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Ecolex II   Us 8011
One-time tape cipher machine

Ecolex-II is an online/offline One-Time Tape (OTT) cipher machine (mixer) for digital teleprinter communication, developed in 1958 by the Dutch Post Office (PTT) and built by Philips Usfa NV in Eindhoven (Netherlands) between 1958 and 1963, as the transistorized successor to the valve-based Ecolex-I of 1953. The machine is also known by the internal Philips designator Us 8011.

The machine was usually connected to a double tape reader (a modified Siemens T-send-77f), a teletypewriter (commonly a Siemens T-68 telex) and the telex line (or a suitable radio interface).

The image on the right shows a typical Ecolex II with model number Us-8011/07, which reveals that quite a few variants were produced over the years. In this case, the /07 suffix tells us that the device has non-standard military connectors. Furthermore it has a modified telex socket at the rear panel, as the circular 6-pin socket has been replaced by a contemporary 8-pin ADo8 socket.
Bare Ecolex II with wiring

The device measures 52 x 23.5 x 14 cm and weights ~14 kg. It is constructed in such a way, that the double tape reader could be placed on the top surface, but is was also possible to place it aside the base unit. The device is based on the so-called Vernam Cipher, in which two inputs (from the two tape readers) are added by means of a binary XOR 1 operation. This is commonly known as mixing, which is why the machine is also known as a mixer. The main advantage is that the same operation is used for deciphering. When correctly applied, the cipher is unbreakable.

  1. XOR = exclusive-or, also known as modulo-2 addition.

Bare Ecolex II with wiring Ecolex II cipher machine Philips Ecolex II with Siemens T-send-61a tape reader Control panel Close-up of the MODE selector Ecolex II seen from the rear right Rear panel Connectors
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Bare Ecolex II with wiring
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Ecolex II cipher machine
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Philips Ecolex II with Siemens T-send-61a tape reader
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Control panel
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Close-up of the MODE selector
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Ecolex II seen from the rear right
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Rear panel
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The diagram below shows the various features of the Ecolex II. All user controls are on top of the device, close to the front edge. All connections are at the rear, with fixed cables for LINE and AC mains power. The machine is constructed in such a way, that the Siemens T-send-77f double tape reader can be placed horizontally on the top surface. It has its own AC mains power cord.

Overview of controls and connections of the Ecolex II

The 30-pin socket on the left side of the double tape reader, is connected to the 30-pin socket at the rear of the Ecolex. It carries the 5-bit parallel digital data from both tape readers and supplies it to the Ecolex. It is also used by the Ecolex to send stepping commands to the tape readers.

Creating a ciphertext message

The simplified block diagram above explains how the Ecolex II works. At the left is the Siemens T-send-77f with its two tape readers. Tape 1 holds the plaintext. Tape 2 delivers the key stream. The outputs from the two tape readers are used as inputs to the Ecolex mixer, which does an XOR operation on the individual bits of the input signals. Note that all signals are processed in serial form (not parallel), so that only one XOR circuit is needed. The resulting signal is sent straight to the line. In online mode, the signal from Tape 1 is replaced by the signal from a teletypewriter.

Deciphering a message

When receiving an encrypted message, the data lines are rerouted so that the teleprinter and the line are swapped. In online mode, the line signal is used as one of the inputs to the mixer. The other input is provided by the key tape reader of the T-send-77f (Tape 2). In offline mode, the incoming data is first punched to paper tape, which is then played back on Tape reader 1.

Mode of operation
The required MODE of operation is set with the MODE selector at the front edge of the device. It consists of five so-called radio buttons plus an ALARM push button. The leftmost button (RESET) puts the machine in the default state (plaintext on line). The following modes are available:

Button Description Lamp  
RESET Teletypewriter directly connected to line  
LINE CRYPTO Ready to receive encrypted text  
LOCAL PLAIN Ready for making plaintext tape (or copy) 1
LOCAL CRYPTO Ready for encrypting or decrypting local tape 1
TAPE PLAIN Ready for sending plaintext tape to the line  
ALARM   Cause an alarm at the remote end 2,3
  1. In this mode, precautions are taken to ensure that no plaintext is sent to the line.
  2. Local machine stays in current mode (with yellow or green lamp ON).
  3. Red lamp will be ON at remote end only. Press RESET to release the alarm state.

