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U-229 Connector
US Army and NATO standard audio and FILL connector

The U-229 is a standard military connector used by the US Army and by NATO for connection of audio equipment to a radio (microphone, headset, etc.) as well as for the connection of key fillers to cryptographic equipment (FILL). The standard connector has 5 pins, marked A-E, but a 6-pin version also exists. In the latter case, the extra pin is marked 'F' and is located at the center.

The image above shows the pinout when looking at the contacts of the female connector (left) and male connector (right). Many different versions of the connector and its mating receptacles are available, such as 5 or 6-pin types, male or female and cable or panel mount. Although each variant has its own specific type number, they are often referred to as U-229. The table below shows the correct number for each version. U-229/U is the female cable mount version.
Type M/F Pins Description
U-228/U M 5 5-pin, male, cable mount
U-229/U F 5 5-pin, female, cable mount
U-328/U M 6 6-pin, male, cable mount
U-329/U F 6 6-pin, female, cable mount
U-183/U M 5 5-pin, male, panel mount (chassis)
U-283/U M 6 6-pin, male, panel mount (chassis)
GC-429 F 5 5-pin, female, circular panel mount (chassis)
GC-629 F 5 5-pin, female, square panel mount (chassis)
GC-529 F 6 6-pin, female, circular panel mount (chassis)
GC-729 F 6 6-pin, female, square panel mount (chassis)

The U-229 was initially designed for use as an audio connector on military radio equipment. The female cable part is commonly found on microphones, speakers and handsets. In most cases the 5-pin version is used for this. The radio itself usually has a 5-pin U-183/U male socket.
The image on the right show a typical U-229/U female connector (right) as part of a handset. On the left is the radio set which (in this case) has two identical U-183 sockets. Most radios have two sockets to allow the separate connection of a microphone and speaker. The NATO-standard pinout of this connector is given below. Note that not all radios adhere to this standard.

Some radios, such as the SINCGARS RT-1439, use the 6-pin variant of the U-183 socket (U-283). This allows the same socket to be used as a KEY FILL connector (see below).
Connecting a handset with an U-229 connector

Nearly all US Army and NATO equipment uses the same pinout for this connector, so that all accessories are more or less compatible. The only exception to this rule is the Dutch RT-3600 radio, which has its microphone (MIC) and speaker (SPK) wires swapped. The reason for this is currently unknown, but it caused great problems when trying to connect standard accessories. The table below shows both types of connection. The NATO pinout is the more common one.
Pin NATO RT-3600 Description Note
A GND GND Ground (common wire)  
B SPK MIC Speaker (microphone on the RT-3600) 1
C PTT PTT Push-to-Talk switch (connects to ground) 2
D MIC SPK Microphone (speaker on the RT-3600)  
E EXP - Various expansions. Not standardized. 3
F EXT - Not present on most radios. 4

  1. External speaker sense
    Some radios have a +6V DC offset on this pin. It is used for sensing the presence of an external speaker and (if it finds one) muting the internal speaker.

  2. Push To Talk bus (PTT)
    This pin should be grounded when transmitting. It is neither an input nor an output, but should be considered a BUS. Anyone on the bus could start a transmission by asserting this line to ground. On some radios it is also used for CW (morse), but this is not possible if the radio supports SSB. In the latter case, pin E is used for CW.

  3. External power or fast CW
    The function of pin E is not standardized, so different radios used it for different purposes. On some radios, such as the PRC-68 family, it is used as a 12V power input. Some manpack radios, such as the PRC-74 and PRC-104 use this pin for CW input (morse). In such cases it can also be used for the connection of a high speed burst encoder, such as the GRA-71. On other radios, pin E is used as an extra speaker line, or as a retransmission PTT line in case of a repeater.

  4. Center pin
    This pin is not present on the original U-229 connector. On some radios however a 6-pin socket is mounted, to allow for future modifications and/or additional features. Some radios use it for digital signals (e.g. uploading and downloading of channel frequency assignments) and for retransmission PTT.

Crypto FILL
The 5 or 6-pin version of the U-229 can also be used to connect a key loader to a crypto device, or to a radio with built-in crypto and/or frequency hopping (FH). In many cases this is combined with the audio functionality described above. Such is the case with many of the SINCGARS radios. The 6-pin version of the U-229/U (known as the U-329/U) is most commonly used for FILL purposes, but this is not absolutely necessary, as the center pin (F) is hardly ever used.
The image on the right shows two 6-pin U-283 receptacles on the front panel of the KY-57 voice encryption device. The rightmost one is for connection to a handset, whilst the leftmost one is free for the connection of a key filler.

