly into a standard 19" rack, and contained 16 DB-25P connectors.

It was connected to the DH11 with a pair of BC08S cables, which carried the 'main' signals (data, etc - i.e. non-modem control); these cables were thus required for both 'later' versions. For the modem control signals (Carrier Detect, etc) used with the DH11-AD, four BC08R cables were used.

Later, the H3007 8-line EIA distribution panel, designed for use in later DEC cabinets with improved EMI characteristics, was introduced; the H3006 fits into the standard panel 21xm x 10xm cutout found in these systems.

Design flaw

Some time after it was introduced, it became apparent that there was a design flaw associated with the 'silo alarm' functionality. This mechanism allowed increased processing efficiency by allowing multiple input characters to be processed with a single interrupt; it involved a register (the 'Silo Alarm Level') which contained a count of the number of characters required in the input buffer (the 'silo') before an interrupt was generated.

The problem is that if less characters than that arrive, and then no more, an interrupt is never generated. So, code which uses higher levels for the silo alarm has to periodically check to see if less characters than that have arrived, and are waiting.

(Apparently, the designer admitted that this was a flaw; he indicated that there should have been a timer, such that after the first character entered the silo, if the silo was still non-empty after the timer ran out, an interrupt would be generated.)

Later implementations

The DH11 was later re-implemented as the DHU11, which was mostly programmatically identical, but fitted on a single hex-height card, which plugged into a SPC slot.

For the QBUS, the DHV11 and DHQ11 (again, programmatically identical) appeared; they only supported 8 lines (via two quad breakout panels) each, however.

External links