The diagram below shows the standard setup of an Ecolex II cipher system. The teletypewriter at the right — which is usually connected directly to the line — is rerouted via the Ecolex II. The double tape reader is connected to the 30-pin socket at the back of the Ecolex, and acts as the input device for the plaintext tape and key tape. It should be seen as one unit with the Ecolex II.

Ecolex II default setup (note that the T-send-77f is usually placed on top of the Ecolex II)

The double tape reader was usually placed on top of the Ecolex II, so that the complete system did not require much space. The system is suitable for any type of teletypewriter, but was often used in combination with the compact Siemens T-68, which was a popular machine at the time. The above setup is the most flexible one, as it allows the machine to be used in a an online and offline configuration. When deciphering a message, the teletypewriter is used as a printer.

Ecolex II in online configuration

When used online, the plaintext is entered directly on the keyboard of the teletypewriter and is encrypted and transmitted immediately by the Ecolex II. In this situation, the teletypewriter acts as the input device. Online operation is considered to be less secure than offline use, as the typing characteristics of the operator might reveal useful information to a potential eavesdropper.

Ecolex II in offline configuration

When used offline, the plaintext is first stored on paper-tape (using the teletypewriter), and then played back on the plaintext reader of the Ecolex (i.e. the frontmost reader of the T-send-77f). In this configuration the tape reader acts as the input device and does not reveal any characteristics, as it sends the characters at a fixed speed. Offline is therefore considered to be the safest option. Note that in this mode the teletypewriter is still connected to the Ecolex, as it is used as a printer.

Double tape reader   wanted item
Siemens T-send-77f

A crucial part of the Ecolex II cipher machine is the double tape reader that is used for the two input streams of the Vernam Cipher. In this case, the Siemens T-send-77f was used, which was a standard component at the time. It was modified by Philips to make it suitable for the Ecolex II.

The T-send-77f was also used on other OTT cipher machines (mixers), such as Siemens' own Schlüsselgerät D (cipher machine D), which is shown in the image on the right. It is placed on top of the actual cipher machine, which can be seen as the Siemens equivalent of the Ecolex II.

The main difference between the Siemens and Philips cipher machines is that in the Siemens Schlüsselgerät D the XOR circuits are all relay-based, whilst in the Philips Ecolex II this is done by means of electronic XOR circuits, making the Ecolex more robust and also potentially faster.
Siemens Schlüsselgerät D

Furthermore, the Siemens machine does not have an appropriate alarm function and is only suitable for offline use. The double tape reader is connected to the 30-pin socket at the rear of the Ecolex II. The rearmost reader is used for the key tape which provides the key stream. This is usually a One-Time Tape (OTT) that is filled with truly random characters. This reader has a built-in knife that destroys the tape immediately after use. The tape reader at the front is used for the plaintext message (when encrypting) or for the received ciphertext message (when decrypting).

In 1958, mixer machines were not new. The principle behind them was invented in 1917 in the USA by Gilbert Vernam — hence the name Vernam Cipher — who is also the co-inventor of the One-Time Pad (OTP). The first mixers that made use of the One-Time Tape (OTT) principle, were developed during WWII for unbreakable communication at the highest level, independently and more or less simultaneously by the Germans (Siemens T-43) and the British (Rockex and 5-UCO).

Shortly after WWII, in the early days of the Cold War, mixers became popular items for protection of TOP SECRET information, such as the radio an telephone traffic between the Foreign Office and Embassies abroad, and strategic communication between the various NATO countries. Because of their size and lack of suitable components, they were constantly in short supply at the time.

