What is the convention on data polarity of NMEA tx /rx?

[ProDave]

Current project, I am trying to get a cheap fix for a dead GPS antenna by buying a very cheap GPS receiver module from china.

I am connecting to the data port on the plotter as the default baud rate of the Chinese GPS receiver is 9600 baud. I can configure the baud rate of the comms port but not the dedicated GPS port.

So I have baud rate set to 9600, my Chinese GPS receiver is sending data measured on my oscilliscope but the plotter is not receiving it.

Time for some head scratching and testing.

The data I am seeing from the GPS receiver measured on my oscilliscope is logic high, pulsing to logic low as it sends each NMEA sentence.

If I then turn on the data output pin of the comms port on the plotter, and look at that with my oscilloscope, I see that is logic low, pulsing to logic high as it sends each NMEA sentence.

So my guess is this Chinese GPS receiver in spite of saying it sends NMEA data, is doing so inverted.

Can anyone confirm my suspicions or tell me what the logic polarity should be?

 

[ProDave]

I have answered my own question.

While waiting for a reply, I just tried it and knocked up a simple circuit to invert the logic polarity, and can confirm now at logic low and pulsing to logic high during each NMEA sentence, it is working and seeing the position from my new cheap GPS module.

 

[Refueler]

Initially your first post made me consider you were trying to connect two TALKERS to each other instead of as it should be a TALKER (antenna) to a LISTENER (Plotter).

I would not expect to see any trace from the Plotters NMEA LISTENER cables.

 

[ProDave]

I only have the TX (from the GPS) to the RX of the plotter connected, and it did not work.

As a sanity check I enabled the NMEA output from the plotter (not actually connected to anything) just to observe on the oscilloscope and that confirmed the signal from my GPS was inverted.

All working now. When it's all done and dusted I will do another post to describe what I have done.

 

[Hoolie]

I'm not sure I have really understood your problem. Standard NMEA is a two wire balanced connection with the same but inverted signal on the second wire. If it appears to be inverted you can just swap over the connections.
If you have a two wire source to a cut price single rs232 input you can just connect to the other source if it's inverted at the input. If you have a single wire rs232 source I think you can just connect to the other connection of the two wire input.
If you have a single wire source to a single wire input and it's inverted then you're stuffed and will need some gizmo to invert the signal!

 

[ProDave]

There is a ground, which is the same as the power ground, and a TX / RX. So if the logig polarity is wrong you can;t just swap tx and ground as it's the same ground providing the power which would be rather messed up.

This device was never marketed to connect directly to marine chart plotters, but I have to say I was somewhat surprised to find the logic level needed inverted.

Also this is old school NMEA0183 not NMEA2000 which is a very different animal.

 

[Martin_J]

So NMEA 0183 v1 used 'single ended' signals i.e. a single Tx wire and ground

NMEA 0183 v2 uses 'differential' signalling with Data+ and Data- signalling wires (where one is an inversion of the other)..

No surprise the answers appear to diverge..

Section 3 on signaling...
https://actisense.com/wp-content/uploads/2021/01/Everything-you-need-to-know-about-NMEA-0183-1.pdf

 

[Martin_J]

This extract from the above might be useful for anyone making a connection because it shows a new Tx (talker) talking to both types of listener... v2 and then a v1.

 

[Martin_J]

To the OP... Did this Chinese GPS module happen to have a second output wire?

 

[ProDave]

No as i say this was not sold as a packaged NMEA module ready to plug in. It is a circuit board and an antenna. It has 4 connections, ground, power, tx and rx. Very much old school V1 NMEA0183, except it did not even get the data signal logic polarity right. What do you expect for £3.28 including postage?

It was bought as a cheap experiment to see if it would work, and with a bit of tweaking it does. I will do a write up with photos in a new thread in due course without the complication of working out how to get it to work. but if you want a preview of that, this document describes the module I have bought. In-Depth: Interface ublox NEO-6M GPS Module with Arduino

 

[andsarkit]

If anyone wants to do this cheaply without complications of baud rate or polarity you may remember that I recommended this one for only a few pounds more.
Quescan

 

[Refueler]

I only have the TX (from the GPS) to the RX of the plotter connected, and it did not work.

As a sanity check I enabled the NMEA output from the plotter (not actually connected to anything) just to observe on the oscilloscope and that confirmed the signal from my GPS was inverted.

All working now. When it's all done and dusted I will do another post to describe what I have done.

