NEMA In, NEMA Out.....

[scotte]

Originally Posted by richardhula

True but a single cable could provide this

Single cable, yes - single wire, no (I think this was David M's point - that NMEA0183 is unidirectional, not a buss, like SeaTalk or NMEA2000 is for example).

Originally Posted by richardhula

Very true especially if it's high speed NMEA as with AIS distribution.

Again to be overly nitpicky AIS is, strictly technically speaking, not NMEA0183 because the rate is not 4800 bps. Where it gets interesting is if you do speed conversion from AIS down to 4800bps - without some sentence filtering, it ain't gonna fly.

Originally Posted by richardhula

Note NMEA 2000 is not the same as NMEA 0183 even at 38400 baud.

Absolutely! NMEA2000 is a buss, like SeaTalk - there's a lot more different than just the line protocol, the entire electrical standard is different. For example, you can't plug it into an RS-232 port without some hardware to interface it, unlike NMEA0183 and AIS data.

I'll leave the whole NMEA0183 is not RS-232 bit out, I wrote a long dissertation about that elsewhere on here. Minor technical nuances perhaps, but important, at least to me - in the same way I call that bit of line a sheet, and that bit of line a halyard.

[David M]

AIS has a baud rate of 38400

[David M]

Originally Posted by richardhula

True but a single cable could provide this, i.e. connecting to COM port on laptop with db-9 plug (via USB adaptor or direct) pin 2 NMEA+ in, pin 3 NMEA+ out, pin 5 common-.

True again but in practice it's dependant on the working impedance of each device connected & is therefore difficult to predict. For the record I have had six physical devices connected to one GPS without a glitch. In most cases therefore it's not an issue.

Very true especially if it's high speed NMEA as with AIS distribution. Very unlikely in this day and age that any buffers will overflow.

Note NMEA 2000 is not the same as NMEA 0183 even at 38400 baud. NMEA 0183 sentences generally use three upper case alpha characters to define the data type they are sending, preceeded by two alpha characters corresponding to the source, making it easy to read "raw". NMEA 2000 uses six numerical characters to define the data type and the sentences are know as PGN's, hence need for a converter.

Regards

Sorry Richard, I hit the wrong button and changed your post when I responded. We have an EDIT button for all posts. I changed it back though. One of the faults of being a tired moderator.

Yes, NMEA can share a common ground.

Yes the number of NMEA receivers varies, I never said it was a specific amount.

The lower the baud rate, the fewer sentences that can be streamed. 4800 baud is still very common.

NMEA 0183 and NMEA 2000 is not compatible.

[Windjammr Spirt]

Wow....listener, talker, listener, talker, I am very confused!

Thanks by the way for the reference to the great article at Actisense, but I wonder if I can ask a simple question here.

Keeping it simple, I have a GPS that I will use as a talker. Now, this GPS will connect to my laptop via COM1 and my software (Offshore Navigator) will be the listener. I will then connect the laptop to my Autopilot. Here's where I get confused....the GPS is feeding position data to the charting software. I want the charting software to drive my Autopilot on the same COM1 port. So it sounds like I've got 2 "talkers" which from what I've read is not going to work. But the data is separate. Without looking up all the data strings at the moment, the way I see it, the GPS will send the laptop my position data on the "send" wire. The laptop will take this data in on the "receive", then plot my course based on the chart software, then send the course to steer, XTE etc thru the "send" wire to the AP. Since the GPS is not going to be sending course data to the AP, the signals that the AP receives from the chart software should be taken in and complied with. The net result is that my GPS determines where I am, I plot the course on my laptop, and I send the final result to the AP. All of this will happen on one cable running from my GPS (only using the output wire from my GPS) coming into my laptop on Pin 2 (data in), then another wire from my laptop using only Pin 3 (data out) to my AP.

Am I out to lunch on this?

Oh, and this is older equipment, no SeaTalk or advanced versions of NMEA, it's all NMEA 0183.

Thanks....Steve

[David M]

Steve....thats it!

That's exactly how my GPS talks to my Nobeltec which talks to my autopilot...all using NMEA 0813 at 4800 baud. They use a few more sentences than you described but you got the right idea.

