NMEA 2000; Some Limitations and Cautions

The intent of this article is to familiarize the boat owner with some limitations that I and others have found with NMEA2000.

NMEA2000 technology was borrowed from the non-marine industry (CAN bus) and adapted to the boating industry. Unfortunately, that has left it with some short comings which I will attempt to explain in this article. Also along the way, some manufacturer's have seen fit to produce products that don't fully comply with the nmea standard.

So what is NMEA2000?

The idea for which the bus was developed, is effectively to use a display from manufacturer A, and possibly a sensor from manufacturer B, connect them both to the NMEA2000 network, and the display will show what the sensor detects. In the ideal network, this should be possible. Even NMEA 0183 could do this in a limited way.

Prior to the development of NMEA2000, the boating industry used an older standard for interconnecting equipment; NMEA 0183. This standard was meant to provide a point-to-point connection between two devices. Obviously the point-to-point scheme between just two devices, is very limited. NMEA 0183 is great for simple applications. It’s so good, in fact, that an updated, high-speed version has been introduced to handle the information from AIS receivers. But a truly integrated system requires more than NMEA 0183 can provide.  NMEA 2000 is 50 times faster than its predecessor, NMEA 0183, so it can handle data from up to 50 different devices, but it’s still not the complete answer. That’s because those message headers can occupy any amount of data up to half of each frame. The result is that NMEA 2000 is too slow and inefficient to be useful for complex data such as video images or cartography.  NMEA2000 is a shared network, not unlike Ethernet, where many sensors, displays, and control units can be interconnected on one common network but has severe bandwidth restrictions to what is available over ethernet. 

Also, when in implementing the standard, manufacturers, for whatever reason, have tended to "go their own way" with NMEA2000. For instance, some of the first generation NMEA2000 hardware used different (and incompatible) connectors, depending on the manufacturer. An example of the first generation incompatible connectors are the Lowrance "Blue" connectors, which will only connect to a Lowrance NMEA2000 device. Fortunately, most manufacturers have seen fit to finally embrace the standard, and Lowrance's "Red" connectors are NMEA2000 standardized, and will interconnect with any other vendor's NMEA2000 connector. 

In addition, Maretron does provide field installable connectors that you can use to correct the early manufactured non-standard connections. For instance, with Lowrance "Blue" devices, I cut the connector off and installed a Maretron field installable connector to make them compatible with the network.

However, there are still some incompatibilities across the various manufacturers as each manufacturer is allowed to develop their own private data packets in addition to the standard (or public) packets defined by the NMEA2000 standard and used by everybody. As well, not every device understands every packet - even the standard NMEA2000 defined packets. That rudder packet may be ignored by a certain display if it doesn't have the capability to display rudder information. For that reason, you cannot simply use any display with any sensor. You may need to review the capabilities of the display unit you wish to use to ensure it can read the sensor. Fortunately, most manufacturers do publish this information.

So this leads me to my first caution...
Because of these "private data packets",  some vendors may purposely use private PGNs (those that only that manufacturer's equipment understands). So a certain device may only be able to be read  that manufacturer's equipment, regardless of whether or not the device has a standard connector.
Also, many sensors must be programmed before use, and this usually requires a display unit of the same manufacturer to program them. However, once programmed - the sensor may be able to be detected by any other manufacturer's product. This incompatibility still exists to some degree, so be aware when mixing products from different manufacturers that you may run across a few problems.

Building a Network
NMEA-2000 initially provided for two network cabling schemes; called Mini and Micro. Maretron now offers a third scheme called Mid. So the first question to answer is which one do I use? While each cabling scheme is identical in their bus configuration, the major difference is the capacity each network will provide. They are:

Since each device on the network requires a different amount of power (i.e. a display unit is likely to demand more power than a sensor), some thought must be given to how much total power the various devices require. NMEA-2000 includes a LEN specification that every NMEA-2000 device must provide. ( 1 LEN = 50mA ) Therefore, if a sensor states that it has a LEN of 5, you know that it's current requirement is 250mA. The idea of the LEN is to add them all up, and they should be less than the maximum LEN of the cable. From the chart, essentially the decision of which cable to use is determined by the length of the network you want to install, and the number of devices you need to support, and their total current requirement.

But there are limitations to the network.
The cable distance between any two points (a point being an electronic product or terminator) must not exceed 250 meters (820 feet) for a system based on the Mini or Mid trunk cable or 100 meters (328 feet) for a system based on a Micro trunk cable
AND the cumulative drop line length is the sum of all drop lines, Mini, Mid or Micro cable in the cabling system; This sum cannot exceed 78 meters (256 feet) and no single device can be more than 6 meters (20 feet) from the trunk line.
This places limits for sail boats with masts greater than 20 feet. Effectively it means the main trunk of the Network has to go up the mast, and  to cover instruments or sensors forward of the mast the main trunk has to do a big "U" back, or use a drop line no more than 20 feet. If one wants to put more than one sensor at the top of the mast, the wiring becomes more of a problem with probably the "T' junctions and terminator living outside the mast in the elements.

