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MA-4231
Morse burst decoder

The MA-4231 was an automatic morse decoder, designed and build by Racal Datacom Ltd. (UK) around 1977. It messages, sent in morse code at various speeds, to be recorded and played back. It was part of a family of devices, including a morse burst encoder, a printer and various power sources. It was intended for use as part of a spy radio set and by Special Forces (SF).

The image on the right shows a typical MA-4231 unit. It is housed in a case that is identical to that of the MA-4230, albeit with far less keys on it. It has two sockets at its right, allowing it to be connected to the MA-4230 burst encoder and to an (optional) printer or teletype unit.

The unit is powered by an internal rechargeable battery, that allows the receiver to be used continuously for approx. 10 hours withut recharging. It can receive morse code signals at normal speed (10-30 wpm) and at high speed (30-160 wpm). This is called Squirt Mode.
  

When receiving morse code signals, the MA-4231 needs at least 32 code elements to stablish the transmission speed. This means that the sender has to include a preample, containing at least 4 dots, 4 dashes, 4 inter-unit spaces and 4 letter spaces, at the beginning of a message. These characters should then be considered lost.

When using the MA-4231 in combination with an MA-4230 burst encoder at the other end, the units are synchronised by a pilot tone sent by the MA-4230 at the start of each message. Once synchronized by this tone, no characters will be lost.

During reception, the received morse code signal is decoded and shown on the red dot-matrix display in the upper right corner, one character at a time. The message is also stored in the internal memory, so that it can be read back later using the CH and BCK keys.

The output can also be sent to a complementary printer, such as the MA-4233. If such a printer is connected, the received message will not be shown on the dot-matrix display and can not be read back using the CH and BCK keys.

Front view of the MA-4231 Close-up of the red dot-matrix display Controls and indicator lights The unit on the side, showing the 2 connectors. Power and data connector The MA-4230 morse encoder and the MS-4231 morse decoder The MA-4230 morse encoder and the MS-4231 morse decode
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Front view of the MA-4231
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Close-up of the red dot-matrix display
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Controls and indicator lights
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The unit on the side, showing the 2 connectors.
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Power and data connector
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The MA-4230 morse encoder and the MS-4231 morse decoder
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The MA-4230 morse encoder and the MS-4231 morse decode

Connections
The MA-4231 has two 5-pin Fischer sockets at its right side. The rightmost socket (male) is used for connection to the MA-4231 morse encoder, whilst the leftmost one (female) allows a printer or a teletype unit to be connected.

Looking into the Fischer sockets
The pins of these sockets are numbered from 1 thru 5, but note that this numbering is different from the numbering inside the matching plug! The pins are wired according to the wiring diagram below. The colours specified in the table are the ones used in the original Racal cables.

Connection to the MA-4230 (right)

Pin Name Colour Description
1 RX Blue Audio from receiver (keyed tone)
2 TX Red Keyed tone from MA-4230 (monitor)
3 PTT White PTT input (controlled by transceiver or MA-4230) *
4 Power Yellow Power supply or battery charger (+ 11 to 30V)
5 GND Green Common connection (ground)
Shield GND Braid Common connection (ground)

A suitable female connector for this socket is Fischer S103 Z054-130+.

*) The operation of the PTT signal at pin 3, is determined by a link on the Keyer board inside the MA-4230. When the link is open, pin 3 is configured as a PTT output. During a tranmission, pin 3 is held low (GND) for the duration of the message plus 1.5 sec. When the link is closed, pin 3 acts as a KEY output (active low), synchronised with the tones at pin 2 [A].

Connection to a printer (left)

Pin Name Colour Description
1 n.c. - No connection
2 OUT Red Serial data out
3 READY White Printer ready (input)
4 Power Yellow Power supply or battery charger (+ 11 to 30V)
5 GND Green Common connection (ground)
Shield GND Braid Common connection (ground)

A suitable male connector for this socket is Fischer S103 A054-130+.

