Interfacing with a PDP-11/05: Sorting the wires

Signal routing for the serial interface inside the PDP-11/05 case

As an experienced DEC hobbyist, you will expect a 40 pin BERG connector to plug the standard serial cable KL8E in.

The KL8E cable is used on PDP-8's and 11's as a link between on board logic and 20mA / RS232 interface plugs. It comes in many length and adapter styles, and carries a lot of serial signals.  See two examples:

Intern 1' KL8E cable with 20mA MATE-N-LOCK connector	Extern 5' KL8E cable with DSUB25 male connector carrying RS232 and modem signals

So you quickly register that the Datapath M7260 indeed contains a 40 pin BERG connector in the near of the UART.

But don't waste time: this is the interface to the blinkenlight console. There is NO connector for the serial interface, the serial signals are routed over the edge connector of M7260 onto the backplane.

The further signal routing depends on the exact 11/05 model:

• In the full size PDP-11/05 case, you have to plug a M9970 adapter board into the next backplane slot. This is simply a passive adapter between flip chip slots and an on-board 40 pin BERG connector.
  You plug the KL8E cable into the M9970 then.
• in the slim size 11/05, theres a plug on the backplane which connects the serial signals to a 40pin BERG on the back side of the BA11 case.

The SCL connector of the slim PDP-11/05

The 40 pin male BERG connector on the backside is called "SCL connector", "SCL" standing for "Serial Communication Line"

In the "PDP-11/05, 11/10 Computer Manual" (Jan 1973, DEC-11-H05AA-A-D) on bitsavers.org it is barely visible due to contrast problems in the scan.
And on a real machine you would not expect an unsecured BERG connector on the outside of a BA case carrying the serial signals, so here is the location in detail:

The pinout for the SCL connector is documented in manual "DEC-11-H05AA-A-D" in Appendix C "Computer Connectors".

Documentation for the SCL connector is very confusing:

• a usual, the wires in the 40 pin flat cable are not numbered, but labled with letter combinations from the "DEC alphabet". The DEC alphabet cleverly uses only letters which can not be confused with other ones... there is no "G", "I", "O" or "Q".
• the SCL connector is mounted up side down, but you have to connect the KL8E cable with its "this side up" label still facing upward. So the KL8E cable has to be plugged in rotated by 180 degrees. The flat cable's red #1 wire must be oriented to the left. On my 11/05, I marked this side with an "A B" label.

• Decoding the DEC alphabet

I got half crazy until I made this conversion chart between DEC pin labels and flat cable wire numbers ... it should be pinned onto every PDP-11 !

And since the SCL is mounted up side down, the same again rotated by 180°:

The big signal conversion table 

So getting from a documented SCL connector pin to a 40 pin flat cable number involves looking up the rotated DEC label of the wire and then converting it to a normal number ... it is a bit like deciphering a cryptographic code.

This is the final translation table. It includes also the connectors of the RS232-to-TTL level converter and the MATE-N-LOCK connector for 20mA TTYs.

Of course you should not trust me: check this table again and again!

 ¹ "20mA Interlock" has to be connected to "SERIAL IN (TTL)" in the cable, if using the 20mA interface.
 ²  Inverted, but labled "SER IN H" in the DEC schematics.