NMEA-0183 Interface

Electrical and electronic topics for small boats
rlboeri
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NMEA-0183 Interface

Postby rlboeri » Wed Apr 06, 2016 7:18 pm

Back to the well again for some help. I just purchased a Lowrance HDS-7 Gen 2 Touch chart plotter and fishfinder combo for the 1988 Montauk. I already own a Standard Horizon GX2000 VHF. I would like to connect the two so that the DSC will transmit my position if necessary. I have no idea how to do this, and the instructions in the manuals are confusing at best.

The radio manual states that the Standard Horizon GX2000 is NMEA 0183 compliant. It says that to connect to a chart plotter which has one NMEA port, the baud rate is 38,400 (I think that the HDS-7 has one NMEA 0183 input on the power cord and one NMEA 2000 plug on the back). I am writing this like I know what I am saying. Not true.

I believe that manual states that for a 38,4000 baud rate I connect the VHF Gray NMEA OUTPUT (+) to the HDS-7 NMEA INPUT (+) wire and the brown NMEA OUTPUT (-) to the HDS-7 NMEA INPUT (-). Here is where I start to get confused.

The HDS-7 Manual says that it has a Transmit A (yellow) and B (blue) and a Receive A (orange) and B (green). I don't even understand what the heck I just wrote... So what goes to what?

Is transmit in the HDS-7 manual equivalent to the INPUT or OUTPUT in the VHF manual? It is like they are speaking different languages.

Next, is it true that the HDS-7 should be considered to have only one NMEA port regarding the VHF?

If I want to connect an engine harness rig to the HDS-7, I know that it goes into the NMEA-2000 port. That looks pretty straightforward. Just buy the harness and run it to the back of the HDS-7 and plug it in. But does that then mean that there are two NMEA ports and the VHF should be set at 4,800 baud, which means that two other wires need to be connected (the blue INPUT and the green INPUT?

See my confusion? Seems like it should be pretty simple. SAVE ME! The dealer wants $440 to do the install, but that seems silly.

Thanks for your help,

Bob

jimh
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Re: NMEA-0183 Interface

Postby jimh » Wed Apr 06, 2016 8:27 pm

The process of interconnecting NMEA-0183 is explained in

Guide to NMEA-0183 Interconnections
http://continuouswave.com/whaler/refere ... A0183.html

Spend five minutes reading that article and you can save $440. That is a good hourly rate, about $5,280-per-hour for your time. So it will be worth it to read the article. The above article is a concise answer. I will give you the long and wordy answer below.

jimh
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Re: NMEA-0183 Interface

Postby jimh » Thu Apr 07, 2016 9:44 am

Bob gave some information about the interconnection of his two marine electronic devices by NMEA-0183, and then said:

I don't even understand what the heck I just wrote...


To help you understand, I can offer some explanation.

UNDERSTANDING NMEA-0183 INTERFACES

The Basics

The NMEA-0813 interface consists of electrical signals which are in two categories: TALKERS and LISTENERS. TALKERS talk, that is they send signals. LISTENERS listen, that is, they listen for signals.

Thanks to the work of NMEA and their NMEA-0183 standard and adherence to the standard by manufacturers, just about every facet and aspect of connecting these devices has already been prearranged for you and will allow this connection to work. All that remains for you to do is to perform the PHYSICAL connection of the signals, that is, you have to connect the electrical wires that carry the signals from one device to the electrical wires that will carry those signals into the other device. This is the simplest and easiest part of the entire process of making the devices communicate. Really, just about anyone can do this.

The key to interconnecting the signals is extraordinarily simple. The signals are not bi-directional. You have to connect them for a one way flow of the signal. To do this, you connect a TALKER to a LISTENER. What could be easier to grasp? Also, you have to understand that for a two-way conversation between devices, you have to make two sets of connections: one device's TALKERS to the second device's LISTENERS, then vice-versa for the other direction.

Of course, you need some way to identify the signals. The signals have names. The names are TALKER A and TALKER B for the talker signals, and LISTENER A and LISTENER B for the listener signals. Just to be clear, each single interface, that is each "port" or "comm port", will consist of both a TALKER A and a TALKER B and a LISTENER A and LISTENER B signals for that one port or comm port or interface. Some devices have more than one set of these interfaces or ports or comports. If a device has multiple NMEA-0183 interfaces, it usually identifies them in some manner, like referring to them by numbers, like "Port 1" or "Comm Port 1" or "Primary" or something similar. But keep in mind that each port or interface will have both A and B signals for their TALKER and LISTENER signals for that interface. That means there will be four wires at each interface or port or comm port. There is a simple rule for connecting them. You connect one interface's TALKER A to the other interface's LISTENER A, and the same for the B signals, first interface's TALKER B to the other interface's LISTENER B. This is such a simple rule that most people don't even have to write it down. You can remember something this straightforward.

