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 DH11 interfaces. Both of these interfaces used a custom system unit backplane, containing multiple cards; flat cables connected this to a separate 5-1/4" high rack-mounted dual-height distribution panel.

There were two different versions of the DM11-BB (unfortunately, there is no accepted naming system for them). The DM11 and early models of the DH11 used the 'early version' of the DM11-BB; it used a DM11 distribution panel backplane. Modular 'line conditioning' units were plugged into the distribution panel backplane to allow support of EIA RS-232 serial lines connected to modems.

Later DH11s used the 'new version' of the DM11-BB, which was program-compatible with the early version, but included built-in EIA level conversion, allowing the use of a passive H317-B 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 single-width M971 cards which plugged into the backplane of the DM11 distribution panel (one for each group of 4 lines). 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 DM11-BB implementation

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). Two flat cables ran from each to the passive distribution panel.