DM11 Asynchronous 16-line Single-Speed Multiplexer

The DM11 asynchronous serial line interface was a very early UNIBUS peripheral which provided up to 16 asynchronous serial line connections. It was in some sense a predecessor to the DH11, in that it used DMA.

However, unlike any other UNIBUS peripheral, much of its internal state was actually stored in main memory, not in registers in the device itself (not even in ones not directly accessible from the UNIBUS), then DMA was used to gain access to it. (Apparently this was early enough in time that gates for flip-flops for registers were too expensive, or took too much space on the board.) The data stored in memory included:

• per-line current output buffer address
• per-line current output transfer length
• per-line input shift registers
• input buffer (a circular FIFO buffer)

(The 'input shift registers' are needed because no UART chips were used; the DM11 pre-dates their creation. The sampling of the incoming serial data stream, and the conversion to parallel form, was done in discrete logic.)

There were separate receive and transmit interrupts, and half-duplex and full-duplex operation are supported in hardware. The 64-character FIFO buffer made input over-runs unlikely.

The implementation of the basic DM11-AA used a custom double system unit backplane, containing multiple cards; a flat cable connected this to separate rack-mounted dual-height DM11 distribution panel backplane (which required its own independent power supply).

Modular 'line conditioning' units from the DF11 Communications Line Adapter series were installed in the distribution panel to allow support of either 20mA or EIA RS-232 serial lines. The alternative DC08CS distribution panel allowed connection to telegraph and Telex lines.

Lines could be connected to modems, provided the correct line conditioning units were installed, but modem control required a DM11-BB Modem Control Option, mounted in the main DM11 backplane along with the rest of the DM11 cards, and connected to the distribution panel via other flat cables which carried the modem control signals. The DM11-BB is logically a separate device from the DM11-AA, albeit one housed in the same DM11 backplane.

The line parameters:

• baud rates (ranging from 45 to 1200)
• character length (5-8 bits)
• output stop bits (1 or 2)

were the same for all lines, and set by jumpers; parity was computed on incoming data. A 'break' condition on the line (i.e. continuous assertion) could also be generated and detected.

DM11-AA Device registers

Register	Abbreviation	Address
Status Register	DMCSR	775000
Buffer Active Register	DMBAR	775002
Break Status Register	DMBSR	775004
Base Address Register	DMBADR	775006

The Buffer Active Register has one bit per line to indicate that has output pending (described by the Current Address and Word Count tables below); the Break Status Register also has one bit per line. The Base Address Register is 8 bits wide, so the in-memory tables must start on a 0400 boundary.

The addresses shown are for the first DM11-AA11 in a system; additional ones (up to 16 total) are normally set to be at 775010, 775020, etc to 775170.

775000: Status Register (DMCSR)

TINT	TO	SOVR	TIE	Unused				DONE	RIE	MEMEX		Unused	MAINT	FHD	RENB
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

• TINT - Transmit Interrupt
• TO - Timeout; non-existent memory referenced
• SOVR - System Overrun; bus access not gained in time
• TIE - Transmit Interrupt Enable
• DONE - Done; input character placed in the Circular Buffer
• RIE - Receiver Interrupt Enable
• MEMEX - Memory Extension
• MAINT - Maintenance Bit; connects line 0 (only) receiver directly to transmitter
• FHD - Full/Half Duplex Select; disables receivers on lines that are transmitting
• RENB - Receiver Enable

Memory tables

The four tables of per-line data kept in main memory were:

Table	Offset (in bytes)	Size (in bytes)
Current Address	0	32
Word Count	040	32
Bit Assembly	0100	32
Circular Buffer	0200	128

The first two are for transmitting, and defined the output buffer; the last two for receiving. The format of the entries in the input buffer is:

Valid	Break	Parity	Channel				Unused	Character
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

The input character, which may be less than 8 bits wide, is stored right-justified.

Implementation

The DM11-AA boards (all quad, except for the Control C, which is a dual) which plugged into the DM11 backplane are:

• M7240 - Control A
• M7241 - Control B
• M7242 - Control C
• M7243 - Transmitter D
• M7244 - Transmitter E
• M7245 - Receiver

and 4 single-width, non-DM11-specific cards:

• M782 - Interrupt control (two)
• M105 - Address selection
• M405 - Clock

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:

	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		Reserved for DM11-BB
4	UNIBUS Out		Reserved for DM11-BB

(NOTE: The document which is the source for this table has two different cards marked as 'M7245', and no 'M7243's. So it clearly has errors, but it's not obvious what they all are; the M7243 here is a guess.)

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

Further reading

• pdp11 peripherals and interfacing handbook, 1972 edition, pp. 131-150
• DEC-11-HDMBA-A-D DM11-BB modem control option manual, pg. 1-5