U-229 Pin Out by Function

© Brooke Clarke 2004 - 2013

Connector Types


There have been a number of audio connectors used on military and civilian equipment.  The U-229 family is the current military standard audio connector.  There are 5 and 6 contact versions of the connector.

This connector started out as just an audio connector since at the time radios were all analog in nature.  But with the advent of digital circuitry this connector started handling digital data in addition to the audio data.  Note that data can be transmitted over radio or wire line audio channels using tones in the range 300 to 3000 Hz, called Audio Data in the SINCGARS radios.

Note that I've been trying to figure out how this connector is used for many years.  My Audio Connectors page was started in 2000, the U-229 Audio Accessories page was was also started in 2000.  The thumbnail photo Military Audio Accessories page is from 2001.  This page was done in 2004.

The different uses for the U-229 type connector needed to have been designed with the idea that although they all used the same mechanical connection, plugging in a device used for one task into a jack for another should not cause damage to either.  This seems to be working, an impressive design.


Pin A Common Ground

Pin A is used as the common ground for audio and is typically the same as chassis ground.  This applies for all audio applications that I know of.
Memory aid: All the grounds are on pin A.

Pin B Handset speaker

Pin B is used for the radio's audio output that is to drive a handset speaker. 

Back when 1/4" phone plugs & jacks were used for headphones the jack could also be a switch so that an internal speaker would be disconnected by the mechanical action of inserting the 1/4" phone plug.  That function is still needed with the U-229 family connectors but it's now done by sensing the resistance between pin-A (gnd) and pin-B (headphone).  For example, with a DC resistance <10 K Ohms (PRC-126) or <3.3 K Ohms (PRC-68) the internal speaker is muted
PS. The problem with 1/4" jacks is that they allow sand and/or water to get inside the radio, not a good thing.  Note the U-229 family are sealed to prevent that.
Memory aid: The capital letter "B" when rotated 90 degrees clockwise looks very much like the schematic symbol for headphones which are on pin B.
U229 Connector memory aid for Pin B Headphones

Pin C Push To Talk

Pin C is the Push to Talk release to listen line.  When grounded it puts the transmitter into transmit mode and when released into receive mode.  Note that this can not be considered as either an input or output, but rather as a buss line.  Any equipment connected to this line can assert (ground) it.

In radios that have a CW mode, like the PRC-70, PRC-138 and HarrisRF-5000 series you use the PTT line for CW keying.  If the radio only supports SSB modes but not CW, like the PRC-104,  you need to use pin E for keying CW since using the PTT line on a SSB mode does not turn on a carrier. 

Memory aid: after you have accounted for pins A, B and D, pin C is the one that's left.  Only 4 pins are used for the audio functions.

Pin D Mike

Pin D is the microphone input to the radio. Dynamic mikes are used with the U-229 family whereas carbon mikes were the common military mike on earlier versions of the connector like the U-77 or 1/4" phone plug

In simplex radio applications the Push To Talk (PTT) switch not only closed contacts to activate the transmit function of the radio the PTT also has contacts that disconnect the mike when released.  This makes is harder to modify a radio so that it can be used to spy on it's operators.  For example the H-250 handset.

In duplex telephone applications the mike and earphone are used at the same time so there is provision for a hot mike as well as a switched mike.  For example the H-350 handset.  The H-350 may also be used with some voice encryption equipment.

Memory aid: The capital letter "D" looks very much like the schematic symbol for a microphone which connects to pin D.
U229 Connector memory aid for Pin D Microphone

Pin E Not Standardized

For classical audio applications pin E is not standardized.  Different radios use it for different purposes.  For example:
PRC-68 Family of Squad Radios uses pin E for "12 Volt" vehicle power input.

The HF manpack radios PRC-74 and PRC-104 use pin E for the CW key input, and it's also used for high speed CW like from a GRA-71.  Note that the pin C PTT line can NOT be used for CW keying on a radio that supports SSB operation, since there would be no RF output.

The VRC-12 series Squad radios uses pin E as the loudspeaker line to drive the LS-454.

The PRC-25PRC-77 AUDIO and the RT-246, RT-524 & R-442 RETRANS use pin E as the retrans PTT to key the other RT with the Mk-456 Retransmission Kit.  (See Retransmission below)

Pin F Not Standardized - DC Power, Retransmission

This is the center pin that is not present on a true U-229 connector.  Many radios install the 6 pin version of this connector, yet do not connect a wire to pin F.  Maybe it's done to allow a future modification, or maybe just to standardize the connector?  Some equipment does use this pin where the other pins are used in the above audio functions, for example:
The PRC-68 Family, except the PRC-68 and PRC-68A, use this pin for digital up and down loading of the channel frequency assignments and as the retransmission PTT output.  This supports a retransmission cable and cloning cable that are different than the Mk-456. 