That's all there is to it. If the manufacturers of marine electronics just followed the rules, we'd be done.

Decoding Signal Names

For some reason--I have no idea why but it perhaps it is due to some sort of general lack of rigor in preparing technical documentation that occurs in many consumer electronic products--the signals on a NMEA-0183 interface are almost never clearly identified with TALKER and LISTENER, the names recommended in the standard. Instead of using the clear, intuitive, standard names of TALKER and LISTENER, the manufacturers give the signals all sorts of names, probably the names their engineers used as shorthand names when they designed the product. In my article, Guide to NMEA-0183 Interconnections (that I mentioned earlier in the thread and which is a bit more concise in its writing), I give a table of translations so you can decode the non-standard or shorthand names used by the manufacturers into the standard names. These translations are not that hard, actually. They are as follows:

--if the manufacturer's name for a signal is "in" or "input", that signal is a LISTENER.
--if the manufacturer's name for a signal is "receive" or "receiver" or "rx" or "rcvr" or any form that looks like it is derived from "receive", that signal is a LISTENER.

--if the manufacturer's name for a signal is "out" or "output", that signal is a TALKER
--if the manufacturer's name for a signal is "transmit" or "transmitter" or "tx" or "xmit" or "xmtr", that signal is a TALKER

But the manufacturer's weren't done creating confusion. They decided they would polarize their signal names and give them positive and negative designators. This is completely baffling because these signals do not really have any sort of DC polarity to them at all. (To be technically accurate, the A signal is the logic true or non-inverted signal, and the B signal is the logic inverse or inverted signal of the A signal; sending electrical signals like this is called differential signaling.) They are not positive and negative like the terminals of a battery. (The signals are logical inversions of each other.) You have to unravel this further misidentification in the naming of the signals from the manufacturers, but the secret decoder key is really simple. Here it is:

--if the manufacturer's name for a signal is "positive" or "plus" or uses the plus-sign (+), that is the A signal

--if the manufacturer's name for a signal is "negative" or "minus" or uses the minus-sign (-), that is the B signal

Again, these translations are not really that hard. You may not have to write these down to remember them, either.

Three Rules for Interconnections

Now that the signals are identified, all that needs to be done to interconnect those signals is as follows:

--(1) connect a TALKER to a LISTENER with the same A/B designation

That is our first rule. Maybe write it down to help remember it.

But the manufacturer's weren't done creating confusion. Instead of following the standard that has been in effect since 1992, which means for 24 years, the marine electronics industry again went off the main channel and into the weeds, and decided they would randomly OMIT some of the signals. This probably saved them $0.25 in costs per unit, but it created a billion dollars of confusion. But we can handle this problem, too. The omitted signal is always the B signal, so we have to make an allowance for some interconnections between devices to have the B signal missing. That means we need a little more complicated rule for how to handle a B signal when it has no mate to connect to. We need two rules because we treat the missing signals differently, depending if a TALKER or a LISTENER is missing. These two rules are probably the most complicated rules in the whole realm of this interface problem, but they are simple, too. Here are the second and third rules:

--(2) if there is a LISTENER B and no corresponding TALKER B to connect it to, then connect LISTENER B to the power common (that is, the black power wire circuit or the device's chassis circuit or the battery negative circuit);

--(3) if there is a TALKER B and no corresponding LISTENER B to connect it to, then insulate the TALKER B and don't connect it to anything.

So there are a total of three rules for interconnecting, and they are very simple. That is it. We have figured out how to interconnect any situation with NMEA-0183 interfacing. Now we go on to the actual problem of accomplishing the interconnection.

Making the Actual Connections

At this point, we know that there are two kinds of signals, and the manufacturers hide them with a bunch of different names. By now we have figured out how to decode the manufacturer's names to get to the standard names. And we know how to interconnect the signals because we followed the three simple rules of interconnection given above. Now we want to interconnect the signals, that is, we want to find the wires that have these signals, and connect those wires together physically so they make an electrical connection.

How do we find the wires? You won't believe this, but those manufacturers decided to throw in more confusion. Instead of using a standard connector with a standard number of pins and a standard arrangement of the signals on those pins, most NMEA-0183 interfaces are provided on a wiring fan out with a bunch of individual wires coming out of a little multi-conductor cable.

In a situation where we are provided with a bunch of wires, the only sure way to distinguish one wire from another is by the color of the wire insulation. You may not believe this, but even though there is a published standard for the wire insulation color to be used for a particular NMEA-0183 signal, the manufacturers once again went off on their own and made up wire insulation color patterns for the signals as they pleased. They went so crazy that sometimes on the same product they would change the wire insulation color from production run to production run, so that their own literature about wire color coding was not even correct. There is absolutely no rhyme or reason to the colors that are used. There is no point in trying to discover a rule or pattern--there is no pattern or rule to this madness. Every piece of NMEA-0183 gear seems to use a completely new and unique choice of colors to identify the wires.