One of the first affordable and reasonably-sized mixers to arrive on the scene in 1952 was the ETCRRM of the Norwegian company STK. Philips followed a year later, in 1953, with the Ecolex I.
Early prototype with single Creed paper tape reader

Both the ETCRRM and the Ecolex I were made with thermionic valves (radio tubes). By 1955, the ETCRRM was produced at a rate of 200 machines per month, at a unit cost of just US$ 1200. In 1963, the ETCRRM was even used on the Washington-Moscow Hotline. In contrast: only 25 Ecolex I machines were ever built, and the price for a single unit was between US$ 3000 and US$ 6000.

In 1957, the Dutch PTT's research lab started the development of the Ecolex II, which would be built with the new transistor technology that had just become available. Like its predecessor – the Ecolex I – it would be built by Philips Usfa NV.

Various experiments were carried out and quite a few prototypes were made, such as the one shown in the image above that uses a single 5-level paper tape reader made by Creed in the UK. In the final design it was decided to replace the single reader by a double one made by Siemens in Germany, as shown in the image on the right.
Three production versions of the Ecolex II on the test bench

A total of 120 Ecolex-II units were built by Philips Usfa [2]. They were sold to the Dutch Army, the Dutch Ministry of Foreign Affairs and the German Ministry of Foreign Affairs [2]. On 23 May 1958, the Ecolex II was approved for NATO traffic of all classifications [3]. Just before the Ecolex-II was introduced in 1959, Philips had already started its own development of OTT cipher machines, resulting in the Ecolex III 1 and the Ecolex IV, both of which offered full signal synchonisation.

  1. Within NATO, the Ecolex III was known as Ecolex II-B.

Unknown device 1 Unknown device 2 Ecolex-II, photo by Jan Lispet
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Unknown device 1
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Unknown device 2
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Ecolex-II, photo by Jan Lispet

Ecolex II interior

Getting access to the machine's interior is only possible with a physical key that has to be inserted into a cylinder lock at the rear. The cylinder lock actuates a complex locking mechanism – using sideways rotating locks – that keeps the bottom panel and the outer case shell in place.

The machine consists of three basic mechanical parts: a bottom plate, a metal case shell, and a frame that holds the actual cipher machine. After rotating the key clockwise, a metal pin – located below the lock – pops out and unlocks the case.

The upper case shell can now be removed, after which the interior is exposed, as shown in the image on the right. The machine is built on a strong metal frame, which can be divided into three sections: a power supply unit (PSU) at the rear, a control panel at the front, and the digital electronic circuits with their wiring at the center.
Interior - seen from the front left

The electronic circuit consist of 10 modules in a so-called cordwood 1 construction, in which the components are mounted vertically between two printed circuit boards. Each cordwood module measures approximately 185 x 55 x 25 mm and is mounted to the metal frame from left to right.

The wiring to the 10 modules is located at the top and is bundled towards the right side of the machine, so that each cord­wood unit can easily be lifted from the frame in case of a repair. The bottom side of each cordwood module can be accessed from the bottom of the machine, after lifting the frame from the metal bottom plate.

The image on the right shows a close-up of the cordwood modules, as seen from the left side of the machine. The brown parts are resistors, and the black ones are OA85 Germanium diodes. All transistors are held in a brown isolation sleeve.
Cordwood circuit - close-up

The complete mixer consists of 30 sub-circuits — spread over the 10 cordwood modules — that are built from 62 Germanium transistors (OC76) and no less than 330 Germanium diodes (OA85). An overview of the 30 sub-circuits is given in the table below. The sub-circuits represent logical functions (e.g. AND, OR, XOR, etc.), switches and analogue power circuits. The circuit diagram plus a detailed technical description of each sub-circuit is available in the technical manual [D].

  1. Cordwood modules, or cordwood constructions, refers to a manufacturing method for electronic circuits, in which conventional parts like resistors, capacitors, transistors and diodes, are mounted vertically between to parallel printed circuit boards, with the aim to save space. Cordwood structures are used for example in the NSA's FLYBALL modules - used extensively in the KW-7 cipher machine – and CIA bugs like the SRT-52.