BUT in addition to this, excessive voltage drop can be an issue when using Micro cable, due to the small size of the power wires within the backbone. Especially on long cable runs you can have excessive voltage drop, even if the total power required by all of the devices is less than the LEN rating of the cable. To determine the voltage drop, an easy formula can be used: Voltage drop = LEN X Cable Length(in meters) X Cable Resistance(in ohms)/100

Clearly for sailboats the micro cable probably can't be used, but one could use a Micro network with a Mid backbone cable. Essentially the only difference is that the Mid cable uses 16 AWG power wires, where the Micro cable uses 22 AWG. Otherwise, all of the connectors and terminators are identical. This is the most cost-effective solution as Mid cables do not really cost much more than a Micro cable, and you can still use the less expensive Micro connectors and drop cables.

One important item that needs repeating is voltage drop.  Attention must also be given to the feed wires. The wiring route from the battery to the NMEA22000 power tee should be minimized to reduce voltage loss. This is sometimes difficult as often, the power switch for the network logically belongs at the helm, however (especially with a center-feed network), the power tee may be located some distance from the helm. One solution is to install a relay near the power tap, which is switched at the helm. The relay would allow a remote switch at the helm to turn the network on or off with minimum voltage loss.

Now my second caution......
Now consider this. If there is a problem with the NMEA2000 back bone, every piece of gear on that network that is dependent on it, stops working. Everything, that comunicates with each other, becomes non effective.

• The GPS stops working, including the plotter
• The fluxgate compass  doesn't work
• Engine instruments don't work
• AIS doesn't show anything
• the autpilot stops working because it doesn't get a heading from the compass 

Get the picture? Now we all know this will happen at the most inopportune  moment.  One may start (trying to find the fault) by unconnecting and reconnecting every connector on the back bone to make sure that they are properly connected. There can be meters of cabling in hard to reach and dar,k places on your boat and all it takes is just one of those connectors to fail and it's "lights out". And if the problem is intermittent, as it very likely will be, then the trouble shooting is going to be ten times more difficult. Then if cleaning and reseating connectors or terminators cannot solve a problem, then by all means seek the professional help of an NMEA-certified electronics technician. Good luck with that, in some of the locations we go to!

What we need to think about is what a network failure will do to us on a dark night in a tricky situation. We must have a backup plan. And because of the issues with trouble shooting these complex systems in remote places, it would be good to have a plan for continuing our cruise without all this interconnection potential problems. Here's some things that could help

•  Have a plotter with a separately connected GPS sensor, or GPS with paper charts, available. Or consider an iPad with it's own charting system. We use it all the time now, to compare two separate chart systems. It has it's own GPS
• There is a separate AIS unit with its own screen and direct connection to its own GPS within sight of the helm. Some of the latest up market VHF have AIA receivers on the screen.
• There are backup hard wired, or mechanically connected, engine instruments.
• And don't forget a properly swung magnetic compass. 

 Now it's not that I'm against NMEA2000. I like the idea of just one cable running the length of the boat. But consider this; since the 14 years NMEA2000 has been out, we now find a class of products specifically marketed to users of existing navigation software that offer to gateway from NMEA2000 physical networks to USB, translating NMEA2000 packets on the fly to NMEA0183 sentences that existing software can read. This reveals that NMEA2000 adds little information and little value to the contents of an NMEA0183 stream of navigation data. The coupling of NMEA2000 to a proprietary physical network is so tight that there is no standard for shipping it over USB, RS232, Ethernet, or any of the other physical networks commonly used in the general computing market at the present time. However, in the near future,  NMEA is going to release a standard called "OneNet" for interfacing NMEA2000 to ethernet and I suspect the bridges to USB etc will appear around the same time.

So when that comes about, are boats then going to run another cabling system around their boat to accommodate for the higher bandwidth of devices today, that NMEA2000 can't handle? It sort of defeats the purpose of running just one cable. NMEA2000 specification itself is proprietary and expensive. Ethernet is more of an open standard and open source and open protocols grow markets and create opportunities. NMEA2000 sharply illustrates the obverse of this point. It is tied tightly to a specific physical networking scheme, and has made the specification expensive and proprietary. And so in 2014, I look at NMEA2000 and see how atypical and archaic it looks. Binary packets instead of HTTP?  Proprietary physical layers that don't lead to a nice plug and play? And bandwidth handicapped! This is not the direction the rest of the Web-enabled  world is moving. Imagine having google earth directly overlayed on to your charts, integrated web tracking of your AIS information as standard on your chartplotter; just to mention of few.

NMEA 2000 operates at 250 kbits/second, very slow compared to Ethernet. Because of this bandwidth limitation, sonar and radar overlays can't be sent over the NMEA 2000 networks, that is why all the major manufacturers have their own proprietary networks for sonar and radar integration with their large display devices. But, NMEA2000 has a few advantages over Ethernet, cost, a collision-less system, time sensitive point to point delivery to name a few. It was developed to function in electrically noisy environments and to have a predictable delivery time for messages. Ethernet does not always provide a real time predictable delivery time or quality of service capabilities.

goal for OneNet is to "Transport NMEA 2000 network messages on Ethernet in a standardized manner" or as the release's subtitle says "Think of it as NMEA 2000 on steroids." That's because OneNet will break out of N2K's speed and node limitations big time, like increasing the maximum number of devices from 50 to over 65,000! (Now, that would be quite a vessel.) - See more at: http://www.panbo.com/archives/2012/08/onenet_nmea_finally_creates_a_marine_ethernet_standard.html#sthash.chOhWjdS.dpuf

Now, I'm not going to comment much about OneNET (as it still hasn't been released in it's final form), but it's stated goal is to transport NMEA2000 packet data on Ethernet in a standardized form. Think of it as NMEA2000 on steroids. This will break out nmea2000 speed limit and node limitations by a huge amount. The number of devices would increase from 50 to over 65,000! I don't see OneNET replaceing NMEA2000 or NMEA 0183 in the near future, but I am left wondering why NMEA has made this protocol so late, with an internet PC centric world everywhere. Also, I suspect, at least initially, it won't play nice with with existing ethernet devices, and I'm pretty sure it won't carry both NMEA2000 packets as well as NMEA0183 data sentences at the same time. You will need to use another gateway for that. However, once they convert NMEA to Ethernet the possibly of connecting to the outside world is endless.

