Connecting the Dots

Today's tools for collecting, mixing, and presenting electronic data make the networks what they are

Plugged in across the backbone of the networks that Frank Cassidy describes are all the tools that see and interpret our fluid environment, then convert the information into bites we can digest. To take a fix on where the state of the art in marine electronics finds itself, we've explored the aisles of the season's boat shows to offer you this firsthand look at what's new. Where possible, we identify street prices; where those aren't available, we note MSRP, for manufacturer's suggested retail price.

Gathering the Facts

At the front line of the electronics revolution are the sensors, those eyes and ears that gather the electronic facts. Some of these do their gathering from inside the boat; others talk to the stars. In the first category is a diagnostic tool called the N2KMeter ($900) from Maretron. The company, founded in part by engineers who supplied cutting-edge electronics to auto makers, provides all the necessary cabling and connectors to create an NMEA 2000 network, among other things. For any sailor

NETWORKING UNPLUGGED: The Uniden WHAM is a wireless mike that controls a fixed-mount DSC radio.

TOUCHSCREEN CONTROL: The Northstar 8000i is a fully integrated video nav and entertainment system.

who plans to have an NMEA network aboard and wants the ability to troubleshoot it, this meter is purpose-built to identify opens and shorts, bad nodes, poor shield connections, and other flaws or failures. If you've ever spent hours crawling around a bilge tracing down a fault, you'll appreciate the speed of the answers this meter offers. To monitor the boat's electrical system, tanks, and switches, Krill Systems has developed an Ethernet and wireless network called KrillNet. Sensors from all these areas send data to the Krill DU840 display, by both Ethernet and wireless up to 100 feet. That's right: both; redundancy is

built in to the network, with no single points of failure. With this system, every one of a boat's gauges is brought to a single display.

Wireless technology is at the heart of NKE's NMEA-compatible Gyropilots. Together with the TL 25 display and remote, a sailor can drive from anywhere aboard the boat. A man-overboard module makes this unit especially popular among sin-glehanded sailors.

To collect signals from off the boat—in this case, to boost cellphone signals—

WHOLE BOAT AT A GLANCE: Krill Systems' DU840 display uses both Ethernet and wireless technology to monitor tankage and electrical systems.

Digital Antenna has developed its line of PowerMax repeaters and amplifiers. The repeaters ($560 for a 40-deci-bel unit; $700 for a 60-decibel unit) are sold as complete systems including a 12-volt-DC power converter, a 110-volt-AC power supply, an internal omnidirectional antenna and external antenna, cables, and connectors. For the amplifier ($300), the cell antenna and adapter cables are sold separately.

In the field of VHF radios, Icom recently introduced the Continued on page 49

Continued from page 46 own language, were not compatible with other products. Generally, they were manufacturer-specific, and they weren't capable of serving a boat's diverse applications— engine control and monitoring, AC- and DC-power control and monitoring, navigation, and security-alarm functions, for example.

And they may or may not have been capable of handling the real-time data flow needed to control engines and autopi-

lots. Today, there are so many equipment makers that you shouldn't have to expect that your boat's GPS, autopilot, radar, engine monitor, inverter, and battery monitor will all come from the same company. If you have to replace an autopilot, you shouldn't be stuck with the same brand. Neither should you have to buy and install a special gateway between two network systems that aren't compatible.

These are some of the arguments in favor of an open standard. To address them, NMEA set out in 1995 to develop what would become NMEA 2000. From its experi ence with NMEA 0183, NMEA knew how hard it could be to gain wide acceptance of a new standard. For this reason, NMEA 2000 was shaped around proven, existing technologies and standards, an approach that would also bring the cost down. In the mid-1980s, Bosch and Intel collaborated on an integrated circuit that would provide a serial communications protocol for real-time control applications. The result was the "control area network," or

CAN. It was originally intended to replace wiring for realtime engine and transmission control and anti-skid braking systems. CANbus networking soon spread to other industries, and today the documents are in the public domain and freely available on the Internet. It's also documented as International Standards Organization standard ISO-11898.

Virtually every major maker of integrated circuits provides chips that do all the hard work. The chip determines whether the network is busy, gains access to the network, sends all the data with the right timing, and performs error checking (limiting the chance of occurrences to less than one in 20 billion). Perhaps most important, it arbitrates disputes when data streams collide. (More on this later.)

All of this complicated business is handled by the CAN chip. Individual equipment designers don't have to worry about it, and the rest of us don't have to worry about whether it's been implemented correctly. The equipment designer now can focus on what's really important: the data that's sent and received.

