Adding Beitian GPS module to B&G V50 Radio

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chris-s

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One of this years upgrades was to fit a fixed VHF radio and whilst we don't really have need for it, we purchased a model with AIS & GPS, however, what I didn't realise was that the GPS required a separate input.

As it happens, I have a number of Beitian GPS modules from previous forays into autonomous model aircraft, so I figured it should be simple enough to take the output (NMEA0183) and feed to the radio. About the only useful information I found on the googler was that the baud rate should be 4800, so that was easy enough to set, but no joy, the radio just tells me that no GPS info is available, The GPS input has a GPS+ and GPS- connection, I'm using the GPS+, grounded the GPS- and tied the GPS ground to the radio ground (the GPS module is powered from a 5v BEC). I know the module works and the LED's indicate satelite lock when on the boat.

The only thing I can think of is the logic level required, the module is 3.3v.

I could take the easy option and go out and buy a plug-and-play solution, but it's not critical and where is the fun in that.

Ideas?

Chris
 
I was advocating this a month or so ago ... and it should work ... I have my Beitian module still in packing .. just haven't got round to it yet ..

You've given me a stir to have a look and get started.

Which module do you have ... maybe we can help each other ?
 
Different manufacturers can be very loose when applying the NMEA 0183 specification so it is often a bit of trial and error to see what works. In theory your single ended (ground referenced) signal will not work into the differential VHF input but in practice it is often OK.
Some information here: NMEA0183
You can get RS232 to RS422/485 converters on Ebay or Amazon, some of which get power off the signal lines and some require an external 12v supply.
An added complication is that differential signals are called A & B or + & - and you connect A to A and B to B whereas single ended signal are connected Tx to Rx and Rx to Tx. Often the only way to get it working is trial and error as different manufacturers use different conventions.
 
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As well as the hardware electrical side, you may need to check all the right NMEA sentences are being transmitted.
4800 sounds like a very low baud rate this century.
Is there a manual online?
The inputs of the B'n'G may be optically isolated. Or even transformer. You might need a pull up resistor on the GPS module output. It may even be a true differential input which needs the Rx+ to go below the Rx- for the zeros?
A differential driver IC might help? It could also change the levels to 5V.
Can you look at the data coming out of the GPS using Hyperterm or similar and a usb/serial converter? Can you fire sentences into the the GPS the same way?
 
Thanks for the replies!

Yes, the input is differential, yes it is 4800, yes I can see the data coming out of the gps module at 4800.

I hadn’t looked at any kind of differential driver, that might help.

Chris
 
chris-s said:
As it happens, I have a number of Beitian GPS modules from previous forays into autonomous model aircraft, so I figured it should be simple enough to take the output (NMEA0183) and feed to the radio. About the only useful information I found on the googler was that the baud rate should be 4800, so that was easy enough to set, but no joy, the radio just tells me that no GPS info is available, The GPS input has a GPS+ and GPS- connection, I'm using the GPS+, grounded the GPS- and tied the GPS ground to the radio ground (the GPS module is powered from a 5v BEC). I know the module works and the LED's indicate satelite lock when on the boat.

The only thing I can think of is the logic level required, the module is 3.3v.

I could take the easy option and go out and buy a plug-and-play solution, but it's not critical and where is the fun in that.

Ideas?

Chris
Click to expand...
Electronically speaking, it is quite simple to derive In+ and In- signals from the unipolar signal, as given by your GPS module. The modern 74HC04 logic inverter has a logic threshold of 2.5v, so 3.3v will be reliably interpreted as a logic 'high'. With a +5v supply, the output will be +5v for a logic 'high'. The output from the 74HC04 will form the In-. The output from your GPS could be fed via a non-inverting buffer similar to the 74HC04, so that the logic 'high' on this line will be of the same magnitude as on the In- line but without being inverted, thus providing the In+ line. I can't recall the full part number of the 74HC buffer off the top of my head.

Soldering iron, 2 ICs, veroboard, 12v plug, 12v to 5v buck converter, required interface connectors and a suitable box; looks like a bit of a project.

Edit:- It would probably be best to get the 5v supply via a linear power supply driven by the 12v, rather than using the buck converter. Buck converters, being a variety of switch mode power supply, are more efficient than linear power supplies but may impart RF interference to your VHF.
 
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HissyFit said:
Electronically speaking, it is quite simple to derive In+ and In- signals from the unipolar signal, as given by your GPS module. The modern 74HC04 logic inverter has a logic threshold of 2.5v, so 3.3v will be reliably interpreted as a logic 'high'. With a +5v supply, the output will be +5v for a logic 'high'. The output from the 74HC04 will form the In-. The output from your GPS could be fed via a buffer similar to the 74HC04, so that the logic 'high' on this line will be of the same magnitude as on the In- line but without being inverted, thus providing the In+ line. I can't recall the full part number of the 74HC buffer of the top of my head.

Soldering iron, 2 ICs, veroboard, 12v plug, 12v to 5v buck converter, required interface connectors and a suitable box; looks like a bit of a project.
Click to expand...

Why the converter - he's already using a BEC which will take voltage of most sources such as 12v and provide steady reliable 5v.
 
You might get away with biasing the Rx- input to say 1.5V with a couple of resistors.
If you've got an inverter IC, there's generally several gates in there, so the non inverted can be generated by re-inverting the inverted out put if you see what I mean. I doubl the delay will trouble you at 4800 baud.

I have a vague memory of doing something very similar and only needing to invert the data with a transistor, but that would have been a ublox module.
Do you have a scope to look at the actual signals?
A soundcard scope might be enough, but I haven't used one myself, you don't when you own 4 oscilloscopes!
Even looking with a mutleymeter on AC volts might confirm things are or are not happening?
Last week I threw away some old modem cards which would have provided all the right bits!
 
TernVI said:
If you've got an inverter IC, there's generally several gates in there, so the non inverted can be generated by re-inverting the inverted out put if you see what I mean. I doubl the delay will trouble you at 4800 baud.
Click to expand...
Thumbs up on the use of two spare inverters to make an un-inverted signal. There are six in a standard 74HC04 package. I got caught up on the availability of single gates.
 
You only need to use the yellow + in and the green - in. If the GPS module uses ground for the - out, you should be OK grounding the B & G green wire. This will normally work OK, might be an idea to run it my B & G though. You didn't say which module you have, some of the Beitian GPS modules default to TTL output and need an RS232 converter cable to work with NMEA 0183.
 
Thanks again guys. The gps module works at ttl levels, I thought that was sufficient but have an rs232 level shifter somewhere!
Going to have another go at the weekend.
 
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