Some modern radios use the center pin F as a source of DC power for things like speaker mikes.  If you have one of these radios would you tell me the open circuit voltage on pin F and the voltage when a load of 470 Ohms is applied to pin A (Ground)?

The RF-5800V-HH hand held radio has 3.3 V through a 1k resistor on pin F.
According to the data sheet for the L3Harris RF-5963-HS10X maritime Headset the following radios support 5V on Pin F:
Harris Falcon III®
RF-7800M-HH and
AN/PRC-159 radios.

The URC-200 uses pin F for not squelch (retransmission).


This is the connector that's now used for filling the crypto keys into all kinds of equipment.  The SINCGARS AUDIO/FILL connector is probably the same as most U-229 family fill connectors.  The CV-4228 PC to SINCGARS Fill Cable uses these pins.  The SVM-68 (KYV-2) does NOT use this pin out, maybe because it was a first generation device that has serious security issues.
not used for SINCGARS fill na
Fill Request-Acknowledgment from Radio
(might be the PTT button on a hand set connected to the radio)
Fill Data into radio Data
Fill Clock into radio
(probably in the 1 kHz to 4 kHz range, to match manually pulling a paper tape)
not used for SINCGARS fill na

DS-102 is a fill data protocol for red keys and specifies the logic levels to be used.  The logic levels are 0 volts and -6 volts.
Note that the BA-1372 (now BA-5372) battery is "backwards", that's to say the the Tip is negative and the flat bottom is positive.

DS-101 is a newer fill data protocol for red keys and uses standard TTL level (RS-485) logic levels.

The format of the fill data is classified.  Some ideas about it are:

First, a device that uses fill data needs to know if it has valid fill data.  For example the SVM68 (KYV-2) secure voice module for the PRC-68 family radios will function when no key is loaded.  This is a huge problem in that an enemy could just zeroize a KYV-2 and the operator would think they were crypto covered but were not.  One way to accomplish this is to use odd byte parity (Wiki: DES) this way an all ones or all zeros data set would violate the parity.

Second, a device that uses fill data needs to know that there was not a data glitch during the fill operation.  Serial ASCII data (for example RS-232) has provision for parity bits so that each byte can be tested for validity.  In addition blocks made up of some number of bytes can have a check byte added that not only detects some number of errors it can also correct a smaller number of them.  This is called cyclic redundancy check (Wiki: CRC)

Third, a device that uses fill data needs to know that the key is intended for it and not some other cryptographic equipment.  For example the key used for a GPS receiver (DAGR, PLGR) should not also work to key a voice encryption box (KY-57).  There are a number of ways this might be accomplished.  The design of the CRC could be different.  One or more of the data blocks might contain a registration key that's specific to the device, etc.  It's important that this be a very robust CRC for applications where Over The Air Rekeying (Wiki: OTAR) is used and bits are commonly lost or garbled.  Also see (Wiki: DES)

Forth, every bit that's used for CRC or device specific ID is a bit not used as a key variable.  The size of the key variable determines the difficulty of a brute force attack.  DES uses 7 bytes (7 * 8 = 56 bits of key and 1 byte (8 bits of CRC) for a total of 8 bytes per block (64 bits).

Fifth, to minimize the amount of key material the same key is used for encryption as for decryption (Wiki: Symmetric-key algorithm).


The SINCGARS, and probably other radios, have a U-229 family connector marked ADUIO/DATA.
When an audio accessory is connected, like the H-250 handset, the above AUDIO pin assignments are active.
Note that the SINCGARS radio has two AUDIO connectors, one marked AUDIO/FILL and the other marked AUDIO/DATA.  So the digital functions for Fill and Data are completely different circuits.  Audio can be used on either connector, but the digital modes only work on the connector labeled AUDIO/DATA.


Analog data is like an FSK modem or FAX machine that uses audio frequencies between 300 and 3000 Hz.  The Tacfire and PSG-2/5 may use these tones.  This method can be used with either HF or VHF radios, depending on the baud rate and so is a popular way to send low speed data.  In the SINCGARS these are the AD2 (also called TF for TacFire) and AD1 modes.


