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Old 13-02-2010, 21:43   #16
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Originally Posted by Waterwayguy View Post
With the original posters limited knowledge of the NMEA, I am pretty sure that after a few more posts he will be much more confused than he originally was
Probably true, but that's how we learn this stuff. I usually have to get myself pretty confused before I force myself to actually pay attention to the details. Most details are irrelevant, but some are crucial, and figuring out which are which is what gets me where I need to go.

Having got there, I can safely forget most of the details again!
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Old 14-02-2010, 01:29   #17
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I agree with that! RTCM is not a signal format. It is the Radio Technical Commission for Maritime Services which set standards for communication and navigation equipment. RTCM SC-104 is the standard for the transmission of ground based differential GPS signals.
Getting a little off topic here. I stated in my first post that RTCM was a format, I am no expert, but that info was taken from the user manual for the Fugro Seatex Diff Signal Demodulators we use.
These demodulators receive GPS differential corrections through both the Inmarsat satellite network, and the Spot Beam sat network. These demodulators can be set up to receive the diff signal in either RTCM or raw format. Up to a few years ago, Fugro also transmitted corrections in SCF (it was not SCM my earlier mistake), SCF was Super Compressed Format.
I also found this quoted on one site

"The format of the correction information varies. There are now two public formats, the RTCM-104 and the WAAS. The RTCM or Radio TeleCommunications, Marine, is a standards organization. The format was generated by its special committee number 104. The WAAS was designed by a similar industry/government organization, the RTCA. In addition many manufactures of high end equipment have a proprietary format. The manufacturers formats are often aimed at the more precise DGPS method called Kinematics."

Quite a few other sites refer to RTCM format

Admitedly, the equipment I'm refering to here is commercial use only, our GPS can utilise corrections signal via satellite (with corrections from up to 10 shore stations being used), also using the SBAS system, and IALA, (which is corrections via radio signal) In addition, the equipment uses what is revered to as XP and HP corrections. The GPS computer can measure phase diferences of the raw GPS satellite signal, and the quoted accuracy is 15mm horizontally, and 10mm vertically.

If I have caused confusion by stating that RTCM is a format, I apologise, I was only passing on information gleaned over a few years of using DGPS, and what I have read in various manuals and reference books, and what was passed on to me at various courses I have attended concerning DGPS and Dynamic Positioning systems
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Old 14-02-2010, 07:55   #18
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Operator manuals and web sites are full of erroneous information. That's how these misconceptions get propagated and become gospel. RTCM is an organization, RTCM 104 are the standards which include the signal format for Differential Global Navigation Satellite Systems. RTCM 101 is a standard defining minimum functions for DSC transceivers in the U.S. RTCM without reference to a standard is meaningless. We don't call WAAS the RTCA format. RTCA (Radio Technical Commission for Aeronautics) is another organization that develops standards such as WAAS. Ever heard of a DB-9 connector? You know what the "B" stands for? It denotes the number of pins which is 25 for "B". So we have a 25 pin 9 pin connector. But DB-9 is written in tons of manuals and web sites so it must be a fact, right, or just another misconception. Sorry, I'm an electronics technician by profession.

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Old 14-02-2010, 08:27   #19
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Originally Posted by goboatingnow View Post
The other posters actually all gave correct information
Blue Stocking stated "+ and - are voice and hearing modes, so are connected" infering that you should connect + to - which is incorrect.

Paul Elliot stated "NMEA (actually NMEA-0182, as NMEA-2000 is totally different), uses what is called a differential connection". NMEA 0182 is an old version which transfered data at 1200bps, 0183 is the latest and most used before NMEA 2000. While the 0183 standards recommend manufacturers use an rs-422 differential electrical interface, many manufactureres do not do this and use single ended input/outputs instead. With these connections, you use ground for one of the wire pairs. Some manuals will show this as the - connection and others don't. This is where much of the confusion comes from.

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Old 14-02-2010, 09:04   #20
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Originally Posted by fairbank56 View Post
Blue Stocking stated "+ and - are voice and hearing modes, so are connected" infering that you should connect + to - which is incorrect.


