DIY GPS receiver

[ctva]

Following on from my recent question on getting a GPS receiver with both nmea 0183 and 2000 to supply a new B&G instrument network and an old Icom m505 DSC. VHF, it became apparent that the cost of the proprietary converters would make it cheaper to buy a new radio, I came across this...
https://proto-pic.co.uk/gps-receiver-gp-20u7-56-channel/?gclid=CKGDrpLrhMwCFY8y0wodR9QD2A

Now I may be barking up the wrong tree or just being totally naive, but has anyone either the experience of using this receiver with its nmea0183 output to provide the gps signal for an Icom or similar radio? From all the enquiries that I have made with the supplier, the datasheet and other articles on the Internet, this should work, but it is only £15 so rediculiously cheaper than any other solution. Am I missing something? Any help of info would be appreciated.

Thanks

Chris

 

[Alpha22]

The only thing I can see as a problem for that one is it runs on 3.3v logic and not 5v as your radio probably expects. Other are available that run on 5v, you need to find one of those to try.... you will need some way of stepping down the boats 12v system to 5v to power it up....consider electrical 'noise' as many cheap voltage regulators and USB cigarette lighter adaptors will totally 'wipe out' your VHF reception.

In theory it should work.... these are the chips that are embedded in all modern electronics.

 

[maby]

They are decent devices - I'm using one in the anchor-drag alarm that I'm building. I've also been thinking of using one to provide the GPS imp input to our DSC radio. I think the output voltage is lower than a standard GPS, but probably within range.

 

[prv]

The receiver for my VHF and AIS is essentially a 5v one of those, in a simple plastic housing, for £5 more.

I've also made a standalone speedometer (for use on the Contraption) using a similar module talking to an Arduino.

I think the module defaulted to a speed of 9600bps which your VHF might not like, but it will be possible to reconfigure it to 4800 using some PC software available for download.

In summary, yes, it's definitely an option. But the 5v version is probably a better choice for signal compatibility with the radio.

Pete

 

[ctva]

The receiver for my VHF and AIS is essentially a 5v one of those, in a simple plastic housing, for £5 more.

I've also made a standalone speedometer (for use on the Contraption) using a similar module talking to an Arduino.

I think the module defaulted to a speed of 9600bps which your VHF might not like, but it will be possible to reconfigure it to 4800 using some PC software available for download.

In summary, yes, it's definitely an option. But the 5v version is probably a better choice for signal compatibility with the radio.

Pete

Dumb question here... The only connection to the VHF would be the NMEA+ and - so is this what needs the 5v rather than just powering the GPS receiver itself?

Also, can you let me know the 5v version that you have used?

It is a looooong time since I've played with electronics like this so I'm very rusty but keen to give it a go.

Many thanks for all the help above too. I'll get there yet!!

 

[prv]

Dumb question here... The only connection to the VHF would be the NMEA+ and - so is this what needs the 5v rather than just powering the GPS receiver itself?

I'm not an expert particularly, but I'd expect the GPS module's NMEA + voltage to be the same as its power supply voltage. The VHF is officially expecting 12v; its tolerance will be wide, but it still seems to me better to choose the voltage which is closer to that (5, not 3) given that both are available.

Also, can you let me know the 5v version that you have used?

So I bought a packaged receiver like this one:

http://www.ebay.co.uk/itm/Globalsat...234947?hash=item5426626303:g:uXgAAOSwq7JUBTqG

The "PS/2" plug is a red herring, it doesn't actually conform to any particular standard, and you cut it off to expose two 5v power supply wires, an NMEA out, and an NMEA in which can be used for configuring the GPS but which isn't needed for our purposes.

But you can also get bare modules similar to the one you originally linked to.

Pete

 

[maby]

Pete is pretty much right on this...

Strictly speaking, NMEA0183 uses RS422 signalling which is balanced with the signal voltage swinging both sides of earth. In practice, almost all NMEA0183 devices are actually built to work with RS232 signalling which is unbalanced and requires a minimum level of 3v for the input but still expect the input signal to swing both sides of earth.

These little GPS modules can only provide an unbalanced output - they have a single power supply which may be either 3v or 5v, but they can never drive their output line below 0v, so they cannot be balanced. Driving a balanced input from an unbalanced source will make it a lot more vulnerable to interference, so keep the wires short and the power supply clean.

Do also bear in mind that some DSC radios output on the NMEA0183 port as well as receiving positional data as input on it. This can, for example, provide positional data to a connected plotter to display the location of an emergency call. The radio is likely to output a much higher voltage than your little GPS module can accept, so don't be tempted to connect the NMEA0183 output line of the radio to the input line of the GPS - you will probably fry it!

 

[prv]

Do also bear in mind that some DSC radios output on the NMEA0183 port as well as receiving positional data as input on it. This can, for example, provide positional data to a connected plotter to display the location of an emergency call. The radio is likely to output a much higher voltage than your little GPS module can accept, so don't be tempted to connect the NMEA0183 output line of the radio to the input line of the GPS - you will probably fry it!

And even if it didn't, the connection wouldn't achieve anything useful

Pete

 

[elton]

Little projects like this are very satisfying, when they work. But it's a lot easier, and possibly cheaper, to get an old 2nd hand Garmin handheld off ebay, and wire it straight in. Most can power off 12V, and there are no interfacing issues, other than the plugs. Also, they have the advantage of having a display, and all the functionality you would expect.

e.g. http://www.ebay.co.uk/itm/Garmin-GP...311976?hash=item3d16f75128:g:mXAAAOSw0QFXCTyx

99p ! That's what I've got wired to my Navman 7100

 

[prv]

99p !

It's an auction on its first day, that starting bid bears no relation to its final price.

Does it come on automatically when you apply 12v?

Seems far more elegant to me to just have a discrete black box glued to the underneath of the deck behind the panel, than to have a device with an unused screen and buttons similarly tucked away.

The modules use modern GPS chips which have a much faster time to first fix, and are probably more sensitive too. My VHF with GPS puck is next to a Garmin GPS128 used with the Yeoman, so I can see the difference between old and new.

If it works for you then great, but I wouldn't choose or recommend an old handheld as a black-box position source for a VHF.

Pete

 

[rogerthebodger]

Even I have 3 old Garmin's I went for a GPS mouse to feed my OpenCPN PC , VHF radio and Angus Crew Saver. The one supplying GPS to my PC is USB and the other one driving the VHF and CrewSaver is RS232 with the power from a DC DC converter 12VDC in 5VDC out to the GPS.

Having a steel superstructure I have to have both mice outside else I don't get a signal.

 

[weustace]

Played with a similar module (except rated to 5V) a while ago with my XM DSC VHF, but despite correct wiring and 4800bd baud rate, I could get no recognition of the data in question from the machine. GPS still working, though, and sitting happily in a Raspberry-Pi based nav computer...

 

[AngusMcDoon]

Strictly speaking, NMEA0183 uses RS422 signalling which is balanced with the signal voltage swinging both sides of earth.

That's not correct. RS422 uses +5V/0V for one logic level and 0V/+5V for the other. Negative voltages are not used.

These little GPS modules can only provide an unbalanced output - they have a single power supply which may be either 3v or 5v, but they can never drive their output line below 0v, so they cannot be balanced.

A single +5V power supply can drive RS422 and NMES0183 differential signal levels, although the outputs are often single ended as you say.