connected to an autopilot— and no one would think of substituting a different compass unit. But with the introduction of digital electronics in the early 1980s, it became possible to interchange units. That's when people did start to care. Digital Loran-C became a consumer product, and even while it provided only time-difference (TD) numbers and had to be used with special charts overprinted with TDs, companies started connecting Loran-C receivers to autopilots. Thus the age of onboard networking was born. In those early days, there were several Loran-C manufacturers and several autopilot manufacturers—each with its own proprietary data languages, and each struggling to keep up with all the specialized and changing data formats. Stuck in the middle was the sailor, paying the price in frustration.
Fast-forward to 2005: A watershed moment in marine networking occurred at last year's Miami boat show when
A WELL-CONNECTED YACHT: The NMEA 2000 open standard lets you link dozens of devices from a host of different manufacturers in one networked system. Physically, NMEA 2000 uses a single four-wire cable (two data wires, two power wires) and standard T-connectors to link new devices. It uses battery power of 9 to 16 volts, so many low-draw instruments can tap power right off the network. The cable is shielded, and the interface circuits are isolated from ground to reduce interference and malfunctions caused by noise and current surges. As many as 50 devices can be connected and configured with up to 252 addresses, or separate functions.
about 40 different products built by more than a dozen manufacturers operated together on a single cable. The event, called Connectfest, was sponsored by the National Marine Electronics Association as a showcase for its NMEA 2000 standard—a standard of which I was a principal originator.
From the very beginning, NMEA 2000 was intended as an open system that a sailor could expand on his own boat as new equipment from a variety of manufacturers became available. This technology makes possible multifunction displays with a mix and match of data in different parts of the boat. Complex operations— like true wind and velocity made good, which rely on two or more inputs—can be calculated seamlessly; the adjusted lighting on one unit can simultaneously show up on others; damping adjustments can be made from various places; alarms can be controlled and heard remotely.
The idea behind NMEA 2000 is that all devices are capable of settling into their role on the network and that they can describe to others who they are and what they can do so that other devices can work with them. "Plug and play" was always the goal. It was never considered that a central controller would be needed to set up the network and make assignments or to control how the network operates. Nor was it ever required for proper network operation that a boat's particular configuration would be planned in a laboratory and then installed on the boat. Beyond that, it was to be a simple parallel-wire bus using standardized
connectors so eq uipment could be added at will without resorting to a bag of special connectors or having to reconfigure the network.
Before we examine today's onboard networks, let's take a brief look at how our common language has evolved. In 1980, as marine electronics were becoming more prevalent on boats, the National Marine Electronics Association introduced a standard interface that addressed the aforementioned language problems between different Loran and autopilot models. Called NMEA 0180, it provided nothing more than a way to exchange cross-track error data. This step, though small, was a crucial beginning.
Digital electronics quickly blossomed, and a whole new range of gear came along, most notably Loran-C receivers working in latitude and longitude that could output speed, course, range, and bearing. To receive all this new data, there were electromechanical position plotters, autopilots, and video depth sounders. But as the gear proliferated, so did the proprietary formats from different companies, each one speaking a different language. Some-
PLUG AND PLAY: Standard cables and connectors simplify adding new gear.
times this was for expediency, sometimes to discourage sailors from buying competitors' products.
In 1983, NMEA introduced its more comprehensive NMEA 0183 standard to make all these new tools more compatible. It took a while, but gradually 0183 prevailed, and now it's considered the universal interconnection standard of its type. In fact, it's been adopted by the International Electrotechnical Commission in Geneva as IEC 61162-1.
NMEA 0183 is an interface standard, not a network standard. Compared to today's technologies, 0183 is slow, and the interface wires are connected to only one data sender (called a "talker"), with the possibility of connecting up to 10 "listeners." It's a simple broadcast of serial data in a computer-friendly format called ASCII; you can display it on your laptop using Hyperterminal. (The parameters are 4,800 bits per second, 8 data bits, no parity, and 1 stop bit.) Or you can display it using a diagnostic menu on such equipment as chart plotters. By either of these methods, you can read the "sen tences" in plain letters and numbers; with a little familiarity, you can recognize the data you're looking for.
That was fine for its time. But consider a contemporary boat fitted with a fluxgate compass, rudder-angle indicator, heading sensor, depth sounder, and speedo. To use or display this data, a radar or chart plotter would need an NMEA input and wires coming from each of those senders separately; alternatively, it would have to employ some form of an NMEA 0183 multiplexing box. Also, because the system is based on a single talker, two devices can share information only if another NMEA interface is introduced between them; this adds its own set of wires. The installation and associated maintenance of such an interface can quickly get out of hand.
Between the "interfacing" of NMEA 0183 and the true "networking" of NMEA 2000, we've seen interim solutions. For example, there's equipment today that automatically multiplexes NMEA data. In this scenario, input data on a receive-0183 port is merged with the locally generated data; this combined data is then transmitted at the NMEA 0183 output. GPS data, say, is passed to the depth sounder, which in turn passes GPS and depth to the speedometer, which in turn passes all the data to the wind instrument, which passes it on to the radar for display. A wind instrument standing alone can calculate only apparent-wind speed and direction, but the same wind instrument in possession of boat-speed data can now calculate true wind. Next, the speedometer can take data back from the wind instrument on a second channel and calculate the boat's velocity made good.
Of course, there's a limit to this, and such a system will handle only so much NMEA 0183 data before some sentences must be dropped. This last system is using networking, albeit with slow speed and extra wires.
To network, according to the dictionary, is "to link so as to operate interactively." It's a two-way flow. Networks that allowed two-way traffic on a single wire pair first appeared in the mid-1980s, and they continue in use today. The problem, though, was that these early networks were designed for only one manufacturer's equipment and, because they were speaking their Continued on page 48
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