DM11-BB Modem Control Option

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The DM11-BB Modem Control Option worked with several early UNIBUS asynchronous serial line peripherals to allow the lines to be connected to modems. It could be set to interrupt the CPU when a modem control line (Ring, Carrier, etc) changed state.

The DM11-BB is logically a separate device from the serial line interfaces, albeit one housed in the same backplane. It was used with the DM11, and early models of the DH11 interfaces.

Both of these used a custom system unit backplane, containing multiple cards; a flat cable connected this to separate 5-1/4" high rack-mounted dual-height DM11 'distribution panel' backplane. Modular 'line conditioning' units were installed in the distribution panel to allow support of EIA RS-232 serial lines connected to modems. (20mA units were also available for use with non-modem lines.)

Later DH11s used a new version of the DM11-BB, which was program-compatible with the early version, but included EIA level conversion, allowing the use of a passive distribution panel.

DM11-BB Device registers

Register Abbreviation Address
Control and Status Register DMCSR 770500
Line Status Register DMLSR 770502

The addresses shown are for the first DM11-BB in a system; additional ones (up to 16 total) are normally set to be at 770504, 770510, etc to 770674.

770500: Control and Status Register (DMCSR)

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

770502: Line Status Register (DMLSR)

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00


The early version of the DH11-BB was connected to the DM11 distribution panel via flat cables which carried the modem control signals, which ran from the M7247 card (two per card) to a single-width M971 which plugged into the backplane of the DM11 distribution panel. The DM11-BB boards (all dual-width) were:

  • M7246 - Scan
  • M7247 - Data Mux (two)

and 2 single-width, non-DM11-specific cards:

  • M7820 - Interrupt control
  • M105 - Address selection

DM11 board locations

Board locations in the DM11 backplane (as seen from the board insertion side of the backplane, not the wire-wrap pin side, as is common in DEC documentation) were:

Slot A B C D E F
1 UNIBUS In M7240
2 M105 M7241
3 Power M405 M782 M782 M7242
4 M974 Cable M7245
1   M7244
2   M7243
3 Power M7247 M7246
4 UNIBUS Out M7247 M105 M7820

(DM11-BB boards were in the last two slots.)

Early DH11 board locations

Slot A B C D E F
1 UNIBUS In M7247 M105 M7281
2 M7821 M796 M7247 M7246
3 M7278
4 M7277
5 M7289
6 M7821 M405 M7280
7 M7360 M971 M7280
8 M7288
9 UNIBUS Out M7279 M405 M4540

(DM11-BB boards were in the first two slots.)

Later DH11 boards

The 'later' DM11-BB used two quad-width boards:

  • 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' DH11 versions (the M7807 in slot 1, the M7808 in slot 2).