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Message exchange device

MED is a Message Entry Device, or Electronic Message Unit (EMU), developed around 1987 by Hollandse Signaalapparaten (HSA) 1 in Huizen (Netherlands). It was intended for use with military HF and VHF radios, in particular with the SPIDER VHF military radio, which was also made by HSA. The fully self-contained device is also known by its internal HSA designator 9556 304 105xx.

The device measures approx. 254 x 154 x 55 mm and weights 1608 grams including Alkaline batteries. It is powered by four internal 1.5V C-size dry battery cells that are installed behind a large circular screw-terminal at the right side.

It has a 4 x 40 character liquid crystal display and is suitable for text-based messages, which can optionally be encrypted. It is suitable for connection to virtually any brand and model of military radio, and supports baud rates from 75 to 1200 baud. The version 2 shown here, has a 10-pin socket for connection to a SPIDER radio.
Message Exchanged Device (MED)

The device is intended for use in combat net radio and field wire 3 systems. It features Forward Error Correction (FEC) and has an RS-232C interface (available on an NF-7 socket) for connection to a portable printer. It also features burst transmissions to reduce the probability of interception and network traffic load. The version shown in Jane's of 1988 [1], is probably the final design, made of aluminium, as it weights 1.75 kg, whereas the one shown here has a plastic enclosure.

The MED was developed at HSA around 1987, at the same time as the SPIDER VHF military radio set. Given the small number of SPIDER radio sets that were manufactured, it is believed that only a few MEDs were ever made. As the device shown here has serial number 0 MODEL S1009 it is likely that it was part of a prototype series — or null-model — intended for evaluation purposes.

  1. HSA Huizen – informally known as SIGNAAL or SIGNAL – was formerly known as Philips Telecommunicatie Industrie (PTI). It was taken over in 1990 by Thomson, which has since been renamed Thales.
  2. The device does not require the CRYPTO option to be installed in the SPIDER. When available, the encryption was performed by the MED.
  3. Transmission via 2-wire line at 1200 or 2400 baud.

Top view Message Exchanged Device (MED) SPIDER radio cable
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Top view
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Message Exchanged Device (MED)
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SPIDER radio cable

The diagrams below give an overview of the controls and connections on the body of the MED. At the top is a large window, behind which a 4 x 40 character LCD display is located. The window is shielded by means of a metal grid, to avoid any unwanted emanations (TEMPEST). In front of the LCD is a 30-button recessed keyboard, of which most keys have a double function. At the left is the 3-position POWER switch. In the center position, the device is OFF. In both other positions, the device is ON. If the left position is used, the backlight of the display is switched ON as well.

Also at the left – at either side of the POWER switch – are the terminals to the 2-wire interface. It allows two MED devices to be connected directly via a regular WD-1/TT field wire line. At the right are the sockets for connection to a radio — in this case the SPIDER — and an (optional) printer. When unused, these sockets should be covered by the protective caps. Also at the right is a large circular threaded cap, that gives access to the internal cylindrical battery compartment.

Message Exchanged Device (MED) Top view Bottom view 2-wire field line terminals with MODE selector at the centre Radio socket (left) and RS232 interface (right) Sockets covered by protective caps
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Message Exchanged Device (MED)
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Top view
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Bottom view
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2-wire field line terminals with MODE selector at the centre
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Radio socket (left) and RS232 interface (right)
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Sockets covered by protective caps

The diagram below shows the simplest possible setup for communication between two MED terminals. The two devices are linked via a standard 2-wire field line (WD-1/TT), with a length of up to 3 km. As each MED has its own local battery source, communication is possible without any additional equipment, at 1200 or 2400 baud, subject to the length and the quality of the line.

In most cases however, the terminal will be connected to an HF or VHF radio via the radio (audio) interface. In this case, the internal modem will be used to generate FSK tones that can be trans­mitted over a regular audio (voice) channel. The diagram below shows a setup with SPIDER radios.

When used in combination with a VHF FM radio (like SPIDER), baud rates of 75, 150, 300, 600 and 1200 baud are possible. When used in combination with a narrow-band HF radio, the baudrate will be limited to 75 baud. The internal modem generates the necessary FSK mark/space tones.

The device featured here is a prototype, which differs from the final version. It is believed that the final product was housed in an aluminium enclosure, whilst the green case shown here is made of plastic. Furthermore, the final version has text labels near the connectors and the POWER switch.

The control panel is held in place by 20 screws around its circumference. With the prototype these screws are inserted from the bottom side, whilst on the final version they are inserted from the top [1]. After removing these screws, the top panel – which holds the display and the key­board – can be separated from the case shell.