Any device that features a U-229 connector, should be designed in such a way that it never causes any damage, even when connected to the wrong type of equipment. Modern SINCGARS radios uses the same connector for audio, data and key-fill purposes. Connecting a key filler to the wrong connector is an easy mistake.
Fill and Audio sockets

Generally speaking, there are two different protocols for transferring cryptographic keys from a fill gun into a radio or crypto device. They are both developed by the NSA. The oldest standard is DS-102, which features synchronous data transfer at arbitrary speeds. It has gradually been replaced by the newer DS-101 standard which is based on RS-232, but many modern devices support both protocols on the same connector.
This is the oldest protocol used for key filling purposes. It describes the physical specifications as well as the data flow. Unfortunately, DS-102 is not yet in the public domain, so the description below is based on global observations and other sources [1]. DS-102 is a synchronous protocol that requires separate DATA and CLOCK signals. It uses negative logic signals, where a '0' is specified as 0V and and logic '1' as -6V. The pinout is given in the table below.

The advantage of synchronous data communication is that it can be used at various speeds (and even at varying speeds) without the need to configure the receiving device accordingly. This was particularly useful when using the KOI-18 key filler, which had a punched paper tape that was manually pulled through the filler. Whilst doing so, the sprocket hole provided the clock (CLK).
Pin DS-102 Description Note
A GND Ground (common wire)  
B - Not used  
C ACK FILL request acknowlegment  
D DATA Fill data into radio or crypto device  
E CLK Fill clock into radio or crypto device  
F - -  

This is the newer standard that replaces the earlier DS-102. Like the DS-102 standard, it was developed by the NSA and is not yet in the public domain. DS-101 is an asynchronous protocol that is based on RS-232. It has data input and ouput lines (RX/TX) plus handshaking (CTS/RTS). Data rate, format and protocol are unknown at this time.
Pin DS-101 Description Note
A GND Ground (common wire)  
B RTS Request to Send  
C RX Data out of radio or crypto device  
D TX Fill data into radio or crypto device  
E CTS Clear to Send  
F - -  

Earlier SINCGARS radios, such as the RT-1439, feature a FILL connector that can be used to program the Frequency Hopping (FH) tables. This was done using specific devices such as the MX-18290. Voice encryption was achieved by using an additional device, such as the KY-57 or the KY-99. Both these devices feature a special FILL connector that is used to load the crypto keys by means of a KYK-13 or similar. Later SINCGARS radios had built-in COMSEC and allowed a KYK-13 to be connected directly to the radio. The table below shows the pin assignment of the FILL connector as specified in the SINCGARS documentation [1].
Pin SINCGARS Description Note
A GND Ground (common wire)  
B CCD Not used for fill  
C FILL REQ-N Fill request acknowledgment  
D FILL INFO Fill data into radio  
E FILL IA Fill info available  
F MUX MUX Override. Not used for fill  

The 6-pin version of the U-229/U (called U-329/U) is also used for DATA interfaces on SINCGARS radios. Data can be send as analog tones between 300 and 3000 Hz (slow speed, e.g. on HF) or as true digital signals with a variety of baud rates between 75 baud and 16 kbit/s (ASYNC/SYNC).
The image on the right shows the RT-1439 SINCGARS radio, which has two identical 6-pin connectors on its front panel. One connector is marked AUDIO/FILL, whilst the other one is marked AUDIO/DATA. Although any analog audio device (e.g. a handset) can be connected to either connector, they are completely different from a digital point of view.

The upper connector shares the FILL function and allows a key filler to be connected. The lower connector shares the DATA interface and allows the connection of a digital serial device.
AUDIO/FILL and AUDIO/DATA connectors on the front panel of the RT-1439

A rotary switch to the left of the lower AUDIO connector is used for selection of the appropriate serial speed (baudrate). It can be set to a number of speeds between 75 and 4800 baud (4K8) in asychronous mode, or 16000 baud (16K) in synchronous mode. When in synchronous mode, one of the pins of the connector carries the clock signal (CLK). The table below shows the pinout of the connector in DATA mode.
Pin SINCGARS Description Note
A GND Ground (common wire)  
B RX Data from radio  
C PTT Grounded when transmitting  
D CLK Clock out (in synchronous mode) 1
E DIG Digital Data Model selected (when grounded) 2
F TX Data into radio 3

  1. Analog data input
    In Analog mode, this pin is used as input for the data tones. Analog mode is selected by grounding pin F. This probably bypasses any filters in the audio path of the transceiver. In synchronous digital data mode, this pin carries the clock signal (CLK).

  2. Digital data mode
    The excact functionality of this pin is currently unknown. Grounding this pin seems to select digital (data) mode. When both pin E and F are high-impedant (default) analog voice mode is selected.

  3. Analog data mode
    In digital mode, this pin is used as the DATA input (into the radio). When pin F is grounded, analog data mode (i.e. tones) is selected.

  1. Brooke Clarke, U-229 Pin Out by Function
    Retrieved from the website in December 2011.

  2. WPI General Connector, Interconnect Systems for Tactical Communications.
    Audio Connectors. pp. 6-13.

Further information

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Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Wednesday, 21 December 2011 - 14:57 CET
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