Digital data is like form the UGC-74 teletype, a computer serial port, etc.  The upper speed limit for this type of data is 16 kb/s, which is the rate for the KY-57 and the maximum rate for the SINCGARS radios.  Baud rates are: RT-1439: 75, 150, 300, 600, 1200, 2400, 4800, 16000 and for the RT-1523 the rates are: 600, 1200, 2400, 4800, 16000 bps.

MIL-STD-188-114, 6V polar, NRZ  differential.
Definition: (Formerly "RS-449") An EIA standard for a 37-pin or 9-pin D-type connector (functional- and mechanical characteristics), usually used with EIA-422 or EIA-423 electrical specifications.  RS-422, RS-423, and RS-449 - A Compatible Improvement of RS-232-C
Note that RS-422 is differential, but can be directly connected to an unbalanced RS-232 port and I expect that MIL-STD-188-114 works the same way.  So some interfaces use just one of the differential lines. When it's an output the unused line is left floating, and when an input the other line is grounded.

However when a data device is connected and the proper mode has been selected the pins have new meanings as follows:

Data Connector Pin Functions

For a SINCGARS radio:
A = ground
B = MIL-STD-188-114 Digital Data out of Radio
C = PTT to key radio (ground for Talk) [this is a buss and any device can ground it]
D = Analog Data Input for transmission or Digital Data Clock Out
E = Digital Data Mode Select - Gnd = Digital Data, Open = voice or Analog data (handset leaves this pin open)
F = Analog Data Mode Select - (ground for digital data) or Digital Data for Transmission input (handset leaves this pin open).

Possible Configurations (just a guess)

Data Rate Selector
Pin D
Pin E
Pin F
Mike in
hi open
hi open
Analog Data (Tones)
baud rate < 16000
Tones in
hi open
asynchronous Digital Data
baud rate < 16000 n.c.
Data In
asynchronous Digital Data2 any baud rate
hi open
Data In
synchronous Digital Data baud rate 16000
Clock Out
Data In
I think that when a baud rate is selected, other than 16 kb, that asynchronous data transfer is used.  But when 16  kb is selected the clock lines are needed. 

Note1 - Note that when both pins E and F are open (high) the AUDIO/DATA connector on a SINCGARS radio is expecting pin D to act as a mike input.  If low level digital data is fed into pin D there will be some strange modulation on the output, but it will not be usable by a receiving station.

Note2 -  Another possible way to differentiate between asynchronous and synchronous would be to leave pin E open.   If you know please email me.

Since the KY-57 would need to interoperate with a SINCGARS radio in both voice and data modes, I think it also responds to data on it's AUDIO connector using the above format.

Typically a special cable will need to be made up, see TM 09726-13.


Since the RT-1439 uses a separate connector just for retransmit I'm making a new paragraph here for this function.  When a U-229 type audio connector is used for retransmit ground is on pin A, the audio out from the radio is on pin B, PTT on pin C and audio in on pin D just like a standard audio connector.  The special signal needed in addition is a squelch closed signal to drive the PTT on the other radio (note PTT is a ground true signal so I called the output signal squelch closed meaning not squelch open asserts PTT).  Pin E is used for the retransmission output on the PRC-25PRC-77, RT-246, RT-524, R-442 RETRANS with the Mk-456 Retransmission Kit.  The SINCGARS RXMT connector also uses pin E for the retransmission output and in addition uses pin F as an A/D select (Analog pin open and Digital when grounded).

The VRC-12 series radios may also use AUDIO pin F for the retrans keying output.

SINCGARS Missing Functionality

The PRC-68 Family, except the PRC-68 and PRC-68A, use pin F for digital up and down loading of the channel frequency assignments (cloning) and as the retransmission PTT output.  As far as I know there is no way to load a SINCGARS radio with Single Channel frequency assignments, i.e. there's no cloning cable.  This seems strange that a more modern radio does not support cloning.  My guess is that there's a way to clone a radio but it's not documented.

Also it seems strange to me that there's no computer remote control function.  For example almost all modern test equipment that contains a micro processor has a remote control mode of operation where you can do anything that can be done on the front panel using RS-232 or IEEE-488 as the interface protocol.  Again I think there's a way but it's part of a "factory test mode" and not in the manuals.

The Harris radios, like the PRC-138, support remote control using a hardware RS-232 terminal (like a glass TTY or hardware TTY with an RS-232 port) or any computer that has a terminal program.