Power supply
The unit is powered by an internal rechargeable NiCd battery that can be charged via any of the connections at the right. Please note that the batteries must be fully charged before the unit can be operated, even when an external voltage is supplied. Also note that the batteries in most of these surplus devices, are either dead or worn-out, even when the device (and in some cases the battery) looks brand new. These units were built around 1980 and its battery life time has long expired. Whithout a healthy battery, the unit can not be operated properly.

The charging voltage is supplied to the same pins of the sockets on all devices of the MA-4230 family. These pins (4 and 5) are all interconnected, allowing a battery charger or external power supply to be connected anywhere in the chain. Generally though, a battery charger would be connected to the printer (if present). When connected to a suitable radio, power would generally be supplied by the radio (connected to the MA-4230 unit).

Please note that, when using an external power source for charging the batteries (e.g. power taken from a transceiver), the battery charger should be disconnected.

Connecting a printer
The MA-4231 allows a variety of printing devices to be connected to the leftmost Fischer connector, ranging from 5-bit teletype devices (ITA2) to modern telex units and printer with an 8-bit serial interface (ITA8). A set of jumper wires on the Interface Board of the MA0-4231, allow the data format to be configured. The following can be set:

  • Baud rate (50, 75, 110, 134.5, 150, 200, 300, 600 or 1200 baud)
  • Parity (on/off, even/odd)
  • Word length (5-bits or 8 bits)
  • Stop bits (1, 1.5 or 2 bits)
The baud rate can be configured by adding or removing 4 wire links on the interface board. All other options are selected by links in the form of 10K and 100K resistors. The drawing below shows the position of the various links on the interface board.

Interface Board
The MA-4231 supports 9 different baud rates that are selected by 4 wire links on the interface board. The links are numbered S0 (top) to S3 (bottom). The table below shows the relation between the wire links and the baud rate. A black dot indicates the presence of a wire link.

Baud rate links
Please note that when a printer is connected to the MA-4231, the message can not be read from the message buffer manually using the CH and BCK keys.

Interior
The MA-4231 is housed in a ruggedized die-cast aluminium case, similar to that of the MA-4230 burst encoder. The case consists of 2 halves (shells), with a rubber gasket in the middle. The two shells are kept together by 4 hex-head bolts at the bottom.

After separating the two halves of the case, the interior is revealed. It consists of a metal frame, holding three PCBs: the Processor Board, the Audio Board and the Interface Board. The latter is in between the two other boards and can easily be pulled out. Setting of the serial baud rate and word format is described above.

The boards are connected together at one end by means of a backplane. This backplane is part of a flex PCB that is also connected to the keyboard PCB (mounted in the top half of the case). The keyboard also holds the display.
  
Taking out the Interface board

According to date codes on the various components, the MA-4231 shown here was manfuctured in 1979 or 1980. The unit is powered by a 6V rechargeable battery that is mounted in the bottom half of the case. Although the battery in the image looks brand new, it should be considered lost after so many years. More about the batteries and power supply above.

6V re-chargeable battery Keyboard wiring Backplane Audio Board Processor Board Stack of 3 PCBs Taking out the Interface board Interface Board
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6V re-chargeable battery
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Keyboard wiring
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Backplane
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Audio Board
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Processor Board
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Stack of 3 PCBs
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Taking out the Interface board
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Interface Board

Documentation
  1. Technical Manual MA.4230 MA.4230S Morse Encoder (with Battery Charger MA.4232)
    Racal-Datacom Limited. 1 October 1980, issue 1.10.80-100.

  2. Technical Manual MA.4231 Automatic Morse Receiver
    Racal-Datacom Limited. 2 February 1980, issue 2.2.80-100.

  3. Conversion notes for the Racal MA4230/4231
    John's Radio, Conversion from Arabic to English. Date unknown.
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Crypto Museum. Created: Thursday 26 August 2010. Last changed: Saturday, 01 July 2017 - 11:12 CET.
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