Inside the top panel are the display electronics – probably provided by Hitachi – and the processor board (CPU). The CPU is built around a 80C31 micro-controller made by Harris, with 8KB static RAM and a 32KB EPROM that holds the firmware.
Processor and memory

The display board is connected to the CPU by means of fixed wiring. The CPU board is connected to the keyboard by means of a 17-pin header that protrudes the top panel. After removing the four screws in the corners of the board, the CPU board can be lifted from the keyboard connector.

Inside the case shell are two further PCBs. The one that is visible immediately after opening the case, is the interface board (I/F) with its solder side up. It is connected to the processor board by means of fixed wiring (i.e. 26 individual wires) and is held in place by 4 screws at the corners. It holds a single-chip FX409 FSK modem, made by CML in the UK [A], and an RS232 interface [B].

After removing these four screws, the I/F board can be taken away, revealing the power board at the bottom of the case. Note that the I/F board is connected to the power board with a header.
Wiring to the connectors

The image on the right, shows the part of the power board that holds a buzzer and a supercap. The power board converts the voltage from the batteries into several stabilized voltages that needed for the CPU, the display and the various interfaces. To the left of the power board, is the wiring to the radio socket and the RS232 socket, both of which should be connected to the I/F board. To the right of the power board are the 2-wire line terminals and the POWER switch.

Display and processor boards Interface board Power board Wiring to the connectors Mode selector (wiring) Processor board Processor and memory Keyboard connector
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Display and processor boards
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Interface board
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Power board
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Wiring to the connectors
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Mode selector (wiring)
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Processor board
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Processor and memory
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Keyboard connector

Help required
At present, the operation of the MED is not known. Furthermore, it is unclear whether the current firmware supports encryption and decryption of the text. If you have any information about this device, such as circuit diagrams and/or operating instructions, please contact us.

When starting the device after it has been switched OFF for several hours (to ensure that the supercap has been discharged), it starts with the following text on the display:

Once the device has been configured (i.e. all questions have been answered and valid parameters have been entered), the current settings are retained in static RAM. A supercap, which is placed on the power board, preserves the contents of the static RAM when the batteries are removed.

Radio   X1
MED has a 10-pin CANNON NF10 socket that has the same pinout as the 10-pin NF10 socket on the front panel of the SPIDER radio. Use a 10-way 1:1 wired cable to connect the two devices. A 180kΩ resistor inside the MED (connected between H and ground) informs the radio that an MED is connected. Below is the pinout of the NF-10 socket on the SPIDER (looking into the socket).

  1. GND
    Common ground
  2. SPK
    Audio output
  3. PTT
    Transmit contact
  4. MIC
    Audio in
  5. SQL
    Squelch contact
  6. NOGO
    'NOGO' information (active low)
  7. not connected
  8. ADR
    Peripheral address 1
  9. PWR
    +12V supply for peripheral
  10. DATA
    'Data mode' information (active low)
  1. A 180 kΩ 2% resistor between this pin and ground, informs the radio that an MED is connected.

RS-232C   X2
The MED has an RS-232C serial port which is intended for connection of a (portable) printer. It is available on an NF-7 socket at the right side of the device. The pinout of this connector is currently unknown.

  1. ?
  2. ?
  3. ?
  4. ?
  5. ?
  6. ?
  7. ?
2-wire interface   X3 & X4
At the left side of the device, at either side of the POWER switch, is a spring-loaded push-in terminal for connection of a 2-wire field line. This wiring does not have a specific polarity.

  • Power
    6V DC (4 x 1.5V C-size dry battery)
  • Baudrate
    75 — 2400 baud  see list
  • Display
    4 x 40 character Liquid Crystal Display (LCD)
  • Standard
  • Dimensions
    254 x 154 x 55 mm
  • Weight
    1.75 kg (including batteries)
Baud rates over HF radio
  • 75
  • 150
  • 300
  • 600
  • 1200
Baud rates over 2-wire
  • 1200
  • 2400
  1. FX409 FSK modem, datasheet
    CMS, February 1985.

  2. MC145406 EIA 232-E and CCITT V.28 interface, datasheet
    Motorola Inc., Rev. 4, January 1995.
  1. Jane's Military Communications 1988, Message Exchange Device (MED)
    Jane's Publishing Company Ltd. ISBN 0-7106-0856-X. p. 428.
Further information
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Crypto Museum. Created: Tuesday 18 December 2018. Last changed: Thursday, 20 December 2018 - 14:21 CET.
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