The burden of figuring out signal identity from wire insulation color falls to the installer. The installer has to find some documentation in the manufacturer's literature that identifies what wire insulation color is used for what signal. I cannot help you with this, because there is no consistency in the use of these colors. You will just have to pore over the literature and find the page or drawing that shows the colors. And, just to keep up the insanity, sometimes this information is only given on special addendum sheets or in special installation instructions, and is omitted entirely from the main documentation like the operating guide or instruction manual. Don't throw out any literature that comes with a marine electronic device because that little slip of paper may be the only place with the wire color codes.

This completely insane behavior about wire insulation color is why I NEVER tell people how to connect devices by telling them to connect a certain color to another color. It is a crazy way to give directions. It is like giving traffic directions by telling someone to turn left at the green light and to turn right at the red light. It makes no sense. So I never offer advice like that. I offer advice like I have given you above. I teach you to read the roadmap, not turn left at the red light. Some organizations, like the USCG, tried to collect all possible combinations of devices and describe the interconnections by wire color. That does not work. The first time I looked up data on their chart for two devices, I found it was wrong. That is another reason to avoid advice that comes to you with a recommendation to connect a certain color to a certain color: there is a good chance it is bad advice. Figure it out yourself; you are probably smarter than the guy giving you advice.

Working with Small Wires

Eventually, let me assume you find a wire with a certain color and want to connect it to another wire. The manufacturer's weren't done making this task hard. They usually provide these signals on very small wire, probably 22-AWG or 24-AWG. Most boaters are not very handy when it comes to working with small wire gauges because it is very unusual to find any electrical conductor on a boat smaller than 16-AWG or perhaps 18-AWG. That has occurred because those gauges are the recommended minimum gauge for any electrical conductor on a boat. But the marine electronics manufacturers did not let those recommendations get in their way; they just use 24-AWG for their NMEA-0183 signals.

Making physical and electrical connection of 22-AWG or 24-AWG conductors can be a bit difficult. This is one point where a boater might want to get some help from a good technician. These small wires cannot support themselves, and if you make a connection between two conductors with small wire like this, the wires must be held in place physically by some means other than their electrical interconnection, otherwise there is a good chance that, over time and with movement and vibration, the connections will fail.

There are many ways to make electrical connections between small wires. One popular way is to use a very small terminal strip with compression contacts. You can get these small terminal strips with compression contacts from almost any supplier. (More on this below in a follow-on posting.)

I have my own method of interconnecting NMEA-0183 using a five-pole connector arrangement. I wire every device the same way to the five-pole connector. Then I construct a motherboard with mating connectors that is pre-wired to interconnect the devices. My invention of this method employs a universal wiring arrangement for the device-to-five-pole connector, and a universal arrangement of the five-pole connector to the motherboard. With my method, interconnecting devices is just a matter of connecting each device to the five-pole connector in a standard way, then just plugging devices into the motherboard to interconnect them. I explain the method in detail in an illustrated article. See

Universal NMEA-0183 Interface
http://continuouswave.com/whaler/refere ... A0183.html

It took me a while to develop the Universal NMEA-0183 Interface. There is nothing like it from any of the manufacturers or from the NMEA organization. If they had come up with this 24-years ago, no one would have ever asked a question about how to interface NMEA-0183 devices.

Now, as for the notion that you will have to pay someone $440 to figure this out, I want to caution you. There are a lot of people employed by someone as a marine electronics installers who won't get this correct. There is a good chance you might pay a dealer $440 and when your radio and chart plotter are installed, they won't communicate via their NMEA-0183 interface.

Signal Rates: Fast Talkers

Oh, one last thing. When I said you only had to figure out the physical interconnection, I was leaving out something: the signal rates. Usually all NMEA-0183 signals are sent at the standard rate, but--as you might expect--there is an option for a different signal rate, a faster rate. The normal signal rate is called "4800". This works almost all the time, and, for once, the manufacturers usually have their gear pre-set to this rate and you never have to worry about it. The faster rate is called "38400" and is used only in special situations. The most common special situation is for the TALKER from an AIS receiver. If you have an AIS receiver it will normally be set up for the faster rate because it is possible that an AIS receiver could have so much data coming into it from its two radio receivers that all that data could not fit into the NMEA-0183 TALKER stream of data in the time available if the lower signal rate were used. Or, perhaps simpler to understand, the AIS receiver is a FAST TALKER. When you have a FAST TALKER you have to set the LISTENER to match its fast talking rate. You'll have to carefully read the operating instructions for the LISTENER device to find where the controls are for setting the signal rate.

