U-229

Pin A Common Ground

Pin A is used as the common ground for audio and is typically the same as chassis ground.

Pin B Handset speaker

Pin B is used for the radio's audio output that is to drive a handset speaker. In most radios with an internal speaker, like the PRC-68 family, there is a DC signal imposed on top of the AC audio signal so the radio can detect if a handset/speaker has been connected and if so the internal speaker is muted.

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.

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 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.

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 output.

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

The PRC-25, PRC-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.

Pin F Not Standardized

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.