View source for DC11 asynchronous serial line interface

The '''DC11 asynchronous serial line interface''' is an early [[UNIBUS]] [[peripheral]] which provides an [[asynchronous serial line]] connection to a [[modem]] (Bell 103 and 202 models), using an [[EIA RS-232 serial line interface|EIA RS-232]] interface. Both input and output used [[programmed I/O]] (with separate receive and transmit [[interrupt]]s); there was no [[buffer]]ing.

The line parameters:

* [[baud rate]]s (4 speeds, separate for transmit and receive, ranging from 50 to 1800, depending on the clock module )
* character length (5-8 bits)
* output stop bits (1 or 2) 
* input parity (even or odd)

could be set under program control. A 'break' condition on the line (i.e. continuous assertion) could also be generated and detected.

The speeds available with various models were:

{| border=1
! Model !! Speeds
|-
| -AA || 300, 150, 134.5, 110
|-
| -AB || 1800, 1200, 300, 110
|-
| -AC || 1200, 600, 150, 110
|-
| -AD || 150, 134.5, 110, 50
|-
| -AE || 150, 134.5, 110, 75
|-
| -AG || 1200, 300, 150, 134.5
|-
| -AH || 1800, 1200, 134.5, 110
|-
| -AX || Other (up to 19.6K), 150, 134.5, 110
|}

Other [[DF11 Communications Line Adapter]]s could be used in place of the EIA RS-232 interface card.

==Device registers==

{| border=1
! [[Register]] !! Abbreviation !! Address
|-
|Receive Status Register        || DCRCSR || 774000
|-
|Receive Buffer Regist          || DCRBUF || 774002
|-
|Transmit Status Register       || DCTCSR || 774004
|-
|Transmit Buffer Register       || DCTBUF || 774006
|}

The [[address]]es shown are for the first DC11 in a system; additional ones (up to 32 total) are normally set to be at 774010, 774020, etc to 774370.

==Implementation==

The implementation used a single custom [[system unit]] [[backplane]], containing two independent DC11's, each consisting of:

* M794 Interface ([[DEC card form factor|quad]])
* [[M782 Interrupt Control‎|M7821]] - Interrupt control
* [[M105 Address Selector|M105]] - Address selection
* M594 EIA level converter
* M957 data set cable connector

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 || M957 || M594 || M7821|| M105
|-
| 2 || G8000 filter || M454 clock || colspan="4" style="text-align:center;" | M794
|-
| 3 || Power|| @ || colspan="4" style="text-align:center;" | M794
|-
| 4 || colspan="2" style="text-align:center;" | UNIBUS Out || M957 || M594 || M7821|| M105
|}

@ = M205 for -AD, -AE; M405 for -AX; or blank

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

==Further reading==

* ''DC11 Asynchronous Line Interface Engineering Drawings'' - not online

==External links==

* [http://www.bitsavers.org/pdf/dec/unibus/DEC-11-HDCD-D_DC11_Asynchronous_Line_Interface_Sep73.pdf DC11 asynchronous line interface manual] (DEC-11-HDCD-D)

[[Category: UNIBUS Asynchronous Serial Interfaces]]