There is one more consideration about NMEA-0183 and fast and slow TALKERS, but that is more of a graduate school level topic, so I will let it rest for now. I hope my little essay has been helpful to you and now you won't have to say:

I am writing this like I know what I am saying. Not true.


[Note to readers: I updated this article several times from the first posting, so I recommend re-reading it if you got the first-draft version--jimh]

porthole
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Re: Connecting a Lowrance HDS-7 Gen 2 Touch to a Standard Horizon GX2000 - HELP!

Postby porthole » Thu Apr 07, 2016 11:48 am

I like this description the best so far, although I still have the "Guide" printed out, but I am adding it to my printed copy.



Just a thought, perhaps you can add the above advice to your original NMEA link

Guide to NMEA-0183 Interconnections

or, add it to this

Universal NMEA-0183 Interface

or, combine all three descriptions for one concise location for all things NMEA 0183
Thanks,
Duane
2016 World Cat 230DC
1999 Outrage 21, Yamaha SW Series II 200
1997 Outrage 18, Yamaha 125
1983 15 SS, Honda 50
1980 42 Post
1983 34 Luhrs 340 SF

jimh
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Re: Connecting a Lowrance HDS-7 Gen 2 Touch to a Standard Horizon GX2000 - HELP!

Postby jimh » Thu Apr 07, 2016 2:16 pm

Many people use a small compression terminal strip, sometimes called a Euro-style (probably because it resembles some DIN terminations with compression terminals). RadioShack may have these under their part number "RadioShack 12-Position European-Style Mini Terminal Strip Catalog #: 2740680."

My concern with this type of terminal strip with compression contacts is the small wires will be crushed under the compression force and soon break off.

rlboeri
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Re: Connecting a Lowrance HDS-7 Gen 2 Touch to a Standard Horizon GX2000 - HELP!

Postby rlboeri » Thu Apr 07, 2016 2:26 pm

Jim et al,

You are brilliant! This is exactly what I needed. Within a few minutes I understood perfectly what I needed to do. If you all don't already do it, you need to be teaching a college course. I am sure you would be great at it. The guide will now be enshrined in a frame on my wall.

One question regarding the installation. I want to be able to easily remove the radio and HDS-7 from the boat at the end of the season. What do you think about using a trailer light style 4 pin connector to join the 4 wires?

Thanks again and have a great day!!

Bob

jimh
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Re: Connecting a Lowrance HDS-7 Gen 2 Touch to a Standard Horizon GX2000 - HELP!

Postby jimh » Thu Apr 07, 2016 4:54 pm

Re using a four-pole trailer light connector: I am opposed to that choice because I have never found any source of those connectors which were not already pre-wired, and usually pre-wired to some wire of 16-AWG or larger size, usually with wire that is rather stiff and has insulation characteristics designed more to tolerating being exposed to road mud, rain, debris, and who knows what, rather than being designed for electrical interconnecting circuits that originate on 24-AWG conductors.

Using pre-wired connector involves more mapping of wire insulation colors because the pre-wired connectors have their own color coding. That would make the connection even more confusing in my mind.

And because the trailer connector wires are connected to connectors of opposite gender, the color code mapping would be even more confusing as it would depend on which connector gender you were using for which side of the connection. In short, a very confusing and inconsistent use of wire insulation colors on top of an already inconsistent and confusing use of wire insulation colors. But I am sure you could use it and it would work. It would just take you a day to re-invent it if you had to troubleshoot the connections.

There is a general problem in data communication wiring to a standard connector that involves the notion of which side of the circuit is being connected to the connector. This is usually referred to with the designators DTE or DCE, which stand for Data Terminal Equipment and Data Communication Equipment. Terminal equipment are the devices that original signals or listen for signals from other devices. Communication equipment is just devices that pass on signals to terminal equipment. The normal method of wiring is to connect all the terminal equipment to the standard connectors in the same way. Then, if you want to make a direct connection between two connectors from terminal equipment (wired DTE), the signals would collide, that is TALKER would connect to TALKER. So you have to use a device called a Null-modem connector or a frame-cross connector, which acts like the missing communication equipment and is wired as DCE on both its connectors, and flips the TALKER to LISTENER and vice versa. That is exactly what my motherboard does. I wire all the devices the same way to their connectors, then connect them to a null-modem or frame-cross connection via the motherboard.

One possible way around this would be to use bi-gender connectors that can mate to themselves. This has been discussed in detail in an earlier thread. See

http://continuouswave.com/ubb/Forum6/HTML/003058.html

rlboeri
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Re: Connecting a Lowrance HDS-7 Gen 2 Touch to a Standard Horizon GX2000 - HELP!

