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

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

The image above shows the pinout when looking at the contacts of the female receptacle (left) and male receptacle (right). Many different versions of the connector and its mating cable parts exist, such as 5- and 6-pin variants, male or female and cable or panel mount. Although each variant has its own specific type number, they are commonly incorrectly referred to as U-229. The table below shows the correct nomenclature. U-229/U is the 5-pin female cable mount part.

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 cryptographic 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 is the Dutch RT-3600 radio, which has its microphone (MIC) and speaker (SPK) wires swapped. The reason for this is unknown, but it often leads to confusion and frustration 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 can 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 that case, pin E is used for CW.

  3. External power or fast CW
    The function of pin E is not standardised, so different radios used it for differentpurposes. On some radios, such as the PRC-68 family, it is used as a 12V power input. Some manpack radios, like 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 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 easily made 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 endorsed 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 through the same connector.

This is the oldest protocol used for key filling purposes. It describes the physical specifications as well as the data flow. In the early 2000s, the DS-102 protocol was freely available on the internet, but has since been removed. From surviving documents it is known that it is a synchronous data protocol with negative logic, in which a '0' is represented by 0V and '1' is represented by -6V.

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 - -  
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 provides the clock (CLK).

 More about DS-102

This is a later (now current) standard that superceedes the earlier DS-102. Like DS-102, it is endorsed by the NSA and its description is not available in the public domain. It is a serial asynchronous protocol that runs over an RS485/RS232D interface, with data input and output lines (RX/TX) plus handshaking (CTS/RTS) and runs at 64 kb/s using the HDLC data protocol.

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 - -  
The advantage of using asynchronous serial communication is that it can be integrated more easily with applications running on modern computers, either directly from an RS-232 port (COM port) of an older Personal Computer (PC) or via a suitable USB-to-RS232 adapter.

 More information about DS-101

Earlier SINCGARS radios, such as the RT-1439, have a FILL socket for programming the Frequency Hopping (FH) tables. This was usually done with a specific transfer device like the MX-18290. The table below shows the pinout of the FILL connector as specified in the SINCGARS manual [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  
On non-COMSEC devices, voice encryption was achieved using an external voice encryptor, like the KY-57 or KY-99. These devices have a separate FILL connector for loading the cryptographic keys, which requires a KYK-13 key transfer device, or similar. Later SINCGARS radios have built-in COMSEC facilities and allow a KYK-13 (or equivalent) to be connected directly to the radio.

DATA   U-329/U
The 6-pin version of U-229, known as 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. The other one is marked AUDIO/DATA. Although any analog audio device (handset, microphone, speaker, headset) can be connected to either connector, the digital signals carried by the data pins are completely different.

The upper connector shares the FILL function and allows a key fill device 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 filtering 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-impedance (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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Crypto Museum. Created: Tuesday 20 December 2011. Last changed: Monday, 23 January 2023 - 14:45 CET.
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