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Enigma G111
A rare version of Zählwerk Enigma G31
By Paul Reuvers & Marc Simons

Enigma G111 is an Enigma cipher machine model G31, also known as a Zählwerksmaschine (counter machine) or Enigma G, developed around 1931 by Chiffriermaschinen AG (ChiMaAG) in Berlin (Germany) and sold to the Hungarian Army. It is a rare variant of the Enigma G which features a 28-pin socket for connection of a printing device (which was usually an Enigma H29).

In May 2009, the G111 turned up at aution House Hermann Historica in München (Munich, Germany). The machine was offered for sale and was scheduled for auction in October 2009. Its condition was not too well. On 1 July 2009, on a visit to the acution house, Crypto Museum was given the opportunity to take a closer look.

As far as we know, it was the first time that this version of the Enigma G was investigated. Our findings are presented here, and in more detail in our paper G111: A rare version of Zählwerk Enigma G31, which can be downloaded here [A].

We would like to thank Frode Weierud in Switzerland (now: Norway) for his help in sorting out the history of the G111 and the family of machines it belongs to. His research in the archives has proven to be invaluable, and has prompted the Enigma history to be rewritten more than once [6].

 Download the full paper

— In February 2018 we received a letter from the original owner of the G111, who has meanwhile restored the machine to operational condition. He has send us a complete description of his restoration work, along with photographs and a few corrections to our G111 report of August 2009. We will process these remarks and release an updated version of the G-111 report in due course. Watch this space...
The socket at the left rear of the machine
The socket at the left rear of the machine
The wooden box
The leather carrying strap
Left perspective view of the machine
Close-up of the lamp panel
Moving reflector
The three coding wheels
1 / 8
The socket at the left rear of the machine
2 / 8
The socket at the left rear of the machine
3 / 8
The wooden box
4 / 8
The leather carrying strap
5 / 8
Left perspective view of the machine
6 / 8
Close-up of the lamp panel
7 / 8
Moving reflector
8 / 8
The three coding wheels

Wooden case
The case of the G111 is nearly identical to the case of the standard Enigma G31. Its dimensions are 25 × 27 × 16.5 cm. The only difference with the standard case is the presence of a hole towards the rear of the left side behind which a 28-pin socket is located. This socket, which is visible in the image above, allows an external printing device to be connected to the Enigma G.

The table below shows the wiring of the rotors of the G111, the entry disc (Eintrittswalze, ETW) and the reflector (Umkehrwalze, UKW). The column 'Turnover' shows which letter is visible in the window when the rotor causes a turnover of the rotor to its left. Please note that these positions are different from the actual position of the notches on the circumference of the rotor (given in the column 'Notches'). Also note that only rotors I, II and V were found with this machine.

IV ? ? ? ?  

As becomes clear from the above table, the number of notches as well as the turnover positions of rotors I and II are identical to those on the same rotors of other Zählwerk machines (17 and 15 notches respectively). We therefore assume that rotor III had 11 notches that are at the same positions (here shown in red) as on rotor III of (all) other Enigma G machines. So far, rotor IV hasn't been found, so we don't know the number and positions of the notches on that wheel.

  1. Wired as per standard commercial enigma.

Printer socket
A inique feature of this particular Zählwerk Enigma, is the presence of a socket at the left side, close to the UKW. So far, this feature has not been found on any other Zählwerk Enigma before. The expansion is described in patents DE536556 [3] and DE595075 [4]. It allows another – much larger – printing Enigma (such as the Enigma H29) to be connected and used as a printing device. It is believed that the printer connection was an option that was also available for the Enigma I.

When used in this way, the Enigma H (shown in the picture on the right) would only function as a printing machine; its cipher capabilities were disabled. Connecting a printer allowed a cipher clerk to decipher any incoming messages much faster then when reading-off the lamps and writing down the letters. He could not use the Enigma H directly, as it wasn't compatible with the Zählwerk Enigma or the Enigma I.

On the G-111, the socket for connection with the Enigma H, is located at the lower left side of the machine. A hole of approx. 50 mm is present in the lower part of the metal body. It gives access to the socket which is mounted behind the hole. The hole can be closed with a metal cover.

When the machine is mounted inside the protective wooden box, a circular hole in the side of the box provides access to the socket. The socket is actually mounted slightly off-centre, but the hole in the metal body is large enough to accommodate the connector.

To the right of the socket is an oddly-shaped metal flap that pivots around an M5 bolt. It can be used to close the socket and protect its contacts when it is not in use. The flap has two small folded edges that allow it to be moved with, say, a finger nail. The upper edge reaches just above the lower part of the wooden box, so that the flap can be lifted even when the machine is mounted inside the wooden box. The illustration above shows how the flap can be moved.

Switching plug
The Zählwerk Enigma has 26 lamps on its lamp panel. These lamps are specified at 3.5 V or lower and are normally powered by a 4.5V battery. The current trough the lamps is approx. 200 mA. When the printing device is attached however, the voltages and currents are much higher, as the machine has to drive the solenoids inside the Enigma H directly, blowing the lamps as a result.

Removing the lamps was not considered an option, as one had to remove them each time the printing device was attached and put them back in again for standard operation. Therefore a solution was developed which allowed all lamps to be switched off when the printer connector was placed in the socket of the machine. This solution is described in patent DE595075 [4].

Cross-section of connector and socket. Copyright 2009, Paul Reuvers.