Postby rlboeri » Thu Apr 07, 2016 5:20 pm

Thanks Jim. I appreciate the input (no pun intended). Trailer plug is a no go.

Bob

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Re: Connecting a Lowrance HDS-7 Gen 2 Touch to a Standard Horizon GX2000 - HELP!

Postby jimh » Thu Apr 07, 2016 9:12 pm

Bob--you were a good LISTENER--pun intended.

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Re: NMEA-0183 Interface

Postby jimh » Fri Apr 08, 2016 9:08 am

Bob also commented about making an NMEA-2000 connection:

If I want to connect an engine harness rig to the HDS-7, I know that it goes into the NMEA-2000 port. That looks pretty straightforward. Just buy the harness and run it to the back of the HDS-7 and plug it in.


Bob--this is not quite right. NMEA-2000 is NOT a point-to-point wiring interface. NMEA-2000 is a networking protocol. You build a network backbone with power. You connect devices to the network. This topic is covered in my article

NMEA-2000 Networks and Modern Outboard Engines
http://continuouswave.com/whaler/refere ... A2000.html

The above article will give you advice on making a NMEA-2000 network and connecting an outboard engine to it.

rlboeri
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Re: NMEA-0183 Interface

Postby rlboeri » Fri Apr 08, 2016 9:29 am

Got it Jim. Thanks. I didn't understand that at first, but it now makes sense (building the backbone). I may hold off on that for now. Seems a bit expensive just to see the operating parameters of the new engine. Maybe a project for another day. Also, there isn't a whole lot of room inside the Montauk console to maneuver around and I like clean installations.

Thanks again for the help.

Bob

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Re: NMEA-0183 Interface

Postby jimh » Fri Apr 08, 2016 9:59 am

Re NMEA-0183, FAST TALKERS, and AIS:

I will explain more about the use of the NMEA-0183 HS (high speed) connections or what I call FAST TALKERs. As I mentioned earlier, the use of the high speed 38400 connection is usually found with AIS receiver interfacing. The typical AIS receiver will be set to use 38400 for the signal rate. The reason for the higher speed is to avoid losing data from the AIS receiver. It is possible in theory for there to be so much data coming from the AIS receiver that it cannot all fit into the available time to transmit it via the slower 4800 rate. To prevent the possibility of losing data, the AIS receiver comes set to the higher speed data rate of 38400. This causes problems when interfacing to a chart plotter in two realms.

The first problem is the change to higher speed. The data rate of the TALKER and LISTENER must match. If the TALKER is sending at 38400, the LISTENER must be set to that same data rate. The usual solution is to set the chart plotter interface to the higher speed, to 38400. This is the most common solution.

There is another option: set the AIS receiver back to the standard 4800 data rate. If the AIS receiver is set to the slower data rate, then it is possible that some data might be lost. However, unless you are located in the middle of the English Channel and are surrounded by very fast moving vessels all sending AIS data, it is probably not likely you will lose any data. The typical situation for a recreational vessel to receive AIS signals is for there to be only a couple of AIS-transmitting vessels in range at any time. AIS vessels transmit their data at rates proportional to the speed they are making. The faster the vessel is travelling, the more often it sends AIS data about itself. In my experience, I have never been around a really fast-moving AIS vessel. Most of them a moving along at moderate speeds. That means they are not transmitting practically continuously; they transmit at intervals. And there usually are not more than two or three vessels in range. All of this means that the chance of losing any data if the AIS receiver is turned down to 4800 is probably very low. It may be possible to interface to the AIS receiver at the standard 4800 speed and not be at much risk of data loss. Whether or not an AIS receiver has the option to send at the slower rate probably depends on who made it.

The second problem that arises when AIS comes into the interfacing of a radio and a chart plotter is a limited number of ports on the chart plotter or on the radio. Let's review the usual arrangement of VHF communication radio with digital selective calling and its interface to a chart plotter.

The first interface between radio and chart plotter is for the chart plotter to send position data to the radio. This will give the radio a continual update on vessel position, and the radio will be able to send the vessel position using digital selective calling, either in a distress alert message to all stations or in a vessel position message reply to another vessel or station. In this arrangement the chart plotter TALKER is connected to the radio LISTENER.

There is a second interface between radio and chart plotter for the radio to send data to the chart plotter. This interface will allow the chart plotter to plot and display the position of another vessel that has been received from that vessel by the radio using digital selective calling. In this arrangement the radio TALKER is connected to the chart plotter LISTENER.

Now we have bi-directional communication between radio and chart plotter, using up the facilities of one NMEA-0183 port. This interface is usually made at the standard data rate, 4800.

