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Author Topic:   Diagnosis of GPS Receiver to Radio Transmitter NMEA-0183 Communication
Tanglefoot posted 06-09-2009 10:03 PM ET (US)   Profile for Tanglefoot   Send Email to Tanglefoot  
I have my GPS connected to my Icom radio as was explained in a previous post (at least, I'm pretty sure I have) but I still get the message on my radio, 'no gps connected'. Any suggestions how I should go about troubleshooting the connection or the gps signal output?

Thanks, Tom

Bella con23 posted 06-09-2009 10:46 PM ET (US)     Profile for Bella con23  Send Email to Bella con23     
What model Icom and GPS?
jimh posted 06-09-2009 11:36 PM ET (US)     Profile for jimh  Send Email to jimh     
This is something of a classic problem in diagnostics. We have two devices, a GPS receiver which puts out serial data, and a VHF Marine Band transmitter, which listens for serial data. When the two are connected we do not get the anticipated result. How can we diagnose the problem?

We need a third device. When you have two devices which are trying to communicate, and if there is no communication, you need a third device to help distinguish which of the two original devices has the problem. The third device has to be a device that is known to be working properly. It can either be a sender or a listener. You replace one of the two devices under test with the known good third device. If the communication link begins to work, you can reasonably conclude the device that has been replaced was the source of the problem. If introduction of the third device does NOT provide a cure, the results are a bit less positive, but it is likely that the other original device, the one still in the circuit is bad.

In the case of a GPS receiver and a radio transmitter, it might not be easy to find known good replacement devices for the test. In that case we can use some generic test equipment. Insert a test instrument in the circuit that connects the two devices. Observe the signal levels. In the case of serial data communications, the best common test device may be an oscilloscope. Lacking an oscilloscope, a voltmeter ought to provide some indication of electrical activity on the circuit.

Most of these problems are simpler still; you just have the wrong wires connected.

Once you verify the electrical layer, move up to higher layers. Is the serial data communication speed set correctly? Check all the serial data parameters.

Is the higher protocol format set correctly? Are you sending the data you need? This is hard to test without some very specialized test equipment.

Tanglefoot posted 06-10-2009 01:50 AM ET (US)     Profile for Tanglefoot  Send Email to Tanglefoot     
The receiver is an Icom M422 and the GPS is a Lowrance 525C-DF.

I was thinking of attempting to measure electrical activity on the GPS out wire but I imagine the voltage and the fluctuation would be quite small. I'll give it a go tomorrow.


jimh posted 06-10-2009 09:42 AM ET (US)     Profile for jimh  Send Email to jimh     
If you have a laptop computer with a serial port, you can use it as a diagnostic tool to check the output of the GPS receiver serial data. Configure the laptop serial port to the same configuration used for serial data communication parameters used by the GPS receiver. Run a standard terminal application on the laptop which will use the serial port as input and output. Connect the NMEA-0183 signal to the RS-232 serial port input. Run the terminal software and open the serial port for I/O. You should see a stream of serial data displayed in the terminal window. The data would look something like this:


Unfortunately, many of these diagnostic procedures are more difficult to connect and configure than the original serial data link itself. This creates a bit of a paradox in that if the installer has enough skill and knowledge to investigate and analyze the serial data communication failure, they probably would not have the initial problem. In almost all cases I have read, these problems are related to simple mistakes in the electrical connection between the units.

Phil T posted 06-10-2009 10:38 AM ET (US)     Profile for Phil T  Send Email to Phil T     
Have you reviewed the gps manual to see if the gps has a setting to transmit to an external source?
Phil T posted 06-10-2009 11:57 AM ET (US)     Profile for Phil T  Send Email to Phil T     
I located the owners manual for your GPS here:

On page 155, it shows you need configure the communications port for NMEA 0183

Are you sure you used the 4 wire data cable and not the 3 wire NMEA network cable as shown in Diagram A on page 32

Blackduck posted 06-10-2009 04:20 PM ET (US)     Profile for Blackduck  Send Email to Blackduck     
Have you gone to the communications tab, (Comm), and selected either port 1 or 2? You must tell the GPS what port you are using, so the radio can find it. If that doesn't work, most likely wired wrong.
jimh posted 06-11-2009 08:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Here is the serial data output from a GPS receiver:


This is the sort of data you would see if your GPS were sending data on its serial data output using NMEA-0183 protocols and you monitored it with a terminal.

Tanglefoot posted 06-14-2009 01:28 AM ET (US)     Profile for Tanglefoot  Send Email to Tanglefoot     
I have not configured the GPS, didn't realize I had to but I sure will now! Thanks for all the input - I'll post a report.
Tanglefoot posted 06-14-2009 02:14 PM ET (US)     Profile for Tanglefoot  Send Email to Tanglefoot     
That did it!! Things go as they should when you get help from those who know what they are talking about. Thanks a bunch!


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