The first part of this task is handled by a set of rules for requesting data, sending data in chunks when it's too big to send in one message (which is most of the time), and how to acknowledge or "non-ac-knowledge" a request for data. In network terminology, this is called the "data link" layer of the network. Turns out that the Society of Automotive Engineers had specified such a layer for their CAN-based SAE J1939 standard and had released it to the international community as standard ISO

11783-3. Besides being an existing international standard, it's already in use by companies that serve the automotive and marine markets, including engine, engine-accessory, and GPS manufacturers.

Another part of this self-organizing network is called the "network management" layer. This layer sets the rules for devices to establish their addresses on the network. It makes 252 addresses available. This they do themselves; there's no network manager or controller. Handled here also is the situation referred to as "instance," when there are two or more GPS receivers or a dozen remote DC power switches; the network is able to distinguish all of these. Each of the entities that provides data for the network has an address so that it can be identified as the originator of the data and so that requests can be sent to the right place.

Earlier, we mentioned collision arbitration. When there's a conflict between two devices attempting to send data on the network, the address of the device helps determine priority. This address isn't arbitrary. When a new piece of equipment is added to the network, its first task is to claim an address; it can even take one that another device is already using. The new device puts a special "address claim" message on the network that contains its "Name"; the Name includes the function of the device. A better-ranked Name can trump other Names and gain the higher-priority network addresses; an engine controller will beat a lightbulb controller every time.

The basic parts of a network | described above are used in g cars, tractors, factories, or ? boats. Boats, however, have | unique requirements. To cre- i ate NMEA 2000, we added £ special rules to help the net- £ work function smoother in a | plug-and-play manner and to 8

allow the network to send the types of data expected on a boat faster and more efficiently. The rules also describe the way two redundant networks operate when connected to the same equipment.

CANbus vs. Ethernet

If you use computers at home or at work, you're probably familiar with Ethernet. Very often, it's the way our computers are connected to the Internet for high-speed access. NMEA 2000 (based on the CANbus) and Ethernet have many similarities. Both use a single pair of wires that communicate serial data and a sequence of bits designated as 0s and 1s. The bit sequence identifies the message being sent, the talker, and sometimes the intended listener; it also includes the data to be sent, as well as some control and error-checking bits. Both use a set of rules called CSMA (Carrier Sense Multiple Access) that's often likened to the conversation rules used by people sitting around a table: Listen first (carrier sense) before you talk, and that way lots of people can share the conversation (multiple access). NMEA 2000 can transmit 250,000 bits per second and is generally used for fairly short bursts of data, such as latitude and longitude, engine temperature, autopilot commands, and the like. It's limited to distances of 200 meters. Ethernet, on the other hand, can operate at j much faster rates: from 10 million bits per second to more than 100 million bits per second over distances from 100 meters to several kilometers when using fiberoptic cable. At those speeds, Ethernet can send huge amounts of data, including

COMING SOON: The Mobile Internet Receiver from KVH will bring broadband connectivity to boats under way.

Continued from page 47 M72 ($185), the industry's first 6-watt handheld. The radio is sold with a 2,000-milliamp lithium-ion battery, charger, and AC adapter. The M72 is also Icom's first to offer a submersibility rating of IP-X8, which says that the radio should still function after a half hour under 1.5 meters of water; the industry standard is IP-X7, which rates submersibility at 1 meter under water.

Another intriguing VHF development comes from Uniden, with its WHAM line of wireless mikes (starting around $180) for use with Uniden's Polaris and Oceanus fixed-mount radios. Incorporating many of the features you'd expect from a handheld—1-meter submersibility for a half hour, memory channel scan, nickel-metal hydride battery and charger—the WHAM mike gives you all the range of a fixed-mount radio with the ability to monitor the radio from anywhere in the boat. You can even use it to speak quietly from the helm to the foredeck during those anchoring

TRANSMIT CLEARER AND FARTHER: Icom's M72 brings 6 watts of power into a handheld VHF.

sessions that have made unfortunate history in so many marriages.

Later in the year, KVH plans to introduce a Mobile Internet Receiver that will bring broadband connectivity to your boat, even when you're under way. The goodies will include web-browsing, MSN Mail and MSN Messenger, Microsoft Windows Media Player, and Wi-Fi output. The two-way connections will come from broadband EVDO cellular services.

To gather signals from farther away than VHF's line of sight, the range of new satellite-based services is growing quickly. WxWorx, which has delivered graphic weather data through the XM WX satellite system for a couple of years now, has recently expanded its offering. Most notably, its high-seas forecasts, detailing wind and sea conditions and frontal systems offshore, has been extended farther offshore from the U.S. coast. By employing a new GPS input, sailors can set the field of awareness ahead of them as the voyage progresses. WxWorx software is now preinstalled on machines from such marine PC suppliers as VEI Systems, Nauticomp, and Marine PC.