Into this situation we now add a third device, the AIS receiver. Even if the AIS receiver is integrated into the communication radio, it is common that the AIS receiver will have its own separate NMEA-0183 TALKER signals. (An AIS receiver is never a NMEA-0183 listener; it "listens" on its radio for AIS radio signals, and translates them into electrical signals for the NMEA-0183. It never listens for signals on NMEA-0183, it just talks there.) We want to get the AIS data to the chart plotter, and we want it to be at the FAST TALKER data rate, 38400.

If the chart plotter has a second NMEA-0183 port there is no problem in connecting the AIS TALKER. We would just connect the AIS TALKER signals to the chart plotter second port LISTENERs, and set that second port to the 38400 rate. This is a good solution, if your chart plotter has two NMEA-0183 ports, but many chart plotters today only have one port.

Note that in NMEA-0183 interfacing, you can NEVER have two TALKERs connected to one LISTENER. This won't work, as the two TALKERs will talk over each other, unaware they are wired together. So the problem of two TALKERs and one LISTENER cannot be easily solved by just wiring them in parallel.

The only way to accommodate two streams of data from two TALKERs into one LISTENER is to combine the two data streams together into one data stream using a device called a multiplexer. A multiplexer can blend two data streams into one; it does this by intermixing the signals in a way that preserves both signals. To do this the multiplexer has to temporarily store the signals and resend them in a way that avoids any data collisions in the output stream. As a result, a multiplexer is not cheap. It might cost as much as the radio itself.

When AIS receivers were first integrated into VHF communication radios, they tended to have their own NMEA-0183 port, with a separate TALKER output. Connecting a radio and AIS receiver like that to a chart plotter with only one port was a problem, because you HAD TO USE a multiplexer. The radio manufacturer (Standard-Horizon) was very smart, and they soon figured out that they could work around this problem if they themselves just built the multiplexer function into the radio--and that is what they did on later models of their integrated VHF communication radio and AIS receiver. If you have one of those devices, you will find that there is a setting or option in the radio set-up that allows you to combine the radio digital selective calling data and the AIS data into a single TALKER. The usual approach is to speed up the DSC data to the AIS data rate, and send the combined data at the 38400 rate.

Even if the radio does not have its own internal AIS receiver, some radios (notably the Standard-Horizon radios) that have a little mini-chart plotter in the radio to show AIS targets will still have a built-in multiplexer. The AIS radio TALKER is connected to the VHF radio LISTENER, and the VHF radio then combines that data with its own data into one TALKER output to go to the chart plotter. Again, this avoids having to buy an external multiplexer.

Things just get more complicated from here. If the radio multiplexes both streams into one FAST TALKER stream, then you have to set the chart plotter port to the 38400 rate. Since ports typically will always use the same data rate for listening as they do for talking, this means the data coming back to the radio will also be at the FAST TALKER speed. Usually the radio is up to this task, too.

To summarize, when an AIS receiver is added to the VHF radio and chart plotter interface, we have several options:

--(1) if the chart plotter has two NMEA-0183 ports, connect the AIS receiver to one port and the VHF radio to the other port;

--(2) if the chart plotter has only one NMEA-0183 port, use an external multiplexer to combine the AIS and VHF radio data streams into one stream

--(3) if the chart plotter has only one NMEA-0183 port and if the AIS in integral with the VHF and if the radio can combine their data into one stream, use that method.

If it happens that you have separate AIS and radio receivers, or you have an early model of integrated receiver without the built-in combining function and your chart plotter has only one port and you do not want to invest $200 in an external multiplexer, then something has to give. You must decide if you want the chart plotter to see AIS data from the AIS receiver or to see DSC data from the VHF radio. You can't get both. It is a hard decision to make, and I can't really see any special logic or advantage to choosing one over the other, unless perhaps the ability to plot the position of another vessel that has sent a distress alert would trump all other concerns.

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Re: NMEA-0183 Interface

Postby jimh » Fri Apr 08, 2016 10:13 am

To be more complete, I should also mention that the entire problem of NMEA-0183 interface confusion and the entire problem of interface of an AIS receiver and a DSC radio to a chart plotter will both disappear if all those devices support NMEA-2000. You can connect all of these devices to a simple NMEA-2000 network and they will all exchange data with practically no configuration needed.

To reach this nirvana a VHF Marine Band radio with NMEA-2000 is the key element. Until recently it was not common to find NMEA-2000 in communication radios, but that is rapidly changing. For a survey of what radios presently available on the market support NMEA-2000, see my recent VHF MARINE BAND RADIO ROUND UP 2016 article.

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Re: NMEA-0183 Interface

Postby jimh » Fri Apr 08, 2016 1:12 pm

ASIDE to Bob: I looked up the GX2000 radio at the Standard-Horizon website. From what I can tell that radio does not have an AIS receiver. Making note of that may simplify your interface of the radio to the chart plotter.

