DM11-BB Modem Control Option

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 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 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.

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)

RING	CARR	CTS	SREC	CLRSC	CLRMUX	MAINT	STEP	DONE	IE	SCEN	Unused	Line
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

770502: Line Status Register (DMLSR)

Unused								RING	CARR	CTS	SREC	STX	RTS	DTR	LEN
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

Implementation

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:

	Connector
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

	Connector
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).