Another satellite system created to deliver communications and navigation information to boats under way is the SkyMate 200, designed to be used with the PC-based Nobeltec navigation programs. The SkyMate service allows you to send and receive e-mail and to pull e-mail messages from your land-based accounts. NOAA 24hour forecasts and NEXRAD radar images are provided graphically. Monthly service plans run from $16 for 8,000 characters to $70 for 50,000 characters. A fun extra feature is a free weblog provided for each subscriber, so the folks back home can keep up with where you've been.

SURFING CHANNELS: Raymarine's ST 45 satellite antenna promises clear pictures from a pitching platform.

Of course, it's becoming ever more common these days to bring aboard the boat from satellites not only vital nav information but also entertainment. Raymarine recently introduced the ST 45 satellite TV antenna system (MSRP $5,400). The stabilized antenna automatically identifies digital video broadcast satellites by employing several high-tech tricks that Continued on page 51

video images from radar, electronic charts, or DVDs.

So, what gives? Why choose the seemingly limited NMEA 2000 when faster techniques, such as Ethernet, have been used for decades? Both Ethernet and NMEA 2000 employ a message-sending technique that can be compared to the rules of human conversation. Imagine a group of people sitting around the dinner table. One hopes that everyone isn't talking at once! If two people begin to speak at the same time, they usually both stop, there's a pause, and then only one begins to speak. And so the conversation resumes. This works well enough in small groups. But at a large gathering with many conversations, problems may arise. If someone has something really important to say or needs to say it in a hurry, that person may not get the chance. This is one of the problems with the way Ethernet works. Ethernet employs "collision detection" (CSMA/CD): As each device connected to the network transmits, it also listens for a garbled signal, which would indicate multiple transmissions on the network. If there's a "collision" of signals, all devices stop transmitting, wait through a random delay, then

As the table shows, an Ethernet bus delivers much more data faster and farther than a CANbus is able to do, but CANbus allows messages to be prioritized so that the most critical ones (throttle controls, autopilot signals) always get through first.

listen and try again.

There's a cost to this system: When a collision occurs, no messages at all are sent; network time is lost. The time to even detect a collision can be considerable, because a device way down one end of the cable has to continue to detect until a signal from the far other end gets to it before giving up.

More time is lost. There's no way of knowing if someone had something really important to say; it's first come, first served. If there are many devices connected, each transmitting frequently, it makes for a very uncertain situation for the device that has to deliver a message immediately.

How does NMEA 2000 handle this? It uses "collision arbitration" (CSMA/CA), a process that's more like office members in a meeting at work than a family conversation around the dinner table. In the conference room, if the president and vice president start talking at once, the president keeps speaking; when he's finished, others speak in their turn, all the way down to the department manager and the new engineer. NMEA 2000 employs a similar system of hierarchy. The serial bits transmitted in the NMEA 2000 system, 0s and 1s, are classified as dominant and recessive respectively. Every time an attempt to put a message on the network occurs, a message will get on the network; no time will be wasted with delays. But messages designated as critical, by the dominance inherent in their pattern of 0s, will be the ones that get transmitted first.

This too has a cost: For bit-by-bit arbitration to work, it means that all devices connected to the network must be able to read the same bit along the length of the wire before the bit ends. Narrow bits (higher speed) could end at the start of the wire before they travel to the other end; longer networks mean that some bits won't make it in time. Thus there is a tradeoff between network speed and network length.

There's been debate in the marine-electronics field about speed versus control. In the end, NMEA 2000's designers favored the CANbus approach as the network in the boat to be used for real-time control of autopilots, transmissions, throttles, and other equipment with time-critical requirements, functions that can't stand in line waiting for other messages to get through. Autopilots, for example, need compass-heading updates hundreds of times per second; otherwise, steering performance suffers. Lists of way-point names, on the other hand, tend to change slowly, and that information can tolerate getting bumped every now and then. This may not seem to be a potential problem at first glance, but with many devices connected and sending data, critical applications could be slowed down.

Proprietary Networks

Control Networks: As seen below, the network of choice today, except for those requiring the high-volume Ethernet connection, is a network that's functionally NMEA 2000. However, many of these aren't certified to meet all the operating requirements of NMEA 2000, nor do they necessarily use the standardized database of messages that should be available to all. For my part, I hope that one day there'll be only one network standard, with compatible products and connectors. For an up-to-date list of NMEA 2000-certified products listed by company, go to the NMEA website (

Maretron makes only NMEA 2000-certified products and uses no net-like name for its line of interconnected products, PC interfaces, diagnostic tools, and NMEA 2000 cables and connectors that allow equipment from diverse

Was this article helpful?

0 0
How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

Get My Free Ebook

Post a comment