Compare at: http://www.standardhorizon.com/indexVS. ... rodID=1559

It says: "AIS (Automatic Identification System) receiver or transponder must be connected to perform AIS functions." I interpret that to mean there is not an AIS receiver integral to the radio.

Note that the chart plotter is not going to provide AIS reception, either. You need a separate and dedicated AIS receiver for that. You can connect such a receiver to both the radio and the chart plotter via NMEA-0183 as I described earlier by using one TALKER to talk to two LISTENERs. You just wire the two LISTENERs in parallel and connect to the TALKER.

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Re: NMEA-0183 Interface

Postby jimh » Sat Apr 09, 2016 9:27 am

For an example of a situation in which a radio had very specific requirements about which NMEA-0183 sentence it wanted to hear in order to get boat position, see my earlier article about an older radio and its behavior:

ICOM IC-M402: Rehabilitating an older VHF Marine Band Radio
http://continuouswave.com/ubb/Forum6/HTML/003562.html

The above instance of NMEA-0183 interfacing was quite a struggle due to behavior in the radio that was not documented in its literature. The radio could only receive one specific sentence with boat position information, even though its documentation insisted it could work with several others.

That situation was another example of the lax standards in documentation. In some cases of NMEA-0183 interconnections the real-world situation will not be as described in the manufacturer's literature—and even perhaps not as I have described it here.

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Re: NMEA-0183 Interface

Postby jimh » Sun Apr 10, 2016 11:55 am

Earlier I commented that omission of the B signals on some NMEA-0183 devices caused a lot of confusion. I explain this in more detail.

If both NMEA-0183 devices implement their interface with the required A and B signals for both TALKER and LISTENER signals, then there is only ONE way to interconnect the devices and only one simple rule is needed. When a device omits the B signal in its interface, it may omit either the LISTENER B, the TALKER B, or both. This means there are four possibilities or four flavors of NMEA-0183 devices; these are

--a device that has both B TALKER and B LISTENER, i.e., a device that follows the standard;
--a device that omits the B TALKER;
--a device that omits the B LISTENER; or
--a device that omits both the B TALKER and B LISTENER.

If we analyze all possible combination of these devices interfacing with each other, we find that there are 4 + 3 + 2 + 1 or ten possible interface situations. This means that instead of being able to interconnect devices in ONE manner according to ONE rule, we now have to deal with TEN possible interconnection arrangements. All of that from just having manufacturers omit the B signals as they felt like it. The interconnection goes from one simple arrangement which anyone could figure out to ten possible arrangement which practically no one can figure out. It is really amazing that this situation could exist under the aegis of being called a standard. I never heard of a standard that provided for ten possible ways to interconnect signals. That is not a standard.

I suppose one could say that interconnecting marine electronic devices using NMEA-0183 was never intended to be something that a boater could do for themselves, and there was an assumption on the part of NMEA and all the manufacturers that a boater would hire a technician to perform the interconnection. That does raise an interesting question: was that the real intent? That is, was this allowed to be made so complicated that only an experience, trained, qualified, knowledgeable technician familiar with serial data connections would be able to accomplish this interconnection? I can't speak to intent, but looking at the real-world I think that has been the outcome. The average boater just cannot get two random NMEA-0183 devices interconnected because he is facing the possibility that there may be ten different ways to interconnect them.

The Coast Guard of the United States wrote to NMEA and asked them to help simplify this problem. The means to simplify the problem are obvious: make manufacturers comply with the standard.

Now there appears to have been some progress lately in compliance, and I want to again recognize ICOM for their work with their latest radios. In their newer model M506 radio Icom has provided the NMEA-0183 interface with the recommended signals (having both A and B signals for both LISTENER and TALKER) and have followed the recommended color coding of the wires that carry those signals. For more details see my prior article about the Icom M506.

Standard-Horizon has for a very long time been providing devices with both A and B signals in both TALKER and LISTENER, but they used wire colors that differ, as did many other manufacturers.

To be fair, I am not sure exactly when NMEA implemented a color code standard, and that standard may only have been adopted recently. NMEA-0183 V.4.0 was adopted in November 2008, and superseded by V4.1, but I don't know the exact time-line when it was proposed or adopted. I don't know exactly when wire color codes became part of the standard, but I suspect it was in version 4.1.

If the color coding of wires is just a recent part of the standard, then manufacturers have a quandary. Should they continue to be consistent with their past practices and keep their own wire color codes? Or should they break with their past and move to the standard colors? One can make an argument for both positions. For a standard that has been around for more than 25 years, it seems like NMEA-0183 took a long time to come up with wire color code standards.

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Re: Connecting a Lowrance HDS-7 Gen 2 Touch to a Standard Horizon GX2000 - HELP!

