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 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 from the DF11 Communications Line Adapter series were installed in the distribution panel to allow support of EIA RS-232 serial lines connected to modems.

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 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) are:

• 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) are:

	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 are in the last two slots.)