PMY: A Breakdown of Protocol - how boats’ systems converse

Clip source: A Breakdown of Protocol - how boats’ systems converse - Power & Motoryacht

By Tim Bartlett

Updated: Sep 19, 2017 Original: Feb 2, 2012

I have found that most endeavors are easier if you work as a team—everything from splitting logs or catching fish to raising a family or running a business. It’s the same for your electronics. Almost everything on your console is—or could be—a team player, working as part of an integrated system.

There are literally dozens of different ways that electronic devices can communicate with each other: through Ethernet, HSB, NavNet, SeaTalk, SimNet, Smartcraft, and USB, to name a few, but the grandaddy of marine instrument integration systems is NMEA 0183.

NMEA 0183 isn’t a collection of plugs and wires. It’s a rule book, published by the National Marine Electronics Association (NMEA) in 1983 (and since updated) setting out strict rules of spelling and grammar that make up a common "language" that one instrument—"a talker"—can use to send information to one or more "listeners."

The talker produces information in the form of a string of short, low-voltage electrical pulses, rather like Morse code, which are transmitted to each listener through two cores of a multicored cable. Each talker can transmit to several listeners at once. A single GPS, for instance, might send information to an autopilot, a plotter, and a DSC radio. But that isn’t really a network, because each pair of wires can carry messages from only one talker and in only one direction. It doesn’t allow several talkers to share one pair of wires nor does it allow listeners to talk back.

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.

And that’s where NMEA 2000 comes in. It is completely different. For one thing, the NMEA 2000 rule book specifies which wires and cables should be used, making it much easier to connect equipment from different manufacturers. But a far more fundamental point is that it allows as many as 50 different devices to be connected to a single backbone. Any of them can pump data into the backbone, and any of them can take data from it. It’s a true network.

The data, however, is still in the form of electrical pulses, and if two devices transmit at once, their pulses would mix together to produce gibberish. So it’s important to make sure that only one device is talking at a time. To prevent simultaneous transmission, each talker breaks its message up into several pieces, called frames. Each frame includes a header that identifies what information the frame contains and how urgent it is.

Before it transmits, each talker listens to what is happening on the network. If everything is quiet, it goes ahead and sends its frame of information. If the network is busy, it waits. There’s still a potential problem, though, if two talkers begin transmitting at the same moment. In this case, every device on the network, including the two that are trying to talk at the same time, recognizes that there is a conflict, and the one that is sending the lowest-priority message shuts down until the network is quiet again.

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.

Most of us are already using a system that works around these problems in our homes and offices—it’s called Ethernet, and even in its slow form, known as 10BASE-T, it carries information 40 times faster than NMEA 2000 and more than 2,000 times faster than NMEA 0183. Speed is important because it obviously affects the amount of information that can be carried, but there are a couple of other really big differences between NMEA 2000 and Ethernet.

The first is that each frame of Ethernet data includes "from" and "to" addresses in its header, so although it may be received by every other device on the network, each one can ignore information that it does not require.The other is that Ethernet uses a completely different system to avoid the "collisions" that occur when two devices transmit at once. It’s called Carrier Sensed Multiple Access/Collision Detection (CSMA/CD). But it’s best compared to the way conversation at a dinner party controls itself: People speak when there is a lull in the conversation, and if two people accidentally start talking at the same time they both stop. It’s a very efficient way of carrying information between small groups of devices—say, as many as five—but the dinner-party protocol means that there is no guarantee that urgent messages will get through in time. For large groups, in particular, the sheer volume of traffic can mean that individual devices have to wait a long time before their messages get through.

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.

Several major manufacturers use their own systems for interconnecting their own products and their own variants on the generic systems: