U-229

Pin A Common Ground

The common ground for audio and is typically the same as chassis ground.

Pin B Handset speaker

Audio output to the handset speaker.

Pin C Push To Talk

When grounded it puts the transmitter into transmit mode

Pin D Mike

Microphone input to the radio.

Pin E 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.