Ecolex II cipher machine Case lock with key Unlocking the case Top cover removed from the Ecolex II Lifting the bottom panel Bottom panel removed Interior - seen from the front left Interior seen from the front right
Bottom panel (top) Sideways locks Bottom view Interior - left side Front panel with hidden switches PSU detail Cordwood circuits Cordwood circuit - close-up
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Ecolex II cipher machine
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Case lock with key
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Unlocking the case
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Top cover removed from the Ecolex II
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Lifting the bottom panel
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Bottom panel removed
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Interior - seen from the front left
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Interior seen from the front right
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Bottom panel (top)
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Sideways locks
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Bottom view
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Interior - left side
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Front panel with hidden switches
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PSU detail
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Cordwood circuits
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Cordwood circuit - close-up

Block diagram
The block diagram below shows the operation of the complete machine, including the double tape reader. Central to the system are the MODE selector — used to select the desired input — an equalizer — used for re-shaping the signal — and the mixer (XOR). The output of the mixer is delivered to the outgoing TX line and to the receive circuit of the local teleprinter at the far right.

Ecolex II block diagram

At the left is the double tape reader, of which the KEY tape reader (Tape 2) is always connected to the mixer. The MODE switches are used to select the other input to the mixer: the incoming line, the output of the local teleprinter or the other tape reader (Tape 1). In the latter case, Tape 1 is either the plaintext tape (when ciphering a message) or the ciphertext tape (when deciphering). The timing of the entire system is controlled by the selected input signal (via the MODE selector).

Below is an overview of the functions of the 30 sub-circuits that are spread over the 10 cordwood modules. The first column shows which transistors are used for these circuits. In November 1960, small changes were made to some of the circuits, and the entire circuit diagram was reorganised. The second column gives the transistor numbers for Ecolex II versions Us 8011/08 and /09.

  1. Tr 1
    Tr 1-2
    Text signal
  2. Tr 2-3
    Tr 3-4
    Tape reader control
  3. Tr 4-5
    Tr 5-6
    Delay circuit (alarm)
  4. Tr 6-8
    Tr 7-9
    Alarm circuit
  5. Tr 9-10
    Tr 10-11
    Check tape feed
  6. Tr 11-12
    Tr 62-63
    Break trigger
  7. Tr 13-14
    Tr 12-13
    Start/stop command
  8. Tr 15-16
    Tr 42-43
    Tape reader 1 command (plaintext)
  9. Tr 17-18
    Tr 51-52
    Tape reader 2 command (key)
  10. Tr 19-20
    Tr 60-61
    Speed check
  11. Tr 21-22
    Tr 27-28
    Selector command
  12. Tr 23-24
    Tr 29-30
    Square wave generator (clock)
  13. Tr 25-26
    Tr 31-32
    Generator buffer
  14. Tr 27-28
    Tr 33-34
    1st divider
  15. Tr 29-30
    Tr 35-36
    2nd divider
  16. Tr 31-32
    Tr 37-38
    3rd divider
  17. Tr 33-34
    Tr 23-24
    Regenerator (equalizer)
  18. Tr 35
    Tr 22
    Regenerator time pulse
  19. Tr 36-37
    Tr 25-26
    Transmit relay driver
  20. Tr 38-40
    Tr 29-41
    Teleprinter driver
  21. Tr 41-42
    Tr 44-45
    Tape reader 1 buffer (plaintext)
  22. Tr 43-44
    Tr 46-47
    5th element store (plaintext)
  23. Tr 45-47
    Tr 48-50
    Tape feed 1 (plaintext)
  24. Tr 48-49
    Tr 53-54
    Tape reader 2 buffer (key)
  25. Tr 50-51
    Tr 55-56
    5th element store (key)
  26. Tr 52-54
    Tr 57-59
    Tape feed 2 (key)
  27. Tr 55-56
    Tr 14-15
    5 ms delay
  28. Tr 57-58
    Tr 16-17
    20 ms delay
  29. Tr 59-60
    Tr 18-19
    Send/receive switch
  30. Tr 61-62
    Tr 20-21
    40 ms delay (or 170 ms)
Configuration switches
At the front of the device, below the MODE selectors and behind the top cover, are seven hidden switches that can be used to alter the configuration of the machine. On some later units, an extra configuration switch is available on the FUSE-panel of the internal power supply unit (PSU).

