Notes: DEC DL11 Digital Equipment Logo

The DL11 is the basic serial line interface for the Unibus PDP-11. When addressed at 777560 it is the console. The typical Unibus implementation is the M7800 quad height SPC module. The module supports several DL11 versions, -A through -E. The differences were in the cables and the crystal supplied. The module can supply asynchronous serial signals in 3 different types: TTL, EIA (now generally called RS-232), and 20mA. The cable determines the signal type used. Four different crystals were available resulting in four Speed Groups. For modern usage, Groups 3 and 4 are the most useful.

Two rotary switches and the installed crystal determine the baud rate. The transmit and receive rates can be different and are determined by the setting two 8 position switches. The switches are marked as R and T on the board for obvious reasons.

The M7800 I have as a spare has the 4.608Mhz crystal.

Speed Group 1 2 3 4
Crystal Freq. (Hz) 844.8k 1.03296M 1.152M 4.608M
S1, S2 POS. (T, R) Baud Rate
1 36.7 44.8 50 200
2 55 67.3 75 300
3 110 134.5 150 600
4 220 269 300 1200
5 440 538 600 2400
6 880 1076 1200 4800
7 1320 1614 1800 7200
8 1760 2152 2400 9600
Baud rate table.

Berg Pin Description Direction Type Active/
A 40 Ground passive
B 39 Ground passive
C 38 Force Busy Out EIA configurable Active or passive based on jumper settings. J2, J4
D 37 n/c
E 36 Serial Input In TTL active TTL serial in, must be looped in berg, H or M.
F 35 Serial Output Out EIA active RS-232 output.
H 34 20mA Interlock Out TTL active Clamped version of K. Loop to E for  20mA.
J 33 Serial Input In EIA active RS-232 input.
K 32 Serial Input + In 20mA active Diode clamps to +5 and  0.
L 31 n/c
M 30 EIA Interlock Out TTL active TTL version of J. Loop to E for RS-232.
N 29 n/c
P 28 n/c
R 27 n/c
S 26 Serial Input - 20mA passive Pulldown to -15.
T 25 Clear to Send In EIA active
U 24 n/c
V 23 Request to Send Out EIA configurable Active or passive based on jumper settings. J1, J4
W 22 n/c
X 21 Ring In EIA active
Y 20 n/c
Z 19 n/c
AA 18 Serial Output + Out 20mA active Serial data source.
BB 17 Carrier In EIA active
CC 16 Clock Input In TTL active Selected by HH.
DD 15 Data Terminal Ready Out EIA active
EE 14 Reader Run - Out 20mA passive Reader relay sink, 68ohms to  -15.
FF 13 Secondary Xmit Out EIA active Uncommitted control signal.
HH 12 Clock Enable In TTL active Low selects external clock from CC
JJ 11 Secondary Receive In EIA active Uncommitted control signal.
KK 10 Serial Output - Out 20mA passive Serial data sink to -15.
LL 9 n/c
MM 8 n/c
NN 7 n/c
PP 6 Reader Run + Out 20mA active Reader relay source.
RR 5 n/c
SS 4 Serial Output Out TTL active Raw TTL gate output.
TT 3 +5V Out passive
UU 2 Ground passive
VV 1 Ground passive
Berg connector table.

The console cable I have is a BC05-C from cursory examination.


There is also a KL11 that was used as the console. It was 20mA only and consisted of 3 modules: M780, M105, and M7821. I have a hacked version of this, probably to adapt it for RS-232.

The M780 should be in rows C and D, the M105 in row E, and the M7821 in row F.

I have since rebuilt the KL11 to original and used it with an ASR33 as the console. The bit rate generators are analog clocks controlled by pots. The output clock can be adjusted by putting the processor in a loop outputing characters. The input clock can be adjusted by disconnecting the input cable. This sets the input to a start bit and the receiver will clock continuously.

M105 Jumpers

The console terminal is at 17 777 560. The M105 provides jumpers for bits 12 thru 3.  The 18 bit console address in binary is 111 111 111 101 110 000. So the jumpers on the board should be OOO OJO OOJ.

KL11 as a PC11

The KL11 (and DL11) have the same basic CSR layout as the PC11. For the purposes of the bootstrap, they are the same. I have moved my KL11 to 17 777 550 and use it as PR in the bootstrap.


The console DL11 was configured for 8-N-1. This was OK until I started using some diagnostics that set bit 7. This was not compatible with the decserver I was using for my serial interface. I changed the DL11 settings to 7-E-1.

Name Jumper State Description
No Parity NP IN Enable parity bit transmission and reception by installing a jumper.
Even Parity EPS OUT Remove jumper for even parity.
STOP Bit 2SB OUT Used with J9, J10, J11. 9 out, 10 in, 11 out, 2SB out -> 2 stop bits.
Data Bits NB1 IN 7 data bits.