The patent describes a switch that is operated by the connector itself. The cable from the printing device has a connector with 28 spring-loaded contacts. After inserting it into the socket, it must be turned somewhat to lock-in. When doing so, the contacts of the connector slide into position to make contact with the circular pads on the socket. At the same time, an internal connector is moved out of position, so that the connections with the 26 lamps are discontinued.

View of the contacts of the connector. Copyright 2009, Paul Reuvers.

The switch is a complex construction that consists of static and movable parts, both inside and outside the machine. The image above shows the static part of the socket. The connector (not part of the machine) actually behaves as part of the switch. It has 28 spring-loaded contacts, organized in a zig-zag pattern, similar to the contact pads on the socket. At the center is a key that fits the key-hole at the centre of the socket.

The key consists of a rather long cylindrical pin with two 'wings' at the end. In order to prevent the connector from being inserted the wrong way around, the wings have different diameters.

The exploded view below should clarify things somewhat. Please note that during our investigation, we were unable to dismount the switch assembly and are therefore uncertain about the shape of the centre part and the cylindrical contacts. We had to make a few 'educated guesses', based on our knowledge of the German manufacturing skills. The same is true for the shape of the connector which we haven't seen yet.

Exploded view of the complex printer connector/switch. Copyright 2009, Paul Reuvers.

The socket assembly consists of a package of three pieces of pertinax. The leftmost one is square and is visible from the outside. The other parts are disc-shaped in order to fit through the hole in the vertical UKW mounting plate. The three parts are held together with 4 recessed M3 screws, fitted from the rear. The centre part (i.e. the first disc-shaped unit) is smaller in order to accommodate the wires to the Enigma keyboard.

The switch assembly is inserted into the vertical UKW mounting plate from the left and is fixed with three recessed M4 screws. The drawing below shows a cross section of the assembly, the UKW mounting plate and the rotatable switch.

Mounting connector and socket. Copyright 2009, Paul Reuvers.

The spring-loaded contacts of the internal rotatable switch (to the right of the assembly) are aligned in such a way that they touch the contact pads of the assembly when in the default position. This way, the Enigma lamps are each connected to a switch of the Enigma keyboard. The spring-loaded contacts of the connector are displaced by approx. 12.8° so that, when the connector is inserted into the socket, the spring-loaded contacts do not touch the contact pads of the assembly. When inserting the connector, the key – that is a permanent part of the connector – is pushed through the assembly, into the rotatable switch.

Connector fully seated in the socket. Copyright 2009, Paul Reuvers.

Once the connector is fully pushed in, is is rotated approx. 12.8° clockwise until the internal rotatable switch is locked in its second position. A spring-loaded bullet ensures that the switch is kept in place. The contacts of the connector – that were positioned in between the contact pads before – are now resting on the contact pads of the socket. At the same time, the rotatable switch is moved out of position and its contact are now resting in between the contact pads. In other words: when the connector is rotated clockwise, the contacts from the Enigma keyboard to the lamps are broken and are connected to the external connector instead.

Circuit diagram
The printing connector has 28 contacts, whilst the machine only has 26 letters.
The two remaining contacts were used for the ground line (-) and the keyboard-release signal. The drawing below shows a simplified circuit diagram of the Enigma with the printing switch (A) set to the neutral position. The standard Enigma power source is connected and the lamps are enabled. When a key is pressed (W in this case), they current flows in the usual manner through the drum and a light will be lit (Q in this case). The red line shows the current.

Default situation. Copyright 2009, Paul Reuvers.

When a printing device is connected and the switch is enabled (A'), the Enigma is powered from the alternative source and the lamps are disabled. Instead, the output is routed to the printing device. The drawing below shows how the current flows in that case. Please note that all parts of the switch move at the same time.

With external printing device attached. Copyright 2009, Paul Reuvers.

Auction house Hermann Historica in Munich (Germany) announced that the G-111 would be up for sale in the auction of 5 October 2009 at a starting price of EUR 60,000. Unfortunately, the machine didn't sell at that auction. This was likely caused by the current state of the machine, the rather high starting price and the current economic situation. Furthermore, this Enigma model is far less common than, say, a war-time service Enigma, making it less attractive to WWII collectors.

The auction house will review the situation and will probably offer the machine again in a future auction. Whenever we receive news from them, it will be announced on this page and in the events section of this website. Additional information is available directly from the Hermann Historica website and it is also possible to order a full-colour catalogue from them.

  1. G-111: A rare Zählwerk Enigma variant
    Full details of the Enigma G-111 with high-resolution drawings.
    Crypto Museum — Paul Reuvers and Marc Simons. August 2009.
    Version 1.10, 1 May 2013.
  1. German Patent DE534947 (9 November 1928)
    Patent covering most aspects of the Zählwerk Enigma.

  2. German Patent DE579555 (17 November 1928)
    Additional patent covering the multi-notched wheels.

  3. German Patent DE536556 (22 June 1929)
    Patent for the connection between a lamp-based Enigma and a printing Enigma.

  4. German Patent DE595075 (22 June 1929)
    Patent for the self-switching print connector inside the Enigma.

  5. David Hamer, G-312: An Abwehr Enigma
    Cryptologia, January 2000, Volume XXIV, Number 1.

  6. Frode Weierud, Personal correspondence
    Crypto Museum. May 2009.

  7. Original owner, Personal correspondence
    February 2018.
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Crypto Museum. Created: Tuesday 07 July 2009. Last changed: Friday, 30 September 2022 - 13:18 CET.
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