What all this boils down to is that we boaters have three main ways of creating a network, each of which complements the strengths and weaknesses of the others: NMEA 0183 is perfect for very simple systems—it’s tried, tested, and virtually universal. NMEA 2000 is for most current multisensor/multidisplay systems, and Ethernet is suited to handle large volumes of complex data. Your best to consult your marine electronics expert to see what best suits your application. For me, I'm still running mostly NMEA0183, and with that, I have a number of back up solutions which give me confidence that when the lights go out on my main network, I can use the backup to get home safely. By that time, OneNet may be well established, and it could be a good time to upgrade. I'll just have to do my homework on the best system at the time, which is what we all do mostly.

A Comparison of the main types of networks/standards

OneNet will not replace NMEA 2000 or NMEA 0183 within the foreseeable future. Each will have its place on a boat - See more at: http://www.panbo.com/archives/2012/08/onenet_nmea_finally_creates_a_marine_ethernet_standard.html#sthash.chOhWjdS.dpuf

goal for OneNet is to "Transport NMEA 2000 network messages on Ethernet in a standardized manner" or as the release's subtitle says "Think of it as NMEA 2000 on steroids." That's because OneNet will break out of N2K's speed and node limitations big time, like increasing the maximum number of devices from 50 to over 65,000! (Now, that would be quite a vessel.) - See more at: http://www.panbo.com/archives/2012/08/onenet_nmea_finally_creates_a_marine_ethernet_standard.html#sthash.chOhWjdS.dpuf

goal for OneNet is to "Transport NMEA 2000 network messages on Ethernet in a standardized manner" or as the release's subtitle says "Think of it as NMEA 2000 on steroids." That's because OneNet will break out of N2K's speed and node limitations big time, like increasing the maximum number of devices from 50 to over 65,000! (Now, that would be quite a vessel.) - See more at: http://www.panbo.com/archives/2012/08/onenet_nmea_finally_creates_a_marine_ethernet_standard.html#sthash.chOhWjdS.dpuf

Beginners Guide to Raymarine's Seatalk and Derivatives

Seatalk is Raymarine's proprietary communication "language" for interconnecting Raymarine instruments. Unfortunately Raymarine keeps the technical details of Seatalk secret and so the nitty critty of the system often has to be reversed engineered if designing something to work with Ray's different seatalk buses. However, most of you out there just want a plug and play system and I can say that Raymarine has done a pretty good job of that.

Raymarine has produced four communications protocols featuring SeaTalk as the root of the protocol's name. These include:

• SeaTalk (also referred to as SeaTalk1 or first generation SeaTalk)
• SeaTalk2
• SeaTalkng
• SeaTalkhs

Seatalk to nmea0183 and RS232

SeaTalk/Seatalk1 
Seatalk is the same as seatalk1. Ray just added the 1 after developing other more modern buses that they called seatalk2, seatalk hs and seatalk ng. Seatalk1 is ray's propriety version of nmea 0183. Seatalk1 is readily able to be converted to nmea 0183. Raymarine supply an accessory (Part #E85001) for converting Seatalk to NMEA 0183. Also, ShipModul's and Brookhouse's MUXes also convert Seatalk to NMEA 0183. Alternatively, if you have the ST60 repeater, you will find a NMEA output on the device which can connect directly to a SOB COM port.

Seatalk which is often referred to now, as seatalk1, is a multi-talker, multi-listener approach using serial data at low data rates (4.8Kbps). Compare that to nmea 0183 which is predominately a single talker, multi listener unless one uses a multiplexer.

The network used proprietary cables and connectors that were designed for the marine environment. The connectors are three pin molded and incorporate 1/8” automotive spades as mating elements. 

Seatalk cable

The network could be implemented as either a simple daisy chain structure or the more robust approaches of backbone, spur or star design.

SeaTalk uses three wires, connected in parallel to all devices on the bus:

1.  +12V    Supply, red
2.  GND    Supply, grey
3.  Data     Serial Data, yellow

There is no master on the bus. Every device has equal rights and is allowed to talk as soon as it recognizes the bus to be idle (+12V for at least 10/4800 seconds). Low priority messages use a longer or randomly selected idle-bus-waiting-time. This allows messages from other devices with a higher priority to be transmitted first. The different waiting times of all devices make data collisions (two or more devices start talking at exactly the same moment) very rare.

Unlike the Seatalk2 and Seatalk ng communications protocols, Seatalk communications protocol does not feature termination. Any Seatalk instruments and instruments or autopilot components featuring a 1st generation Seatalk interface which does not have a Seatalk ng interface (ex. ST40/60+ Instrument Displays, ST6001, etc.) can be interfaced to one another, in any order, using standard Seatalk cable. Seatalk cables can be joined using a Seatalk Junction Block, splices, marine grade terminal strips, or R55006 Seatalk Auxiliary Junction Boxes. This method of interfacing is referred to as a Seatalk bus. The Seatalk bus will typically be powered via a Seatalk Auxiliary Junction Box or via a Seatalk power cable which has been connected to one of the Seatalk devices.

