DEC asynchronous serial line pinout

DEC asynchronous serial line interface boards which supported one, or a small number of, lines all tended to use one of two physical interfaces: a 40 pin Berg connector .100" header (now often called DuPont or IDC connectors), and later, a 10 pin IDC header; both with a standard pin-out across all instances. Thus, all the interfaces of a given header type all took standard interface cables.

So, for the UNIBUS, the KL11 (M780), DL11 (M7800) and DL11-W (M7856) all used the 40-pin format, as did, on the QBUS, the DLV11 (M7940), DLV11-E (M8017), and DLV11-F (M8028).

On the QBUS, the DLV11-J (M8043), the KDF11-B (M8189, PDP-11/23+), and KDJ11-B (M8190, used in the PDP-11/83 and PDP-11/84), used the 10-pin format.

The 40-pin variant provided both EIA RS-232 and 20mA electrical interfaces (on different pins; EIA- and 20mA-specific cables connected to the appropriate pins). The 10-pin variant provided for both EIA RS-232 and RS-422 (differential pairs) (20mA was obsolescent by the time the cards which used this came out).

Note that the 10-pin variant does not actually include any modem control signals, so one could not connect many actual modems to it.

DEC interface cables for these cards (for the 40-pin header, the cable is the BC05C for EIA operation as a DTE, and the BC05M for 20mA current loop; for the 10-pin header, the BC20N is the EIA null modem DCE cable, and the BC21B is the EIA DTE cable) are now essentially un-obtainable, but they are relatively easy to make; this page provides the information needed to make new cables.

When making new cables, note that both of these interface types require a loop-back between a pair of pins on the Berg connector to operate (although for very different reasons, electrically). Producing a cable which only connects up Transmit, Receive, and Ground alone will not produce a functioning cable.

40 pin header

For the 40-pin Berg/DuPont male 'header' (on the board), when looking at the header face-on (from the side the connector shell on the cable will be plugged into), with the board component-side up, pin 'A' is in the lower left corner, with 'B' above it; they then follow the 'DEC alphabet' ('ABCDEFHJKLMNPRSTUVWXYZ', with G, I, O and Q dropped) for the first 19 pins, and then partially repeat for the next 18 pins, AA-VV.

When looking at a connector shell for the wire, from the side which goes into the header, the pins will be the mirror-image from that, from right to left; from behind the connector shell, where the wires come out, it will be the same as on the header.

The pins needed for an EIA 'simple' serial line cable (i.e. no modem signals needed), along with the connections for a 'normal' serial cable (i.e. DTE, male DB-25P or DE-9P connector, to plug into a modem), and for a 'null modem' cable (i.e. DCE, female DB-25S or DE-9S, to plug into a terminal) are:

Berg pin	Signal	DTE DB-25P pin	DCE DB-25S pin	DTE DE-9P pin	DCE DE-9S pin
A	Ground	1	1	1	1
B	Ground	7	7	7	7
E	Interlock IN
F	EIA Serial Output	2	3	3	2
J	EIA Serial Input	3	2	2	3
M	EIA Interlock OUT

Note that pin E must be connected to M for operation in EIA mode.

Note also that DB25 to DB9 adaptors are easily available at modest cost, so it might be desirable to make all PDP-11 cables with DB25 connectors, so that one can be easily interconnect different PDP-11's; a single DB25 to DB9 adaptor will suffice to connect any PDP-11 serial line to a PC.

40 pin modem control signals

If other signals are needed, here is a more complete table (for the full table of which signals are on which pins, consult the DL11 User's Manual, DEC-11-HDLAA-B-D); again for a 'normal' serial cable (i.e. DTE, male DB-25P or DE-9P connector, to plug into a modem), and for a 'null modem' cable (i.e. DCE, female DB-25S or DE-9S, to plug into a terminal) :

Berg pin	Signal	DTE DB-25P pin	DCE DB-25S pin	DTE DE-9P pin	DCE DE-9S pin
V	Request To Send	4	5	7	8
T	Clear To Send	5	4	8	7
DD	Data Terminal Ready	20	6	4	6
Z	Data Set Ready	6	20	6	4
BB	Data Carrier Detect	8	20	1	4
X	Ring Indicator	22	N/A	9	N/A

Note that since the EIA interface is not symmetric - there is no 'outbound' version of 'Data Carrier Detect' - in most null modems, as here, it is connected to 'Data Terminal Ready', to simulate a modem which is seeing carrier and is ready to accept/provide data.

10 pin header

For the 10-pin male pin IDC header (on the board), when looking at the header face-on (from the side the connector shell on the cable will be plugged into), with the board component-side up, pin '1' is in the upper right corner, with pin '2' immediately below it; pin '6' is missing (as an interlock), and pin '10' is in the lower left.

A complete pin-out for the interface is given below. It can be set up to do either differential, or single-ended; the usual RS-232 usage is to ground the '-' side of the received data, so it can run single-ended. Again, note that this interface does not include any modem control signals.

The table below also gives the connections for a 'normal' serial cable (i.e. DTE, male DB-25P connector, to plug into a 'modem'), and for a 'null modem' cable (i.e. DCE, female DB-25S, to plug into a terminal).

IDC pin	Signal	DTE DB-25P pin	DCE DB-25S pin	DTE DE-9P pin	DCE DE-9S pin
1	Clock input
2	Ground	7	7	5	5
3	Transmit +	2	3	3	2
4	Transmit -
5	Ground
6	Index (missing pin)
7	Receive -	*	*	*	*
8	Receive +	3	2	2	3
9	Ground	*	*	*	*
10	+12V

For normal RS-232 use, the two pins marked '*' (Ground, and Receive-) must always be connected to each other, as described above,

Again, note that DB25 to DB9 adaptors are easily available at modest cost, so it might be desirable to make all PDP-11 cables with DB25 connectors, so that one can be easily interconnect different PDP-11's; a single DB25 to DB9 adaptor will suffice to connect any PDP-11 serial line to a PC.

Note also that the 'Clock input' pin allows the stock KDF11-B CPU (11/23+) console cabinet kit to provide baud rate selection on the back panel (using the 'external clock' option on the card). However, an 11/23+ will work with a stock DLV11-J cable, or a hand-made cable, provided the board is jumpered correctly, to select the desired baud rate. Similarly, the 11/23+ cabinet kit can be used with a DLV11-J, where it will provide external baud rate selection - again, if the board is jumpered correctly (to use the external clock input).