Postby jimh » Sun Apr 10, 2016 12:32 pm

Regarding how to make an NMEA-0183 connection with some sort of simple disconnect, Bob writes:

rlboeri wrote:I want to be able to easily remove the radio and HDS-7 from the boat...


Bob's need for a simple disconnect of the radio and chart plotter from their NMEA-0183 interface connection is a problem that was recognized by the Coast Guard of the USA. They mentioned this problem in their letter to NMEA sent in 2011 (five years ago) requesting that NMEA figure out a solution. Here is an excerpt from the letter regarding disconnecting a radio or chart plotter:

...the VHF radio / GPS interconnect problem cannot be resolved absent a technological solution. While all DSC-equipped radios are required to have an NMEA 0183 interface and most boaters are understood to carry GPS receivers also having an NMEA 0183 interface, most radios remain unconnected from the GPS. Given that many if not most radios designed for the recreational market provide that interface by use of a wire fan out without a connector and that many boaters must remove their radios after each docking to keep it from being stolen, it's not surprising that few interconnect their radios to an existing GPS receiver...

I request the NMEA 0183 standards committee address this interconnect problem as a matter of urgency and consider revising their standard as appropriate.


Although more than five years have passed since the USCG asked NMEA to come up with a solution to NMEA-0183 interconnections that would provide a means to easily remove the connected devices from the boat, I am not aware of any technological breakthrough having been announced by NMEA, the USCG, or any marine electronics manufacturers. As far as I can tell, there has been nothing done by these three groups that is been publicly announced that would resolve the problem raised in the USCG letter of 2011 or in Bob's problem in 2016.

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Re: NMEA-0183 Interface

Postby jimh » Tue Apr 12, 2016 8:06 am

rlboeri wrote:I just purchased a Lowrance HDS-7 Gen 2 Touch ...I already own a Standard Horizon GX2000 VHF. I would like to connect the two so that the DSC will transmit my position if necessary.


Bob--connecting the NMEA-0183 output of the chart plotter to the radio will provide the radio with your own boat position. That can be sent to other boats using digital selective calling, either in a distress alert message or in a non-distress message such as a position poll request reply message. But this is not the only facet of interconnecting the chart plotter and radio for digital selective calling.

Don't Stop with Just Half the Interface Between Radio and Chart Plotter
Why you want to connect radio data to your chart plotter

When a digital selective calling or DSC radio of Class-D type receives a digital call from another boat, the radio will generate some data output on its NMEA-0183 interface. This data can and should be connected to the chart plotter for display. Making this interconnection is the last step in a series of data communication links. Let me diagram these steps in a simple manner:

BOAT #1
CHART PLOTTER ---> DSC RADIO TRANSMITTER ---> radio transmission

BOAT #2
radio reception ---> DSC RADIO RECEIVER ---> CHART PLOTTER


By connecting the DSC RADIO RECEIVER to your CHART PLOTTER, you will get much greater utility from the data your DSC radio has received. While digital selective calling can be used just for that, to selectively call another station, it is also the modern means of making a DISTRESS ALERT, of notifying all stations that a boat is in a distress situation and requests assistance. By completing the last link in the series, by connecting the radio data output to a chart plotter, your chart plotter will show you the exact position of the other vessel in distress. It will also show you the position of vessels not in distress when those vessels send you their position in routine messages.

Being able to see on a chart plotter the exact location of another boat that is requesting assistance in a distress situation is clearly valuable. To appreciate the location of the distress boat relative to your boat is much more easily done if you can see the other boat's position plotter on your chart plotter. This makes the information more usable. You can see the location of the other boat relative to your boat much better on the chart plotter than you could ever conceptualize it from a set of coordinates in latitude and longitude. You can immediately tell from the chart plotter the distance and bearing to the other boat from your location. You can assess the situation much better.

Now it is true that some modern digital selective calling radios offer a feature of a mini-chart plotter display, and may be able to act as a navigation computer and calculate the distance and bearing, but most boaters are not familiar with or accustomed to employing their boat radio as a navigation aid. Most boaters with an electronic chart plotter use that device to navigate. Transferring the data to the chart plotter from the radio will complete the communication circuit; it will put navigation data onto the navigation display and navigation computer that you are familiar with and normally use to navigate your boat. For that reason, when making an interface between your chart plotter and radio via NMEA_0183, I recommend you take the extra step and provide the connection for the radio to send data to the chart plotter. That connection may be critical at some time in a distress alert situation.

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Re: NMEA-0183 Interface

Postby rlboeri » Tue Apr 12, 2016 2:29 pm

Thanks Jim. Great advice. I will wire the two so that it will display on the HDS when receiving a DSC call from another boat.

Thanks again!

Bob