PC Interface for Military Radios Using U-229 Audio Connector

REV.  0.55 - 24 May 2009
This is a work-in-progress.  Please keep this in mind if you want to duplicate this circuit.  So far, testing has been limited to the shop/shack where the equipment is not in the immediate field of the antenna.  The radio ground and PC ground are connected together via the AC ground and the transistor circuit driving the PTT/KEY line.  To date, RF filtering has not been necessary.  Things may be different in a field environment.

If you have comments or suggestions, email me at:        

N3FRQ homebrew interface being used with AN/PRC-104 and Acer Aspire "netbook".  All components fit nicely in the medium Alice pack.

I've been running "Digital Master 780" a part of the "Ham Radio Deluxe" distribution.  

It's free here:  http://www.ham-radio-deluxe.com/
Interface is housed in a small Plano tackle box from Wal-Mart.  All cords, except for the RS-232 dongle, fit inside.
Pushbotton at upper left activates the KEY line for tuning.  Slide switch selects whether the RS-232 Request To Send line activates KEY or PTT.  LED indicates that the computer has activated the transmitter, while VU meter indicates appropriate audio output from the computer.  Output from the Acer is too low to get a good meter indication, but still sufficient to produce full modulation of the PRC-104 with the attenuator shown in the schematic.  Perhaps the meter is not needed, but it reasurring to know the signal is getting this far.  Maybe monitoring the transceiver sidetone is sufficient indication.


Receive audio from the radio is applied to a 600-ohm to 600-ohm transformer.  The 1K:47-ohm voltage divider reduces the signal to an appropriate level to apply to the MIC input of the PC.  

Transmit audio from the computer is applied to a second 600:600 transformer.  A VU meter indicates the signal level from the PC, about 0.8 VRMS in this case.  The 1K:4.7-ohm divider reduces the signal to an appropriate level for the radio's microphone input.  The voltage dividers may need to be adjusted for your particular application, but these values shoud be a good starting point.

The PC software uses a serial-port line, typically RTS, for DC control functions such as PTT or to KEY a CW transmitter.  With most modern PC's, this requires the use of a USB to Serial "Dongle" and appropriate driver software.  The RTS line drives the radio via an opto coupler or transistor.  An LED in series with the RTS line indicates that it is asserted.

A SPDT switch applies the control signal to PTT or KEY as needed.  A momentary switch on the outside of the box can be used to key the transmitter manually, e.g., to force a "tune" cycle.

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