I'm still wondering ...

"I only have the TX (from the GPS) to the RX of the plotter connected, and it did not work."

Surely you connected the TX from GPS to RX wire of Plotter and the GND wire of GPS to the GND / -ve of the Plotter ?

 

[Refueler]

There is a ground, which is the same as the power ground, and a TX / RX. So if the logig polarity is wrong you can;t just swap tx and ground as it's the same ground providing the power which would be rather messed up.

This device was never marketed to connect directly to marine chart plotters, but I have to say I was somewhat surprised to find the logic level needed inverted.

Also this is old school NMEA0183 not NMEA2000 which is a very different animal.

NMEA0183 usually has 3 wires ... TX / RX / GND ...

If the GPS provides NMEA0183 - it should be capable of providing to any item that receives NMEA 0183 ...

I have posted in the past many times about eBay / Banggood GPS antenna's such as Belkiin etc and their use for our VHF's / Plotters ertc.

 

[ProDave]

I'm still wondering ...

"I only have the TX (from the GPS) to the RX of the plotter connected, and it did not work."

Surely you connected the TX from GPS to RX wire of Plotter and the GND wire of GPS to the GND / -ve of the Plotter ?

Yes, do give me some credit for knowing a bit about electronics.

It should have worked, the plotter was set to receive at 9600 baud but it did not work. It turned out to be the logic level was inverted. If you still can't grasp that, look at the diagram in post#8 and it was like I was implementing the second diagram of that post but my talker ONLY has the B (inverted) output. So to get it back to sorts I had to invert that signal.

I now wonder how many have bought this one I have, tried it, and binned it as a "useless piece of c**p that does not work"?

 

[Refueler]

I've set up PC's .. Tablets .. Flight Controllers ... cameras ... all sorts with GPS and I have to say I have never experienced what you have ...

I've bought similar to post #11 ... Multi wire modules such as Belkin and other chinese few $ chip units ...

If you search my ID on here ... you will find my posts on connecting up VHF's etc ...

 

[ProDave]

Yes in hindsight one that has been proven to work would have been a better buy than the one I bought, but I am the guinea pig here experimenting with this one and finding out what was needed to make it work and posting that information for others. Part of my motivation was I am building this one into the housing of the old failed Lowrance unit, to keep the deck mounting and installed cabling back to the plotter in use. So the very small bare board one seemed best for this application.

I started over 40 years ago in industrial electronics when RS232 was common and not only did you have to fight with baud rates but parity bits, number of data bits, stop and start bits and handshake protocol just to get 2 bits of equipment to talk to each other, but never have I found one with inverted data, something I most definitely had not expected to find, and would not have solved that problem without the use of an oscilloscope.

 

[Refueler]

RS232 ... 9 pin .. and all that !! UGH ! the memories !

I have an old Lowrance Globalmap .... sometimes the screen flickers - its a common fault with them - the way the screen is connected is not exactly best ! But it still works ... but I hate that external NMEA2000 Antenna that goes with it ... the cable is so stiff and hard to work with ... I wish I had bought the alternative version with built in GPS.
I have often thought about ditching the supplied antenna and changing to an NMEA puck.

I still use the Lowrance Chart card that came with it !!

 

[ProDave]

That is a similar plotter to ours but I am sure ours is older. The cable to to the cabin mounted antenna (GPS receiver) is 4 core and fairly flexible, non of it is outside but a lot is under the headlining.

Our flickering screen gave up completely so I replaced the L shaped ccfl backlight tube recently, only to find the intermittent loss of GPS signal from the original antenna had become a permenant loss of signal. Hence this thread as a last attempt to avoid replacing the plotter.

This thread would suggest buy the puck recommended above, not the one I just bought.

 

[Refueler]

The Globalmap was also branded Eagle ...

I really like it ... even though its a bit bulky like an old TV ...

Mine has been replaced with an ONWA KP39A with inbuilt AIS Transceiver ...

The Lowrance will be mounted on my motor boat, which will stay mostly on the river - as I was heavily into compiling and vectoring my own charts for it .... I created my own card with the whole navigable part of the river at back of my house ... from Sea Breakwaters to the shallows near Kuldiga..... about 70kms.

 

[st599]

Doesn't NMEA0183 only have two baud rates in the Spec - 4800 and 38400?

Are you sure you can't send an instruction to the GPS mouse to change baud rate?