3D printed Polish Enigma machine plug board wiring.

November 30th, 2017

Someone asked me to explain how I will use the sockets I bought to implement the plugboard on my Polish Enigma double. This film should explain. Sorry it gets blurry from time to time!

To understand how my plugs work you have to understand how the wiring of Enigma works. It’s really quite simple although the number of wires involved makes it look complicated!

To start with consider the three rotors and the reflector as a single thing, I call it a scrambler. That terminology comes from the Turing Welchman Bombe. I talk about that a lot here.

The scrambler can be thought of as having 26 contacts, one for each letter of the alphabet. If you put a voltage on one letter a voltage will come out on a different letter. Which one depends on the the reflector, the rotors, their order, their position, etc but the important thing is for a voltage in you will get a voltage out on a different contact. That was one of the quirks of the Enigma, a letter can never encode to itself. The reason is the reflector. Each pin on the reflector is paired to another pin, 26 letters means 13 pairs. And a letter can’t pair to itself.

The scrambler connects to the entry wheel inside the machine. The entry wheel has 26 contacts, A-Z. This was one thing the Polish worked out that the British did not. The entry wheel letters could have been wired up in any order which would have made it much more difficult for someone trying to reverse engineer a machine without actually having one to examine (and the Polish didn’t). The commercial machines used a different wiring (based on the German QWERTZ keyboard layout) and when the Wehrmacht took over the machine for some reason the order they used was alphabetical. Marian Rejewski figured this out and this was one of the important pieces of information the Polish mathematicians shared with the British from Bletchley Park.

The entry wheel connects to the plug board and the plug board has 26 in contacts and 26 out contacts. I call them in and out but really they go both directions. When no plugs are inserted the plugboard does nothing, the A in goes directly to the A out and then to the A on the entry wheel.

Each key in the keyboard has three contacts, a common, a NC (normally closed) and an NO (normally open). The common connects to the entry wheel (through the plug board). The NC contact connects to it’s corresponding lamp. The other side of the lamp goes to one side of the battery. The other side of the battery goes to the NO contact. Also note I don’t mention positive or negative with the power supply. Because it is all just simple switches and lamps the polarity doesn’t actually matter. The battery/power supply can go either way around. This is one advantage over using LEDs for the lights as LEDs, being light emitting diodes, only work one way around!

When a key is pressed it’s lamp is disconnected and the switch connects to the battery instead. This is another reason (or result rather) of one letter never encoding to itself. With one end of the lamp now disconnected it can’t light up. The voltage (current really but voltage is easier for non electrical people to understand) then goes to the entry wheel, via the plug board, through the scramblers and back out on another letter on the entry wheel. Then through the plugboard again to another switch which isn’t pressed. Since that switch isn’t pressed it goes to it’s matching lamp lighting it up.

The plug board simply swaps pairs of letters around, both on going into the entry wheel and coming out. I am a tester by profession so I think in terms of test cases and minimal ways to test things. This table shows the different types of cases possible.  Remember the scrambler can never turn a letter into itself.


Below are some pictures of the actual plugs and sockets I am using. They are stereo sockets but I am using mono plugs. It just means one set of contacts won’t be used.

IMG_1012 IMG_1013 IMG_1014

The sockets are switching contacts. With no plug inserted the little metal bridges short the two pins for each contact together. When the plug is inserted one set of pins is disconnected and the plug connected instead.

The two contacts on the plug are called the tip and the sleeve. Each cable for the plug board (there are 10 of them) has the tip on one end connected to the sleeve on the other and vice versa. That is how the letters get swapped. On the socket the switched tip and sleeve contacts will simply be wired together.

My diagram possibly makes it clearer.


Another good description of the Enigma wiring is shown here at www.cryptomuseum.com.

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