  1. LINE PLAIN on/off
  2. LINE CRYPTO on/off
  3. Invert output
  4. Teleprinter current 40/60 mA
  5. Line current single/double 5a
  6. Speed 45.5 or 50 baud
  7. 250 Hz frequency adjust
    Configuration switches at the front
  1. This switch controls the possibility to send plaintext straight to the line. In the UP position, with the MODE selector set to TAPE PLAIN, the plaintext from reader 1 is sent straight to the line. In the DOWN position, this operation is blocked. It is always possible to send plaintext directly from the keyboard.
  2. This switch controls the possibility to send ciphertext straight from the keyboard of the connected teletypewriter. In the UP position, with the MODE selector set to LINE CRYPTO, ciphertext can be sent straight from the keyboard. In the DOWN position, this operation is blocked.
  3. This switch can be used to invert the output of the mixer. In the DOWN position, the machine uses the XOR operation for mixing the individual bits from the plaintext and keystream. In the UP position, the output of the mixer is inverted (NOT-XOR). This was the default setting for most Ecolex II machines.
  4. This switch controls the line current that is used for the connected teletypewriter. In the UP position, 40mA is used. In the DOWN position, this is 60mA.
  5. This switch controls the line configuration. The UP position selects single-current operation. The DOWN position selects double-current operation. (5a) From version Us 8011/08 onwards, switch 5 is used to delay the LINE CHECK by 5 ms.
  6. In the UP position, the machine is suitable for 50 baud signals. In the DOWN position, a speed of 45.5 baud is selected, making the machine compatible with American teletypewriters.
  7. This is a TEST switch that should be used when adjusting the generator frequency. In the UP position, a neon lamp is enabled. Using a 125 Hz tuning fork, the generator frequency can be adjusted with the potentiometer to the right of the switch. Once the frequency is correctly adjusted, the switch should be returned to the DOWN position (more about this procedure below).

Adjusting the 125 Hz generator
The internal 125 Hz frequency generator of the Ecolex II can be adjusted easily and accurately, with the help of a 125 Hz tuning fork. After placing switch 7 in the UP position, a neon lamp (to the right of the switch) will light up at 125Hz.

By looking at the neon lamp through the slit of an exited 125 Hz tuning fork, the potentiometer at the right should be adjusted so that only one of the lamp's electrodes is lit. When the light alternates between the two electrodes, the 125 Hz generator has not been adjusted correctly.

 More about tuning forks

Tuning fork used for adjustment of the internal generator

Line interface
An extra switch is located on the internal PSU from model Us 8011/05 onwards. When set to the ON position, the Ecolex II is suitable for connection to a TH-5/TG telegraph/telephone terminal. Note that in this situation, the alarm function of the Ecolex II can only be used when it is connected in a 4-wire configuration [A].

The image on the right shows the TH-5/TG as it is shown on the front cover of its manual [G]. The device is housed in a green metal enclosure and was used extensively by the US Army during the Vietnam war.

 Download TH-5/TG manual

Configuration switches at the front Interior - front view Sealed symmetry adjustment potentiometer 125 Hz generator frequency adjustment, consisting of Switch 7, a neon lamp and a potentiometer. Toggle switch for enabling the external TH-5/TG interface 125 Hz generator frequency adjustment, consisting of Switch 7, a neon lamp and a potentiometer. Tuning fork used for adjustment of the internal generator 125 Hz tuning fork to be held in front of the neon lamp between switch 7 and the potentiometer
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Configuration switches at the front
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Interior - front view
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Sealed symmetry adjustment potentiometer
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125 Hz generator frequency adjustment, consisting of Switch 7, a neon lamp and a potentiometer.
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Toggle switch for enabling the external TH-5/TG interface
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125 Hz generator frequency adjustment, consisting of Switch 7, a neon lamp and a potentiometer.
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Tuning fork used for adjustment of the internal generator
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125 Hz tuning fork to be held in front of the neon lamp between switch 7 and the potentiometer

