ICOM IC-92 and external GPS

The ICOM IC-92 D-Star radio can transmit GPS data while operating a D-Star repeater. In order to receive the GPS signals one would normally have to buy an external microphone with a built-in GPS receiver. This device is rather unhandy and particularly expensive: around 220.00 €. I put the radio into a box on the carrier of my bike. Therefore the GPS mic is not an option. As I already figured out the pinout of the 12pin connector I tried to reverse-engineer the GPS input.

First of all there was the voltage and maximim load to measure. The voltage applied to the external mics turned out to be +5V. In order to not damage the device with too much load I measured the current drain of the external GPS mic. That gave about 50mA.

Pin extraced for current measurement Measurement of current drain

That should be sufficient to drive an external GPS receiver. I ordered a u-blox NEO-6M module which uses around 10mA and outputs NMEA data at RS-232 TTL level. So I would need an extra MAX3232 level converter which also takes 10mA. So this is a good combination to use with the radio.

From sources on the net I learned that the IC-92 could also be supplied with GPS in NMEA format via the data interface that is normally used for programming the radio. For a test I attached the radio to a FTDI USB-RS32 converter and fed the radio from a GPS simulator on the computer. I used gpsfeed+ (see [1]).

Simulated GPS data shown on aprs.fi

Choosing the correct parameters for the serial interface (4800 8N1 no handshake) made the radio immedeately show a valid position and that was also trasmitted to the D-Star repeater. The result could be seen on an APRS website presenting the position data on a map like aprs.fi (see [2]). As I used the default settings of gpsfeed+ I was circling around Athens temporarily.

References

[1] http://gpsfeed.sourceforge.net/
[2] http://www.aprs.fi/