SeaTalk^hs
Seatalk hs is raymarines high speed version of ethernet. It is plug and play.

By using SeaTalk hs networking, you can instantly transfer radar, chartplotter, fishfinder, thermal imaging and navigation functions. Configuring your system is a simple as determining which SeaTalk hs devices you'd like to have (chart, radar, sonar, video) and where you would like to see them.

The most basic system may consist of a C Series Widescreen or an E Series Widescreen with a DSM30, DSM300 or a radar. The components are all auto sensing, there is no complicated setup required. Simply plug them in.

E series integrates nmea 0183, 2000 and seatalk hs

Raymarine E-Series can input/output data via good old NMEA 0183, Seatalk (Ray’s proprietary improvement on 0183), Seatalk2 (sort of NMEA 2000 but seatalk ng is the bus that mimic's nmea 2000), and Seatalk HS (actually Ethernet).  Plus the E’s (and C’s) do something called data bridging where they take data from one bus and put it onto another bus.

Raymarine has network switches when working between 3 to 8 devices. They are.......

HS-5

1. The new HS-5 Seatalk hs Network Switch which has five ports
2. Seatalk hs Network Switch which has 8 ports.

The following devices have Seatalk hs connections and can be easily connected together via the Seatalk hs network switch and Seatalk hs network cables:

1. New e Series Multifunction Displays
2. New c Series Multifunction Displays
3. C Series Widescreen Multifunction Display
4. E Series Widescreen Multifunction Displays
5. G Series Navigation
6. E Series Classic legacy Display 
7. T Series Thermal Cameras
8. DSM30 and DSM300 Fishfinders
9. Super HD and HD Digital Open Array Radar
10. Digital Radome Radar

Seatalk 2

Seatalk 2 is a five conductor system that has some N2K functionality. This network does require terminators at each end.

Seatalk 2 network 

and here is the comparison of the different network cables below.....

seatalk1 cable

 Provided you do not want to connect to NMEA2000, you can connect a Seatalkng system to Seatalk(1) using a suitable Seatalkng bridging product (such as an ST70 instrument) and adapter cables to connect the two systems.

If you intend doing this, note that:

You can connect a single Seatalk(1) network to Seatalkng using an adapter cable and one bridging product (e.g. ST70 instrument).

You can connect two separate Seatalk(1) networks to SeaTalkng using different adapter cables and bridging products (e.g. ST70 instruments), but the two Seatalk(1) networks must NOT be connected together. See below for the diagrams

Seatalk2 cable

Seatalk 2 whilst is not plug compatible with NMEA2000 or SeatalkNG, adapter cables are available. Raymarine A06048 is the part number for seatalk ng to seatalk2 adapter cable. (not shown).

Any data available on the SeaTalk2 network will then be available on the SeaTalk NG network. Only one connection is needed per network.
This adapter enables an existing instrument network, using the SeaTalk2 5-pin connectors, to interface with a SeaTalk NG network.*Note that this cable cannot be used to connect a SeatalkNG network to the Seatalk2 port on the back of a Raymarine E-Series display. Use cable A06061 for that purpose.

Older Style of Seatalk hs cable

Raymarine’s current generation of ethernet cables replaces the RJ45 connector (seen to the left) with a waterproof vibration tolerant twist pin connector that is designed for the marine environment.
The new RayNet (F) to RayNet (F) network cable can be used to daisy-chain 2 adjacent RayNet devices together. It is also useful for joining two adjacent HS-5 Network Switches together on larger systems where multiple switches are necessary.

Rayamrine has a number of adapter cables which most are to adapt the older RJ45 with the newer Raynet cable. They can be seen here

An NMEA standard for the transfer of NMEA 2000 messages using an Ethernet protocol is being developed that will be called OneNet.

Correct method seatalk1 to seatalk ng via ST70 instrument

No ST70 for bridge

Do not link different seatalk1 networks together.

Seatalk ng

SeaTalkng is an interconnection bus for Raymarine products, and comprises a main
backbone to which Raymarine products are connected via spur cables.

Seatalk ng is basically NMEA 2000 and can be connected to a NMEA 2000 network via an adapter cable.

SeaTalkng comprises a single backbone terminated with two terminators, one at each end. Spur cables connect the backbone to individual Seatalkng products.

Small diameter cable connectors are used throughout the system, to make installation easier. Cables and connectors are color-coded to reduce the likelihood of misconnection.

A wide range of different cable lengths provides flexibility and obviates the need for cutting and splicing cables.

Three-way, five-way and in-line connection pieces are available to connect cables, to

deploy Seatalkng as required.

Typical seatalk ng network,

One of the things I like with the raymarine seatalk ng is the 5 way connector blocks. Something I wish NMEA 2000 would have. NMEA 2000 also has this and be seen here.

Plan the route of the Seatalk ng backbone so that it runs as close as is practicable to the intended location of each Seatalk ng product, to keep spur lengths to a minimum.

Products connect to the backbone via spur cables. Spurs connect to the backbone via either a Seatalk ng T-Piece or a Seatalkng 5-Way Connector

Seatalk (sic) has a number of adapter cables, of which one is the seatalk ng to NMEA 2000 adapter cable. Part number A06045 for the female cable and A06046 for the male cable.

