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PDP11/05 : the BERG40 external serial connector is weird


New Member
Jun 3, 2018
Hello PDP masters,
I recently acquired a PDP 11/05 computer, yaaaay.
It's the low-profile one.
It is complete and it runs (after some power supply repairs), but has no peripherals.
So I want to use an M9312 "serial console monitor" card (which I also bought) to interface with it over serial interface, from a PC.

My 1st hurdle is hooking up a USB-to-TTL converter (such as the one frequently used on Arduino Uno boards) to the BERG 40-pin uart/serial/whateveryoucallit connector on the back of the PDP.
I found the nice instructions at https://www.retrocmp.com/how-tos/in...interfacing-with-a-pdp-1105-sorting-the-wires
1. I don't know which pin on my connector is "A". The article above is super confusing, talking about reversed pins, upside pins, etc. They could just tell me where my pin A is :(
2. The "BB" pin should measure -15V. It's the 12th pin on one of the rows. I measured the voltage on the 12th pin from any direction (top row left to right, right to left, then bottom row left to right, right to left, and neither of them measures -15V. So I am quite stuck at this point.

Here are the voltages I measured (in volts) after turning on the PDP:
Top row: 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,5,0,
Bottom row: 0,0,5,0,5,0,0,-15,15,0,0,4,5,-15,5,-15,0,0,3,0

I will appreciate if someone can tell me: do I have a non-standard BERG conector (given the weird voltages above), and which pin is "A"?
Thank you!

Below I am attaching a picture of the BERG slot , with an empty connector plugged into it (so that the pins are more visible).


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Congrats on your new acquisition! I also found the console connector confusing when I first got my '05. To help me wrap my head around it I put together the following doc which shows the connector pin numbering and signal assignments, as seen from the back of the system.


Not sure why you're not seeing -15V on BB/24. Doubt its a power supply issue, as the -15V rail is certainly needed for proper operation of other parts of the system (core in particular).

Also, since you're a fellow 11/05 owner, I'll take this opportunity to plug my PDP-11/05 Console USB Adapter project. It provides both a USB and RS-232 interface for the 11/05 console and makes it possible to run the system at baud rates above 2400.
Another PDP-11/05 serial adapter is described on Jay Logue’s github. His design and documentation is very complete. For example, the pin out you are looking for is:

He uses a Teensy v3.2 board that is no longer being produced but you can find them on ebay or he may have some stashed away. He recently updated the github to provide both a USB and serial connection that can work simultaneously to connect to the 11/05’s 40 pib BERG connector.

Quite a while back I did a 40p flat cable serial connector (as used on the PDP-11 DL11-W and PDP-8 KL8-x serial boards).
So I just run a short piece of 40p flat cable to this PCB which has a 40p IDE cable socket and a DB9 serial connector socket and some jumpers for signals in between.
The layout looks like this. The jumpers allow for using either a male or female DB9 connector as you desire.
The PCB design is shared on OSHpark, it is a simple 2 layer design. Search for 'BERG40toDB9x' in the shared projects area.
Beats having to track down an old DEC BC01V (iirc) cable and a 25p to 9p adapter.
Thank you all for the prompt replies!

Based on the doc at https://github.com/jaylogue/pdp-1105-console-usb-adapter/blob/master/docs/pdp-1105-scl-connector.pdf , pin A is in the upper right corner of the BERG connector (when looking at the back of the PDP).
Great, this is exactly what I was looking for.

For my other question about the voltage on the BB pin, it's still a mistry:
The BB pin is the 9th from the left on the bottom row. I am measuring +15V, but it should be -15V based on all available doccumentation.
It's not that I need to use that voltage; I am just wondering if my BERG connector is wired properly.
I will just assume that the Tx and Rx pins are properly wired, and I will give it a try, hoping there will be no thick smoke.

Jay, I love your Pdp Usb Adaptor project, I am going to read on it tomorrow. Before finding out about it, I was already looking on AliExpress for simple agenerator boards that could speed up the baud rate; but you have solved this nicely, and more.

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