View source for 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 asynchronous serial line interface|DH11]], in that it used [[Direct Memory Access|DMA]].

However, unlike any other UNIBUS peripheral, much of its internal state was actually stored in [[main memory]], not in [[register]]s in the device itself, using DMA to gain access to it. (Apparently this was early enough in time that [[gate]]s for registers were too expensive.) 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 [[First-In First-Out buffer|FIFO buffer]])

(The 'input shift registers' are needed because no [[Asynchronous serial line#UART|UART]] [[integrated circuit|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]].)

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

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 [[DEC card form factor|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 current loop serial line interface|20mA]] or [[EIA RS-232 serial line interface|EIA RS-232]] serial lines. The alternative DC08CS distribution panel allowed connection to telegraph and Telex lines.

Lines could be connected to [[modem]]s, 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 rate]]s (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 [[jumper]]s; 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==

{| border=1
! Register !! Abbreviation !! Address
|-
|Status Register                || DMCSR || 775000
|-
|Buffer Active Register         || DMBAR || 775002
|-
|Break Status Register          || DMBSR || 775004
|-
|Base Address Register          || DMBADR || 775006
|}

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)===
{{16bit-header}}
| TINT || TO || OVR || TIE || colspan=4 | Unused || DONE || RIE || colspan=2 | MEMEX  || Unused || MAINT || FHD || RENB
{{16bitoctal-bitout}}

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.

==Memory tables==

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

{| border=1
! Table !! Offset (in [[byte]]s) !! 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, the last two for receiving. The format of the entries in the buffer is:
{{16bit-header}}
| Valid || Break || Parity || colspan="4" | Channel || Unused || colspan="8" style="text-align:center;" | Character
{{16bitoctal-bitout}}

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

==Implementation==

The DM11-AA boards (all [[DEC card form factor|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
* M7244 - Receiver

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

* [[M782 Interrupt Control‎|M782]] - Interrupt control (two)
* [[M105 Address Selector|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:

{| class="wikitable"
! !! colspan="6" | Connector
|-
! Slot !! A !! B !! C !! D !! E !! F
|-
| 1 || colspan="2" style="text-align:center;" | UNIBUS In || colspan="4" style="text-align:center;" | M7240 
|-
| 2 || || M105 || colspan="4" style="text-align:center;" | M7241
|-
| 3 || Power|| M405 || M782 || M782 || colspan="2" style="text-align:center;" | M7242
|-
| 4 || || M974 Cable || colspan="4" style="text-align:center;" | M7245
|-
| 1 || colspan="2" | &nbsp; || colspan="4" style="text-align:center;" | M7244 
|-
| 2 || colspan="2" | &nbsp; || colspan="4" style="text-align:center;" | M7243
|-
| 3 || Power ||  || colspan="4" style="text-align:center;" | Reserved for DM11-BB
|-
| 4 || colspan="2" style="text-align:center;" | UNIBUS Out || colspan="4" style="text-align:center;" | 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.'')

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

[[Category: UNIBUS Serial Interfaces]]