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. Modular 'line conditioning' units allowed support of either 20mA or EIA RS-232 serial lines.

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, character length, stop bits, and parity) 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), and on input a 64-character FIFO buffer made over-runs unlikely.

Initial implementation

The original implementation took an entire double system unit. It came in 'earlier' and 'later' versions: in the 'early' versions the line conditioning units (to adapt from TTL to either 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 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. For modem control, a single DM11-BB per DH11 was required, which in turn connected to four DM11-DC's.

The 'later' versions only supported EIA-type line conditioning (with or without model control).


The 'early' versions of the DH11 were:

  • DH11-AA - Basic unit (110VAC), requires DM11's in addition
  • DH11-AB
  • DH11-AC - Basic unit (220VAC), requires DM11's in addition

The 'later' versions were:

  • DH11-AD - EIA level conversion, modem control, H317-B passive distribution panel
  • DH11-AE - EIA level conversion, no modem control, H317-B passive distribution panel


The 'basic' DH11 included the following boards:

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