You can see a range from rayamrine, of the adapter cables here.

Raymarine with Wifi and Bluetooth

This is still in it's infancy, but is now working. It can only get better from here. There iOS app has been released and can be viewed here. I wonder if it will work with an iMux instead of their e-series display WiFi?

RayControl transforms your tablet into a full function Raymarine MFD. Remotely control and view electronic charting, sonar, radar, and even thermal night vision, right from your tablet.

RayControl emulates Raymarine MFD’s with touch screen interaction and a virtual slide out MFD keyboard. The slide out virtual keyboard gives you control of all MFD functions and the virtual uni-controller allows you rotate through menus and adjustments effortlessly.

Take a look....

Perhaps in the not to distant future an iMux will no longer be required as manufacturers integrate wifi and bluetooth into their products. I look forward to that, but in the mean time, the cheapest option for someone who has already invested heavily in existing technology is a Wifi seatalk/nmea multiplexer.

Whether you use Navionics or iNavX on your iPad, you have to get the various Seatalk output data from your instruments, GPS, AIS into a format that can be understood by either of those programs. That is exactly what you have to do to get the same information into an onboard computer. Typically, this is accomplished by a multiplexer which is spliced into the Seatalk network. Most of the data comes at a slow baud rate, but the data from an AIS is at a much faster rate. The multiplexer takes all of these data streams and sorts them out and sends them in a controlled output to the computer or if WiFi capable, to your iThing. But now, with MFD's having WiFi integrated, it takes the hassle out of setting up a WiFi multiplexer. Also, it is one less device to purchase, to get WiFi   Looking at the video, RayControl also has some high bandwidth data displayed on your iThing. I'm impressed, and wonder what the other big manufacturers will bring to the market.

One last question. Does the new WiFi enabled e-series display integrate with iNavX for the data? I'm not expecting the chart to come across, but basic nmea 0183 sentences. Perhaps a reader will let us know.

Beginners guide to Nmea 2000, Nmea 0183, and bridging

Most of the new instruments for boats are now coming out with NMEA 2000 (National Marine Electronics Association) as the preferred method of connecting various devices on a boat. The NMEA 2000 transmits data through Controller Area Network (CAN bus). It simplifies the connection and shares information among different devices by using a single trunk cable. Compared with NMEA 0183 in “RS422” interface, NMEA 2000 has better transmission reliability and shares data easily in a network. 
Here's a good primer for constructing NMEA 2000 networks
And Power and Motor Yacht has an article worth reading; " A Breakdown of Protocol: How a Boats' System Converse"
 I'll attempt here to explain some of the data networks and how to connect various different types.

Typical Nmea 2000 Backbone.

The Nmea 2000 Backbone is pretty easy to set up as you can see from the diagram. All you need is the right parts/cables/terminators and follow some simple rules. Read the link above for the do's and don't's. Even hooking in a new device is relatively easy. More hooking up Legacy Equipment to this network later. But here is some tables that illustrate the differences between the old and new nmea standards.

Nmea 2000 verses Nmea 0183

Nmea 0183
Nmea 0183 and it's various versions is the older standard from NMEA. But it will be around for quite a bit longer.
NMEA 0183 is a low-cost, low-capacity, single-transmitter/multi-receiver network for
interconnecting marine electronic devices, also known as a “single talker/multiple
listener” interface. The NMEA 0183 Standard, along with RS-232, RS-422, etc., is often called “Asynchronous Serial” interfaces. This means that data is transmitted serially (bit-by-bit) on a single line. Furthermore, the transmission is asynchronous because no “clock” signal is transmitted with the data.

Single Talker, Multiple Listener

Multi Talker to Single Listener

There are multiple versions of the NMEA 0183 specification in use in the marine industry.  Prior to NMEA 0183 version 2.0, including NMEA 0180 and NMEA 0182, the hardware employed a single ended interface implemented with one signal wire and a common ground. Early versions of the NMEA 0183 Standard specified that “listeners” must be isolated from ship’s ground but allowed ”talkers” to be simple single-ended drivers (referenced to ground). Furthermore, the input impedance of a listener was specified to be greater than 500 ohms, and opto-isolation was recommended. Later versions of the Standard retained the opto-isolated listener recommendation, but revised the talker requirement to be RS-422 compliant. As a result, there exist some instruments (talkers) with single-ended outputs and others with RS-422 (differential) outputs.
All implementations from 2.0 and later employ a differential interface with two signal wires.

Due to the difference between the single ended and differential interfaces 
implemented, older versions of NMEA 0183 prior to 
version 2.0 cannot be connected to equipment supporting NMEA 0183 version 2.0 or higher without proper interface circuitry. Do not connect one of the signal wires of the differential interface to the common ground of the single ended interface!  
Single-ended drivers are perhaps the simplest to design. The desired state indicated by the presence or absence of a voltage above some threshold. For NMEA 0183 (ver. 1.5), a “one” was any voltage less than +0.5, while a ‘zero’ was any voltage greater than +4.0. Besides their design simplicity, single ended drivers need only one wire to transmit data. Since voltages are referenced to Ground, no signal return line is needed.
Interconnecting NMEA instruments can be a real hassle if one is not familiar with the different types of connections. The NMEA-0183 standard specifies the talker ports (outputs) and listener ports (inputs) to be differential. This means that the data is transported by means of voltage levels over two wires, separated from ground. Roughly, the voltage levels swing between 0 and 5 Volt, and both wires are in opposite phase. When one is at 5V, the other is 0V and vice versa
Some  manufacturer's though, would not adhere to the standard. It would have been perfect if you could simply connect the 'A' terminal on a talker to the 'A terminal of a listener and do the same with the 'B' terminal. So what came about were these "single ended" connections which complicated matters for the DYI person.

