Furuno 1724C plus Nasa AIS original plus Garmin  GPSMap 78 hookup


The challenge of buying an old boat includes trying to interface existing electronic navigation gear with stuff you really need and want. Case in point is our recently acquired Nordic Tug 26 built in 1983. She came equipped with a very old nonworking Garmin GPS and a newer Furuno radar unit which were used as standalone units.  The Furuno radar unit did not come with a chart chip or any data cables to allow external GPS attachment.

I had a newer Garmin GPSMap 78 and an old Nasa AIS engine that I wanted to interface with the Furuno radar unit.

The Garmin GPS has an optional cable that supplies power and a serial cable female plug.

The original NASA AIS unit has a female serial port connector built into the box. Supplied with the unit is a serial cable with a male end that has an extra blue wire attached. The other end of the cable does not have a connector. I lost this cable some time ago and replaced it with a 9 pin serial cable from a discount electronics store.

The Furuno 1724C has 3 data ports and one network port, two of the data ports are 7 pin and one is 6 pin. The 7 pin cables can be ordered from amazon   as part number Furuno 000154028 1 x 7 pin Connector NMEA Cable (5 Meter).

Short Version to hook up:

1.      Cut off the Garmin cable serial plug  to expose 3 wires Yellow (RS232 OUT, 4800), White (RS232 IN, 4800) and Black (common ground). The White wire is used to connect to a PC serial port pin 2 to allow data upload and is not covered here. Attach the cable to the GPS.

2.      Modify the purchased 9 pin serial port cable by cutting off the female plug end to expose 9 wires. Use a VOM or DVM to determine which colour wire is attached to which pin in the male plug. You will be using pins 2, 5 and 9 only. Pin-out looking at the male plug end   is 1 2 3 4 5 top row, 6 7 8 9  bottom row …. Left to Right. Attach the cable to the AIS unit. Pin 2 is a RS232 output and Pin 9 is a RS232 GPS input. Pin 5 provides a common ground.

3.      The Furuno cable shipped only has a 7 pin connector. The coloured wires have different functions depending on which socket it is plugged into. The Data 1 socket supports the correct speed for the AIS unit and its colour code is

a.      White(+)  TDA NMEA transmit

b.      Blue (-)  TDB NMEA transmit

c.       Yellow(+)  RDA NMEA receive

d.      Green(-) RDB NMEA receive

e.      Red (+12v)

f.        Black(-12v)

g.      Shield Ground

Please notice that I have used 2 terms to define the wires. NMEA and RS232. RS232 uses one wire for data transfer and a common ground. NMEA or more correctly RS422 uses two wires for data transfer (balanced transmission line) and no common ground. There are converters available for doing the magic RS422 to RS232 conversion but none is required if the distances are short and there is minimal electrical noise (no florescent lights). Confusion exists because RS232 and RS422 are wiring definitions while NMEA is a data content definition which works with both RS232 and RS422. Labelling of RS422 wires can be very confusing since manufactures seem to take great delight in creating their own versions. You can simply connect the RS232 data wire to the RS422 + (A) wire and the RS232 ground wire to the RS422 – (B) wire.

NMEA TDA is the transmit + signal that can be connected to the RS232 data input wire

NMEA TDB is the transmit – signal that can be connected to the RS232 common ground

NMEA RDA is the read + signal that can be connected to the RS232 data output wire

NMEA RDB is the read - signal that can be connected to the RS232 common ground

The basic concept is to feed a Garmin GPS signal into the NASA AIS unit and have it add AIS data, increase the transmission speed to 38,400 and pass the result to the Furuno display. Wiring is simple and using the colours from my cables is as follows:

             GPS yellow (data out) to AIS black (gps data in, pin 9)

            GPS black (ground) to AIS green (pin 5) to Furuno green (NMEA – RD-, pin 4)

            AIS red (AIS data out pin 2) to Furuno yellow (NMEA – RD+, pin 3)

Set up port 1 on the Furuno to handle 38,400 and set NAV Source to all and the port monitor function should show data flow.


That’s all folks. If you need to add LEDS to watch the data transfer then put LEDs with 180 ohm current limiting resisters on the following junctions:

1.      For GPS data traffic to AIS put LED anode to GPS yellow, AIS black connection. LED cathode to ground.

2.      For AIS data traffic to Furuno put LED anode to AIS red, Furuno yellow connection. LED cathode to ground.

For most LEDs the long leg is the anode and the short leg is the cathode.