Connector Types

There are plugs and receptacles.  Any plug will mate with any receptacle. 


A typical use would be for an audio accessory like the H-250 handset.  The older plugs have a flat which should be pointing up just prior to mating.  The newer plugs have a "hump" that points up when the rotation is correct to mate with a panel receptacle.  The "hump" also gives you a longer lever arm for attaching and removing the connector.  The electrical contacts are fixed (not spring loaded).

Cable spring
strain relief
M55116/1, /3
GC 329 6 Contact Audio Plug
old style with flat
Newer stype
                GC329 with Hump
                AP-126 Shell
new style with hump

Cable spring
strain relief
GC 329
M55116/2, /4

Cable Molded
M55116/5, /7

Cable Molded 6

M55116/6, /8 AP-155
GC 729 6 contact Audio Recp MC-729/U
GC 729


The electrical contacts are spring loaded.  A typical use would on the front panel of a radio like the three on the RT-1439.

Mounting #
M55116/9 na
GC 283 6 contact Audio Receptacle  MC-283/U Panel 6
GC 283
M55116/10 na
Cable spring
strain relief
Cable spring
strain relief
GC 328
(GC)Drawings from General Connector web page
(PC) Power Connector
(Nx) Nexus MS-5116 connectors
Cooper Ind. (EATON): catalog.pdf
Amphenol Series 164: catalog.pdf


The most common problem is a dry O-ring.  This will make it difficult to mate and to uncouple.  I like Radio Shack Lube-Gel which is an oxygen free Silicon grease (PTFE).  There are other Silicon greases that probably work the same, but the ones I've found locally cost at least twice as much.

There is an O-ring lubricant sold for use on paint ball guns but so far I have not been able to find out what it is, so can not make a recommendation.  I have the bottle in a ziploc bag in the freezer to see what happens.  After a few days it's still a liquid.  But no MSDS from the maker.

(NSN 5330-00-905-6032)

This has been reported as being a metric 1.5 mm by 15.5 mm OD Chloroprene (generic Neoprene).
A Buna-N version is available from McMaster Carr as p/n 9262K636

Cross Section
Center Hole
0.061 / 0.055
0.615 / 0.605
1.5 mm = 0.059055" 15.5 mm = 0.610236
This looks good. I received the O-rings and they do work.


Advanced Pacific uGATE

This looks like a prototype judging from the circuit modifications.
If you have any info on this let me know.
Fig 1 Operator Panel
Advanced Pacific
                  uGATE Audio Flow accessory
Fig 2 Dual Radio Connection & LED
Advanced Pacific
                  uGATE Audio Flow accessory
Fig 3 Inside Components
Advanced Pacific
                  uGATE Audio Flow accessory
Fig 4 Inside Ultralife UBP002 Li-ION battery
Advanced Pacific
                  uGATE Audio Flow accessory

This is an audio combiner box made by "Advanced Pacific" marked "uGATE J3".
It has an LED and two GC-726 audio jacks, like may be used with a male - female extension cable from a pair of radios bringing their audio to this box.
The other end of the box therea single GC-283 audio jack so looks like the audio connector on either of the radios.
There are a couple of 3-position control switches.
FLOW: (A to B), (A to/from B) or (A isolated from B)
Operator Transmit (OPR XMIT): A, A+B or B

Inside there's an Ultralife UBP002 3.7 Volt Li-Ion single cell battery rated for 3.4 Watt hours.  It was totally dead, but does take a charge.
So, this box needs to be connected to a radio that has DC power on the AUDIO connectors to maintain the internal battery (not a good idea since no user will know that's the case).  The center pin (F) on both the A and B connectors shows 3.2 Volts when the battery is charged to 3.7 Volts.  Applying 12 Volts to any of the audio center pins shows no current draw so it' snot clear how the internal battery gets charged?

Also inside is a PIC24FJ256 microcontroller which is in production as of July 2015.  This is an IC optimized for graphics use, maybe it also can be used for digital audio?

When +20 volts is connected to the center pin of any of the 3 audio connectors no current is drawn.
The voltage on the "A" and "B" audio connectors is 3.2 (slightly lower than battery voltage).
Battery negative is chassis ground.
When an H-250 handset is connected to the operator's connector and PTT pressed, you can hear side-tone in the speaker.
The enclosure is an aluminum extrusion with a central groove for the printed circuit board and no holes (i.e. no place to have a battery charging input.
So, how the battery gets charged is still a mystery.


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page created Aug. 8, 2004.