There are four types of connections possible with differential and single ended.
They are,

• Differential > Differential
• Single ended > Single ended
• Single ended > Differential
• Differential > Single ended

What distinguishes NMEA 0183, RS-232, and RS-422 from each other is their physical voltage and current interface levels. NMEA 0183 originally allowed “single-ended” drive, but was later updated to differential drive (RS-422).  RS-232 is a bipolar interface and RS-422 is differential drive. These are explained in more detail next.

RS232 data is bi-polar. +3 TO +12 volts indicates an "ON or 0-state (SPACE) condition" while A -3 to -12 volts indicates an "OFF" 1-state (MARK) condition. Modern computer equipment ignores the negative level and accepts a zero voltage level as the "OFF" state. In fact, the "ON" state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced. RS-232 interfaces are somewhat sensitive to environmental noise (motors, ignition pulses, etc.) and are therefore usually limited to installations of tens of feet or less.

RS-422 is most commonly called “differential drive”. Two wires, A and B are used for this interface, but neither wire is grounded. A “zero” is produced by making A positive with respect to B and a “one” by making B positive with respect to A. It is therefore the direction of current flow rather than a voltage level that determines the logic state. It is important to note that neither RS-422 signal line (A and B) can be connected to Ground.


RS-232 interfaces are somewhat sensitive to environmental noise (motors, ignition pulses, etc.) and are therefore usually limited to installations of tens of feet or less.
RS422 standard that is widely used for the transmission of high speed serial data. Transmissions using RS-232 are limited in their speed and the length over which data can be transferred. Normally the maximum is 19.2 k baud and the distance 15 metres, although for slow transmission speeds longer lengths can sometimes be used.
It is very immune to external noise, and so can be used over greater distances (hundreds of feet).Under the latest version of NMEA 0183, all talkers are supposed to be RS-422. RS-422 is not strictly compatible with RS-232, but most RS-422 drivers will act like a single-ended driver if only one output line is used. In this way, the RS-422 “A” output can be used to drive either an RS-232 input directly. The “B” line is simply left unconnected.
Fortunately, both types of outputs will drive opto-isolated listeners. What is "optp-isolation"... Opto-isolators are actually quite simple devices. They consist of a light source (LED) coupled to a photo transistor.  The input data signal turns the LED on and off which causes the photo transistor to switch on and off. The LED and photo transistor are completely isolated from each other electrically. The data source (talker) must provide sufficient current and voltage to power the LED. A current limiting resistor in series with the LED is generally used for protection.  The advantage of opto-isolated inputs is that virtually any type of driver can activate them.




Here are the possible connections...

Differential to Differential

Single ended to Single Ended

Single ended to Differential

Differential to Single ended

When multiple listeners of different nature (single-ended vs. differential) are to be connected to a talker port, the same connection rules apply. Here, two differential and one single-ended listener are connected to the differential output of a multiplexer.

Multiple Talker Circuits.

Where a single listener requires data from multiple talkers, a data multiplexer or combiner will need to be installed.  Data multiplexers buffer the input sentences from each talker, and provide a single data stream that may then be connected to multiple listeners.
There are a number of companies that provide multiplexers and these days, you can even get a multiplexer that outputs Nmea 0183 Sentences via Wifi.

There is a practical limit to the number of talkers that can be combined based on the available bandwidth and the number of sentences that each talker transmits.  Products are available that combine up to four talkers and replicate the combined output into four or more new talker circuits, each capable of supporting multiple listeners.   Here you see a typical setup using a Brookhouse multiplexer which has also the capability of converting Seatalk to Nmea as well.

Typical Multiple Talkers Combined to Single Listener

But what do we do if we want to mix our older hardware which runs on the older "bus" of Nmea 0183 and the newer Nmea 2000?
Well, I can only find two devices that do this directly. Presented here are......
Actisense
Here's some of what the company says... 
The Actisense® NMEA 2000 to NMEA 0183 Gateway is the easiest way to link between a boats old and new data networks. The NGW-1 can convert NMEA 0183 data into NMEA 2000 data and vice-versa.

The NGW-1 allows current NMEA 0183 equipment to be kept, or allows the latest NMEA 2000 equipment to be incorporated into the NMEA 0183 system. Once the single cable NMEA 2000 bus system has been installed, data from any node on the bus can be listened to by any other, including any NMEA 0183 instruments connected using an NGW-1.

The NMEA 2000 network acts like a bi-directional multiplexer, allowing all data sources to talk and listen to each other.

At the moment, this seems the simplest way of achieving a bi directional talking of the two Nmea standards.

Amec NK-80

An alternative to Actisense would be this product from Amec
NK-80 is a gateway between NMEA 0183 electronics devices and NMEA 2000 device/network. NK-80 allows users to connect NMEA 2000 network with their existing NMEA 0183 devices. 