[richardhula]

Originally Posted by Windjammr Spirt

Wow....listener, talker, listener, talker, I am very confused!

Thanks by the way for the reference to the great article at Actisense, but I wonder if I can ask a simple question here.

Keeping it simple, I have a GPS that I will use as a talker. Now, this GPS will connect to my laptop via COM1 and my software (Offshore Navigator) will be the listener. I will then connect the laptop to my Autopilot. Here's where I get confused....the GPS is feeding position data to the charting software. I want the charting software to drive my Autopilot on the same COM1 port. So it sounds like I've got 2 "talkers" which from what I've read is not going to work. But the data is separate. Without looking up all the data strings at the moment, the way I see it, the GPS will send the laptop my position data on the "send" wire. The laptop will take this data in on the "receive", then plot my course based on the chart software, then send the course to steer, XTE etc thru the "send" wire to the AP. Since the GPS is not going to be sending course data to the AP, the signals that the AP receives from the chart software should be taken in and complied with. The net result is that my GPS determines where I am, I plot the course on my laptop, and I send the final result to the AP. All of this will happen on one cable running from my GPS (only using the output wire from my GPS) coming into my laptop on Pin 2 (data in), then another wire from my laptop using only Pin 3 (data out) to my AP.

Am I out to lunch on this?

Oh, and this is older equipment, no SeaTalk or advanced versions of NMEA, it's all NMEA 0183.

Thanks....Steve

Confused or not you seem to have it spot on

You just need to check in your autopilot manual which NMEA sentences it needs to steer a course in "NAV" mode. Typically they would be APB, BWR & maybe RMC. The RMB sentence can also be useful as it includes VMG data - speed at which you are closing your destination, assuming your autopilot can interperet & display this.

Your chartplotter should have a configuration screen to turn transmission of these sentences on or off. You may need to experiment. Up to a point better too much data than too little as it will discard/ignore anything it can't work with.

Regards

[Windjammr Spirt]

OK folks, thanks for the responses. Now I just have to wire the thing up!

The existing wiring has the GPS output to the AP input, AP output to laptop. The old owner used the laptop to watch but actually plotted his courses on the GPS. I want it the other way around, so....

GPS Talk to laptop Listen, laptop Talk to AP Listen

Then go thru the different sentences strings as mentioned, turn on the NMEA AP output in my chartplotter software and I think it should work for me.

Thanks for confirming all of this, it's a big help.

- Steve

[sandy daugherty]

Furuno has just introduced a NMEA 183 / NMEA2000 gateway, for $700. That seems steep but we've got to remember that a PGN in N2K does not directly translate to a 'word' in N2K. They frequently come close, but the definitions and applications have evolved.

In the bad old days of NMEA0183 the was more babel than understanding between manufacturer's and even between iterations of a manufacturer's lineup. This is the PRIMARY reason for N2K.

There are other limitations. A talker may be strong enough to be heard clearly by 3 listeners over an average distance. Some more, some less. A listener can't always handle two talkers, they talk over each other. Not very polite. When a listener hears two voices at the same time, it garbles the data, or waits for the next repeat. Messy.
Few listeners pass everything they hear back out. If they did there would be a lot of duplicated data being reduplicated!

For these, and all the reasons listed above, its difficult to give advice to a novice without having all the manuals, and quite a few customer support emails to plan out any installation with more than 6 common devices.

[richardhula]

Originally Posted by sandy daugherty

Furuno has just introduced a NMEA 183 / NMEA2000 gateway, for $700. That seems steep but we've got to remember that a PGN in N2K does not directly translate to a 'word' in N2K. They frequently come close, but the definitions and applications have evolved.

In the bad old days of NMEA0183 the was more babel than understanding between manufacturer's and even between iterations of a manufacturer's lineup. This is the PRIMARY reason for N2K.

Given the price looks like bad old NMEA 0183 has plenty of life left in it!

Originally Posted by sandy daugherty

There are other limitations. A talker may be strong enough to be heard clearly by 3 listeners over an average distance. Some more, some less.

Seems like any wired communications protocol would have similar limitations. If the wires too long the leccy won't come outa t'other end

Note Garmins advice on NMEA 2K is that power is fed to the centre of backbone to reduce voltage drop.

Regards