Eric
That is not what I meant. That is your interpretation. Sorry I was unclear.
I have read this thread with interest, and decided to stay on as a spectator only.
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Old 14-02-2010, 09:13   #21
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Originally Posted by Blue Stocking View Post
That is not what I meant. That is your interpretation. Sorry I was unclear.
That was also the OP's interpretation. "Hi Blue Stocking, so am I correct in saying that by following through on this logic that a '+' should connect to a '-' ? A listener to a talker?" This is what happens, the more people jump in and try to help without making sure they are clear and correct, the more confusing it gets to the OP.

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Old 14-02-2010, 11:20   #22
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Originally Posted by fairbank56 View Post
Paul Elliot stated "NMEA (actually NMEA-0182, as NMEA-2000 is totally different), uses what is called a differential connection". NMEA 0182 is an old version which transfered data at 1200bps, 0183 is the latest and most used before NMEA 2000. While the 0183 standards recommend manufacturers use an rs-422 differential electrical interface, many manufactureres do not do this and use single ended input/outputs instead. With these connections, you use ground for one of the wire pairs. Some manuals will show this as the - connection and others don't. This is where much of the confusion comes from.
True, and I see that I mis-typed "0182" when I meant to type "0183", but I was trying to help the original poster solve the problem, not educate him in all the possible interface options. As it was, I probably injected too much technical trivia, but I find it useful to have a little balance between "what" and "why". What he was describing was a Raymarine setup, and as far as I have seen Raymarine NMEA uses RS-422 balanced NMEA, running at 4800 bps, and possibly at 38400.

But I agree that the details should be accurate, otherwise all sorts of confusion can result.
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Old 16-02-2010, 10:14   #23
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0183 is the latest and most used before NMEA 2000. While the 0183 standards recommend manufacturers use an rs-422 differential electrical interface, many manufactureres do not do this and use single ended input/outputs instead. With these connections, you use ground for one of the wire pairs. Some manuals will show this as the - connection and others don't. This is where much of the confusion comes from.
The NMEA 0183 specification is currently at version 3.01. From version 2.0 onwards the RS-422 differential interface was specified. All versions previous to v2 were allowed to use ground as a reference. Many manufacturers only comply with version 1 because it is cheaper and they can still claim NMEA 0183 status.
Connecting devices for different versions incorrectly can cause damage to one or both of your devices. Most notably connecting a talker -/B to a listeners ground will have the talker drive excessive current into the ground of the listener. Somewhere down the line something will melt. Typically your expensive device is not covered under warranty because you connected it incorrectly.

johnar posted a link to our information sheet, earlier in this thread, containing diagrams and an explanation of all the various versions of NMEA 0183 with some connection dos and don'ts. It also contains some information on sentence headers.

If you are not sure, NEVER just randomly try connections in the hope of success. ALWAYS seek advice. As you can see from this thread there are plenty of knowledgeable people who are willing to give sage advice when needed.
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Old 16-02-2010, 16:07   #24
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I agree that a talker -/B output should generally not be connected to ground as this should not be needed IF in fact you are dealing with a differential output feeding a differential input but just because two wires are labled -A/-B does not make it rs-422. Take the Icom M504/604 VHF. They claim the NMEA output is version 3.01 and the wiring is labled NMEA OUT (-) AND NMEA OUT (+) when in fact, the OUT (-) is GROUND. It is NOT a differential output. You would connect this ground to the (-) differential input of your listener. The NMEA input of these radio's can be fed by either single ended or differential input as it simply goes to an optoisolator whose output feeds a single ended buffer. In other words, these radio's do not use rs422 yet they label their wiring as if it is. Just one example of many as you never know what they are really using til you look at the service manuals. By the way, rs422 drivers are typically short circuit protected. A quote from a Texas Instruments rs422 driver spec:

"With the driver shorted to ground, the magnitude of the output current shall not exceed 150 mA, regardless of the state of the driver output (high or low) at the time of the short. This test ensures that the device is not destroyed by excessive current flowing through the output stage."

Of course, that doesn't mean you should connect it to ground when not necessary. Iv'e been a marine electronics component level repair tech for many years. Things aren't always what they seem to be. Is it really rs422? Is it really isolated?


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