Key Features of NK-80 according to the company: 

‧Upgrade NMEA 0183 equipment to NMEA 2000 
‧Conversion between NMEA 2000 and NMEA 0183 
‧Full NMEA 2000 compliance 
‧Full NMEA 0183 compliance 
‧Compact size, easy for installation 
‧Isolation between NMEA 0183 and NMEA 2000 
‧Fully galvanically isolated for electrical spike protection

One final way that may prove beneficial to some. There are now devices such as MFD's that have both NMEA 2000 and NMEA 0183 in and out. It may seem odd that the Raymarine E-Series can input/output data via good old NMEA 0183, SeaTalk, (similiar to NMEA 0183) SeaTalk2 (actually NMEA 2000), and SeaTalk HS (actually Ethernet).  So the E’s (and C’s) do something called data bridging. It works by taking data, if it is not on a particular level, and converts and sends the data on at the higher level. I suspect it is converted up to the higher level, but not converted down. Perhaps someone can leave a comment and advise as to whether this is true?

That's it for now. Hopefully soon, I'll post about Raymarine and their Seatalk interface, along with it's interface with NMEA2000 and NMEA0183

Cruzpro

Shortly after writing about my Paddle Wheel Dilemma, Mark Corke also wrote about the Cruzpro (also mentioned in my blog) and offered up some pro's and con's. Go over to Mark's blog and have a read. He is going to buy one and install it. Keep an eye on his blog site for updates and reports of this interesting device.

Paddle Wheel Transducer Dilemma

A lot of you out there will know of the frustration when your paddle wheel gets clogged, and fails to work. Pulling the thing out (if it is a thru-hull) to clean is not only a chore, but can let in a bit of sea water.
I have an Airmar DST800 (Depth, Speed, Temperature)  which I bought around 2005. It was for Raymarine ST60 instruments and did the job fairly well. When removing the transducer, I get an ingress of water because it does not have a self closing  valve which would close as the transducer is removed. I believe (I have been told, correctly or incorrectly) this is still the case for the Raymarine  DST800; and yet most of the other transducers of this model (post 2005) have the self closing valve. See their brochure here for the DST800. I was told the transducers were changed after 2005  so that they have the self closing valve, BUT not the Raymarine version. A little research was required.

My instuments (ST60 Tridata) work by taking an analog signal input from the transducer. My ST60 will only display seatalk input if changed to a repeater display. It will not display nmea 0183 or 2000 data, UNLESS one can convert that data back to seatalk (1)

My choices after some contemplation were
 1. Stay as I am
 2. Change to a new DST800  with nmea 0183 output,  and get a nmea 0183 to Raymarine
 converter so I can have the self closing valve.
 3. Purchase a new DST900-EM which has no paddle wheel and picks up speed by electromagnetic induction, also with a self closing valve

Considering those choices, here are the problems.

1. There are plenty of multiplexers out there that will convert Seatalk to nmea 0183; but I've yet to find one that converts nmea 0183 to Seatalk. This conversion would not be needed if the new DST800 has BOTH a self closing valve and analog output. The brochure here says it only comes in nmea 0183 & 2000 outputs.
2. After contacting an Airmar dealer through email, the DST900 isn't on the market as yet. Here's what he said.(Oct. 2012) "The DST900 is still not available yet from Airmar. No date or cost on the release." AND (as the brochure reads only nmea 2000) I would then have to change nmea 2000 to nmea 0183 and then to Seatalk or analog signal. Can't be done, I think. More about that later. I made some more enquiries.

Notwithstanding that statement, I'm sure they will be released soon and have included here a little of what Airmar has to say about the product. For those with nmea 2000, a great choice when it is released!

Here's Airmar's blurb. Check their Brochure here.

Airmar’s NEW DST900-EM is the industries first all-in-one depth, speed, and temperature sensor with no moving parts. The NEW innovative speed sensor circuitry creates a magnetic field in the water. Speed is accurately measured by the voltage created through 

conductive properties of water, combined with the forward motion of a vessel. With no moving parts, the DST900-EM is capable of accurate readings at all vessel speeds. What sets the DST900-EM apart from the traditional speed paddlewheel is the excellent 

accuracy at very low-speeds between 0.5 and 5 knots (0.6 and 6 MPH)—an important feature for sailboat racing. The DST900-EM’s accuracy at high-speeds up to 50 knots (57 MPH) makes it an important sensor for powerboats. By eliminating the traditional paddlewheel, there is no fouling, and drag is reduced to a minimum of 6.3 mm (0.25”) below the hull due to the low-profile housing.  

Rated at 100 W power, the 235 kHz depth transducer provides depth readings in as little as 0.45 m (1.4’) of water and can reach depths up to 180 m (600’). The narrow-beam of this transducer will 

deliver accurate depth readings at speeds over 35 knots (40 MPH). 

Additionally, the 235 kHz frequency prevents mutual interference with other echosounders on the vessel. 

The DST900-EM’s fast-response water temperature sensor provides ±0.2°C (±0.1°F) of accuracy to help in finding the optimum conditions for fishing or swimming. Airmar’s patented housing design incorporates the popular self-closing valve. When the transducer is removed for cleaning, the valve minimizes water flow 

into the boat. 

The lightning-fast processor inside the DST900-EM makes calculations every second, so it can respond quickly to changes in speed, depth, and temperature. This data is then sent to the NMEA 2000®network via a single devicenet cable for an easy plug-and-play installation.

Sounds like it will just support nmea 2000,

But recent enquires put a different light on the question of legacy formats. A recent email had this to say...  "The DST900 most likely will have the self closing valve. The DST900 will be capable of outputting Analog, NMEA 0183 and NMEA 2000 so it should cover all your needs including working with the ST60.

