DC11 asynchronous serial line interface

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The DC11 asynchronous serial line interface is an early UNIBUS peripheral which provides an asynchronous serial line connection to a modem (Bell 103 and 202 models), using an EIA RS-232 interface. Both input and output used programmed I/O (with separate receive and transmit interrupts); there was no buffering.

The line parameters:

  • baud rates (4 speeds, separate for transmit and receive, ranging from 50 to 1800, depending on the clock module )
  • character length (5-8 bits)
  • output stop bits (1 or 2)
  • input parity (even or odd)

could be set under program control. A 'break' condition on the line (i.e. continuous assertion) could also be generated and detected.

The speeds available with various models were:

Model Speeds
-AA 300, 150, 134.5, 110
-AB 1800, 1200, 300, 110
-AC 1200, 600, 150, 110
-AD 150, 134.5, 110, 50
-AE 150, 134.5, 110, 75
-AG 1200, 300, 150, 134.5
-AH 1800, 1200, 134.5, 110
-AX Other (up to 19.6K), 150, 134.5, 110

Other DF11 Communications Line Adapters could be used in place of the EIA RS-232 interface card.

Device registers

Register Abbreviation Address
Receive Status Register DCRCSR 774000
Receive Buffer Regist DCRBUF 774002
Transmit Status Register DCTCSR 774004
Transmit Buffer Register DCTBUF 774006

The addresses shown are for the first DC11 in a system; additional ones (up to 32 total) are normally set to be at 774010, 774020, etc to 774370.


The implementation used a single custom system unit backplane, containing two independent DC11's, each consisting of:

  • M794 Interface (quad)
  • M7821 - Interrupt control
  • M105 - Address selection
  • M594 EIA level converter
  • M957 data set cable connector

Board locations (as seen from the board insertion side of the backplane, not the wire-wrap pin side, as is common in DEC documentation) are:

Slot A B C D E F
1 UNIBUS In M957 M594 M7821 M105
2 G8000 filter M454 clock M794
3 Power @ M794
4 UNIBUS Out M957 M594 M7821 M105

@ = M205 for -AD, -AE; M405 for -AX; or blank

Power comes in on a single-width stub card in the A3 slot (as is canonical in the PDP-11/20 generation of PDP-11s).