We currently have an Ecolex II on loan, and we are aiming to get it up and running in due course. It would be great if it could be made to communicate with an Ecolex IV. This depends on the state of the components, the wiring and the ability to secure a double tape reader. In the meantime, restoration of the base machine has been started. The following has so far been restored:

  • Case lock removed and replaced.
  • Case realigned and paint restored.
  • Locking mechanism repaired and realigned.
  • Grommets of fixed cables at the rear replaced.
  • Strain relief of power cable repaired.
  • Rubber pads added at bottom to avoid scratches.
  • One fuse holder repaired.
  • Mains power switch replaced.
 Missing items

With the early Ecolex machines, the fixed line cord has a 6-pin circular plug at the end. It is wired as shown in the diagram below. At the rear of the machine is a female socket for connection of a teletypewriter, with the same layout. Below is the pinout when looking into the socket from the rear of the machine. Note that the socket has a micro-switch behind contact 1. When inserting a plug into the socket, pin 1 engages this switch, which interrupts the internal 200Ω terminator.

  1. TX relay A (common)
  2. TX relay Z (mark)
  3. TX relay T (space)
  4. 120Ω RX relay (with 6)
  5. Alarm contact (with 5)
  6. Alarm contact (with 4)
  7. 120Ω RX relay (with 3)
  • Single-current: use TX contacts 1 and 2, set RX current to 40 mA.
  • Double-current: power supply via ballast lamps to pins 2 and 4, set line current to 20 mA.
  • 2-wire operation: TX and RX contacts connected in series.
  • 4-wire operation: TX and RX contacts connected separately.
  • The alarm contact is used to activate an external alarm circuit (e.g. lamp or bell).
  • Alternative ADo8 socket
    On Ecolex II units, the existing 6-pin telex plug and socket (see above) was replaced by a more common 8-pin ADo 8 one. The diagram below shows the pinout for that connector type. At present the wiring of the alarm lines is unknown.

    1. TX a (a1)
    2. TX b (b1)
    3. RX a (a2)
    4. RX b (b2)
    5. Bridge to 6 1
    6. Bridge to 5 1
    7. ?
    8. ?
    1. This is a bridge inside the socket (not the plug).
    Alternative 10-pin line plug
    In some cases the fixed cord for connection to the (telex) line does not have a standard 6-pin or ADo8 plug at the end. Instead, it might be fitted with a military 10-pin male plug, of which the wiring is given below, when looking into the socket. This plug was standard on the Us 8011/07.

    1. TX a
    2. Alarm a
    3. TX b
    4. Alarm b
    5. RX a
    6. RX b
    7. not connected
    8. Ground
    9. not connected
    10. not connected
    AC Mains
    Most Ecolex II units have a standard plug for connection to the AC mains wall socket. Some versions however, notably the Us 8011/07, have a 6-pin military plug fitted to its mains cable. The pinout of this plug is given below, when looking into the socket.

    1. AC mains (A)
    2. not connected
    3. AC mains (B)
    4. not connected
    5. GND
    6. GND
    • Power
      110, 127 or 220V AC 1 (selectable with internal voltage caroussel)
    • Fuses
      2 x 500 mA (internal)
    • Mode
      Offline, online, 2-wire, 4-wire, single current, double current
    • Tape reader
      Double, Siemens T-send-77f
    • Speed
      50 baud, 45,5 baud
    • Line current
      40 or 60 mA
    1. Extra taps are available on the mains transformer to adjust the selected voltage by ±5.5V.