At this point we still do not know when it will be released." 

So if I wait long enough, maybe this will be my solution of choice. Sometimes this is called vapor ware.

So, I'm no closer to solving the paddle wheel "blues". However....

Here's another solution. Don't have a transducer at all!!

This solution by Cruzpro here is great if you don't mind SOG instead of STW .

The CruzPro SOG1 takes NMEA 0183 data from a GPS and creates an analog paddlewheel signal so you can display Speed Over the Ground (SOG) on a standard speed log without a transducer.

SOG1 NMEA 0183 to paddlewheel signal converter

The SOG1 creates an analog paddlewheel output signal from a GPS NMEA 0183 $GPRMC sentence so you can display Speed Over the Ground (SOG) on a standard analog or digital speed log.

The SOG1 replaces a thru-hull or transom mount paddlewheel speed transducer and provides Speed Over the Ground in place of Speed Through the Water.

No more cleaning of paddlewheels to remove marine growth or weeds.

Provides a more accurate estimate of time to get to your destination since it is not affected by water currents and provides a resolution of 0.01 knots from 0.00 to 60.00 knots.

The SOG1 is powered by 12VDC and draws less than 0.020 amps.

NOW,

Back to to Airmar's current selection; Thinking that the new DST800(V) isn't available for Raymarine instruments, I came across the following info in this Newsletter. It appears that perhaps they do have a new DST800 that will do the ST60 instruments AND the self closing valve. Read below their blurb....

The DST800 Retractable TRIDUCER® Multisensor is now available with

a VALVE

The industry’s only depth, speed and temperature retractable transducer is

now available with a valve! The DST800V meets customer demand for our

patented valve assembly that prevents water from rushing into the hull when

the insert is removed. And the addition of a valve lets boaters service the new

DST800V while the vessel is in the water, saving time and money!

Offered in NMEA 2000®, NMEA 0183 or Analog versions, the new DST800V simplifies any

installation by delivering three readings while having only one hole drilled in the bottom of the

boat. These three important readings can then be displayed on instruments, echosounders,

NMEA repeaters or Network navigation systems.

The new DST800V includes:

• Airmar’s patented self-closing valve assembly.

• Two O-rings, one at the top and one at the bottom for a water-tight seal.

• Industry standard 2” thru-hull hole size.

• NMEA 2000®, NMEA 0183 or Analog versions available.

Although the new DST800V contains Airmar's popular TRIDUCER® insert, the original DST800inserts cannot be retrofitted into the new housing with a valve. This is because all DST800 insertsmanufactured prior to November 2005 have ribs and an alignment key. If an older style insert withribs is pushed into the new P617 housing (which contains a valve), the insert will damage thevalve assembly.

However, all DST800s manufactured after November 2005, do not have ribs or an alignment keyon the insert. These can be retrofitted into existing installations that have a plastic P17, bronzeB17 or stainless steel SS577 housing. To insure proper sealing when installing a DST800 in aP17, B17 or SS577 housing, the yellow O-ring at the top must be replaced with a black O-ring.

Sooooo; my choices now are, I can wait for the NO paddle wheel DST900 and change out (choosing the analog version). Change to the upgraded DST800 (with self closing valve) but have to pull it out occasionally to clean the paddle wheel ( but not suffer the ingress of water due to a self closing valve), or go no transducer and loose the depth and water temperature in the process. Hmmmm, I think I will just sit and wait another year!
What would you do?

Holding Tank Level Monitor

 I wrote previously about Gobius Tank Level monitoring. You can review that blog here.
On my forward holding tank, there is no way to easily place an inspection hatch for level monitoring (as per my previous blog). That is because to get at the tank, I have to remove a heavy mattress and covering board to see the tank. We also use the forward berth a lot, to store our extra equipment and sometimes supplies. So it can be a real hassle getting to the forward tank.
We use to empty the forward tank whenever we emptied the aft tank; but that was a
waste of tank treatment if the tank was say only 1/4 full.
So for me the solution lay with this new tank monitor device. You can see in the three photo's, the installation of the three main components of the device. The level indicator was placed just over the head electrical switch. The "computer" was placed on the side of a locker, next to the holding tank and the sender unit on the side of the tank as indicated in their web site tank calculator. Everything was easy except the sender unit. I followed their direction to the letter, but it would not stick. Sanded level, made sure tank surface was above 20 degrees C, and wiped surface prior with their special towel. Didn't stick! They supplied a spare sticky patch with the kit, so I tried the spare. Still no stick! So I wrote to their support email and two days later got a reply. The insinuation was that I had not followed with their directions

but they would post a new sticky patch out to me. Well, that wasn't going to help much, being on a boat, "cruising the world". So I took matters into my own hand. This they didn't recommend, but I did it anyway. I sanded the sticky patch that wouldn't stick so that it had a rough surface. Next I applied a very thin layer of 3M 4200 and applied the sender unit to the tank. I had duct tape ready to tape this unit to the tank until the 4200 set up. Next morning, the sender unit was attached "rock solid" to the tank. I proceeded to change the default settings and calibrate the unit as per their directions. And the unit works great. I may at a later date put a buzzer in the analog circuit which is detailed in their instructions. In the mean time, the 3/4 warning light comes on when the tank is 3/4 full. You can add extra sender units to have a 1/4 and 1/2 level light as well, but I'm very happy with this at the moment.