NMEA output on a Garmin 740s

[geoffschultz]

I have a RayMarine C80 as my primary chartplotter which interfaces to a RayMarine SmartPilot S3 via SeaTalk. I used to have a Northstar 6100i as a backup chartplotter, which interfaced to the SmartPilot via NMEA 0183. The NorthStar died and I replaced it with a Garmin 740s.

The Garmin receives NMEA data from the SmartPilot on both NMEA ports without issue, but didn't control the autopilot. To see what sentences the Garmin was transmitting, I connected the xmit to the receive line (on the Garmin) and I see 0 sentences being transmitted. I tried it on both ports and cross connecting the ports, but still nothing. It's like the Garmin isn't transmitting anything. The ports are configured to be NMEA Standard. Garmin specific sentences are shut off. I'm completely at a loss as to why I don't see any data being transmitted. Any ideas?

Of course we're on the Pacific side of Mexico trying to get ready to depart for El Salvador on Monday and Garmin tech support is closed until then.

[galacticair]

It's hard to picture what you've been connecting and what you're trying to achieve. I gather you want the Garmin to talk to the Raymarine Smartpilot, but what NMEA port are you connecting (most equipments have several) and what information exactly are you trying to send?

These issues tend to either connection mistakes, or NMEA settings issues.

A few questions:
- Are you sure both NMEA ports (transmit on the talker and receive on the listener) are set to the same baud rate? (4800 baud is standard, AIS is 38400 baud)

- Are you sure you connected the transmit + wire to the receive +, and the transmit - to the receive - (ground)? That's easy to get wrong.

- Can you tell us what color cable from each equipment you're trying to connect? Then I can look at the manuals and see if you're getting it wrong.

- What NMEA sentences is your Garmin programmed to send? (see NMEA settings under the Chartplotter's networking settings tab)

[geoffschultz]

1) I am trying to connect the Garmin to the SmartPilot.
2) The Garmin's ports are set to NMEA Standard, which is 4800 bps.
3) Port 1 on the Garmin uses Blue (output) and Brown (input) wires.
4) Port 2 on the Garmin uses Grey (output) and Violet (input) wires.
5) There is no ground wire on the Garmin. It uses the common power ground.
6) On the RayMarine the ports are clearly labeled with in and out and ground. I'm not confusing these.
7) I can connect the brown and violet wires to the output of the RayMarine and receive good NMEA sentences
8) I've tried connecting Blue to Brown, Grey to Violet, and cross connecting the two ports to see what the output of the Garmin is (with no other equipment connected) and I see no output from the Garmin
9) The Garmin is programmed to send a bunch of sentences, including GPS position/satellite data, which should output constantly. It excludes Garmin specific sentences.

I'm a computer science/hardware major and built my first computer from chips in 1973. I'm very hardware literate. I just don't understand why I'm not seeing output from the Garmin.

-- Geoff

[galacticair]

Thanks Geoff, that's much clearer (wasn't trying be patronizing, a lot of people have trouble with the basics of NMEA connections and settings).

Sounds like your understanding of the wires and settings is spot-on. However, I'm not sure whether your test concept (connecting two NMEA ports of the same device) would actually work. Seems a bit of a stretch. Even if the Garmin receives it's own data via this loop, it's likely to just ignore it since it already has the same data internally.

I'd suggest connecting your Garmin to a separate device (or if you really want to diagnose its NMEA out, to a PC). On our boat, I've connected 0-15+ year old hodge podge of Garmin, B&G, Nexus, Autohelm, PC and Standard Horizon gear via NMEA 0173, and so long as the wires & baud settings are right, i have yet I see a "transmit" error like you're worried about. Then you can really see what the listeners are receiving.

[goboatingnow]

Check you've configured the 0183 ports in the 740 software , sounds like a configuration issue.

Dave

[smac999]

I'm not sure I'd trust the port crossing. I've never tried to test that way. if you have computer knowledge. you should be checking the port with a computer with serial port and hyperterminal set to 4800. pin 5 ground, pin 2 tx from garmin.

you'll see all the data coming out of the garmin onto the pc screen

there should be lots coming out. gps data etc.

it's also possible there is an autopilot control that is disabled In the menu somewhere. the raymarines have that. you have to turn it on before it'll be allowed to control a pilot.

but if this was the case you'll still be seeing some info on that port.

[galacticair]

Originally Posted by smac999

if you have computer knowledge. you should be checking the port with a computer with serial port and hyperterminal set to 4800. pin 5 ground, pin 2 tx from garmin.

you'll see all the data coming out of the garmin onto the pc screen

Agreed, a PC connection is very powerful for diagnostic purposes.

In terms of software to use:
- Actisense's free "NMEA Reader" also allows you to read/test the raw NMEA 0183 data flow very easily, including visibility into each sentence sent and its contents. Also try Com Check (similar).

- OpenCPN and NavMon PC are PC-based nav software that will allow you to see the processed NMEA data flow from the perspective of a nav program (e.g. wind/nav/speed/etc outputs). This will help you check that the data makes sense.

Later, you'll also need to check your listerner (Raymarine) to make sure it understands the NMEA sentences your Garmin is sending. NMEA standards incorporate a lot of sentences with fairly duplicative data, as manufacturers' equipment often use different sentences for the same purpose.