    • Us 8011
      Philips Ecolex II cipher machine  Versions
    • Us 8900/02
      Modified version of Siemens T-send-77f double tape reader (40mA)
    • ?
      Line connection and filter unit
    • Us 8011/01
    • Us 8011/02
      Release version
    • Us 8011/03
    • Us 8011/04
    • Us 8011/05
      Addition of switch for TH-5/TG interface on the PSU
    • Us 8011/06
    • Us 8011/07
      Military connectors (and modified Telex socket)
    • Us 8011/08
      Upgraded circuits and new circuit diagram
    • Us 8011/09
      Same as /08 but with 4-pin telex socket (rather than 6-pin)
    Help required
    Crypto Museum are still looking for an Ecolex II unit for its collection. If you can help in any way, please contact us. At present, we have an Ecolex II machine on loan, but unfortunately it is not complete. We are currently looking for the following items:

    • Siemens T-send-77f double tape reader (Philips designator Us 8900/02).
    • 40-pin to 40-pin cable for connection of the T-send-77f to the Ecolex II.
    • Full service documentation (the one in [D] is incomplete).
    • Line connection and filter unit.
    1. Gebruiksaanwijzing voor het Philips Usfa vercijferapparaat Ecolex II Us 8011/05
      Philips Usfa NV. Ecolex II User Manual (Dutch). Declassified.
      VTH 11-956/1. KL/GGC-3010. 2 September 1958.

    2. Ecolex II User Manual, Supplement
      Description of interoperability with Lorenz Mixer (Dutch).
      Koninklijke Landmacht. 8A-II-IUB 504427. 15 April 1965.

    3. Bedienungsanweisung Chiffrier-Anlage 'ECO II'
      Unknown author, User manual (German). BA-ECO II. Date unknown. 10 pages.

    4. Nähere Beschreibung für das Philips Usfa Chiffriergerät Ecolex II Us 8011
      Philips Usfa, Technical description (German). May 1959. incomplete

    5. Anlagen bei [D], Nr. 13351/D für Typ Nr. Us 8011/08/09
      Supplement to [D] for Ecolex II Us 8011/08 and Us 8011/09 (German).
      Philips Usfa NV, November 1960.

    6. Ecolex II, Us 8011, circuit diagrams
      Philips Usfa NV, 1—25 May 1959.

    7. Telegraph Terminal TH-5/RG and TH-5A/TG 1
      Organisational Maintenance Manual. US Army, December 1982.
    1. Obtained via Google Books, March 2018.

    1. Elektromagnetische straling van crypto-apparatuur IV 1 Part 2
      Omvang van de stoorstraling (RFI). Ontstoring.
      TNO RVO, 1 May 1965. Geheim (secret).

    2. Elektromagnetische straling van crypto-apparatuur IV 1 Part 23
      Appendix 3 of part 2.
      TNO RVO, 1 May 1965. Geheim (secret).

    3. Elektromagnetische straling van crypto-apparatuur IV 1 Part 3
      Omvang van de informatiestraling (data leaks).
      TNO RVO, 1 May 1965. Geheim (secret).

    4. Elektromagnetische straling van crypto-apparatuur IV 1 Part 32
      Appendix 2 of part 3.
      TNO RVO, 1 May 1965. Geheim (secret).

    5. Elektromagnetische straling van crypto-apparatuur IV 1 Part 4
      Bestrijding van de informatiestraling (solutions).
      TNO RVO, 1 May 1965. Geheim (secret).
    1. Electromagnetic radiation of cryptographic equipment, volume IV. A series of reports (in Dutch) about unwanted emanations (TEMPEST) of an Ecolex II machine – resulting in data leaks – and possible solutions. Written by the Dutch Research Laboratory TNO RVO.

    1. Philips Usfa, Photographs of Ecolex II
      Crypto Museum Archive #CM300637.

    2. Philips Usfa, Internal Memo L/5636/AvdP/JG
      23 August 1982, page 5.

    3. NATO, ECOLEX MARK II, Transistorized Version
      SGM-318-58. 23 May 1958. NATO SECRET.
      Declassified by NATO in 2006 (IMSM-0001-2006).

    4. Anonymous, Philips Ecolex II, Model Us 8011/07, S/N 205
      Accessed March 2018.
    Further information
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