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NMEA 2000

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NMEA 2000, abbreviated to NMEA2k or N2K and standardised as IEC 61162-3, is a plug-and-play communications standard used for connecting marine sensors and display units within ships and boats. Communication runs at 250 kilobits-per-second and allows any sensor to talk to any display unit or other device compatible with NMEA 2000 protocols. Electrically NMEA 2000 is compatible with the Controller Area Network ("CAN Bus") used on road vehicles and fuel engines. The higher-level protocol format is based on SAE J1939, with specific messages for the marine environment. Raymarine SeaTalk 2, Raymarine SeaTalkNG, Simrad Simnet, Furuno CAN are rebranded implementations of NMEA 2000, though may use physical connectors different from the standardised DeviceNet Micro-C M12 5-pin screw connector, all of which are electrically compatible and can be directly connected.

The protocol is used to create a network of electronic devices—chiefly marine instruments—on a boat. Various instruments that meet the NMEA 2000 standard are connected to one central cable, known as a backbone. The backbone powers each instrument and relays data among all of the instruments on the network. This allows one display unit to show many different types of information. It also allows the instruments to work together, since they share data. NMEA 2000 is meant to be "plug and play" to allow devices made by different manufacturers to communicate with each other.

Examples of marine electronic devices to include in a network are GPS receivers, auto pilots, wind instruments, depth sounders, navigation instruments, engine instruments, and nautical chart plotters. The interconnectivity among instruments in the network allows, for example, the GPS receiver to correct the course that the autopilot is steering.

History

The NMEA 2000 standard was defined by, and is controlled by, the US-based National Marine Electronics Association (NMEA). Although the NMEA divulges some information regarding the standard, it claims copyright over the standard and the contents thereof are thus not publicly available. For example, the NMEA publicizes which messages exist and which fields they contain but they do not disclose how to interpret the values contained in those fields. However, enthusiasts are slowly making progress in discovering these PGN definitions.[1]

Functionality

NMEA 2000 connects devices using Controller Area Network (CAN) technology originally developed for the auto industry. NMEA 2000 is based on the SAE J1939 high-level protocol, but defines its own messages. NMEA 2000 devices and J1939 devices can be made to co-exist on the same physical network.

NMEA 2000 (IEC 61162-3) can be considered a successor to the NMEA 0183 (IEC 61162-1) serial data bus standard. It has a significantly higher data rate (250k bits/second vs. 4800 bits/second for NMEA 0183). It uses a compact binary message format as opposed to the ASCII serial communications protocol used by NMEA 0183. Another improvement is that NMEA 2000 supports a disciplined multiple-talker, multiple-listener data network whereas NMEA 0183 requires a single-talker, multiple-listener (simplex) serial communications protocol.

Network construction

The NMEA 2000 network, like the SAE J1939 network on which it is based, is organized around a bus topology, and requires a single 120Ω termination resistor at each end of the bus. (The resistors are in parallel, so a properly terminated bus should have a total resistance of 60 ohms). The maximum distance for any device from the bus is six metres.

Typical NMEA 2000 Network Installation

Cabling and interconnect

The only cabling standard approved by the NMEA for use with NMEA 2000 networks is the DeviceNet cabling standard, which is controlled by the Open DeviceNet Vendors Association. Such cabling systems are permitted to be labeled "NMEA 2000 Approved". The DeviceNet standard defines levels of shielding, conductor size, weather resistance, and flexibility which are not necessarily met by other cabling solutions marketed as "NMEA 2000" compatible.

There are two sizes of cabling defined by the DeviceNet/NMEA 2000 standard. The larger of the two sizes is denoted as "Mini" (or alternatively, "Thick") cable, and is rated to carry up to 8 Amperes of power supply current. The smaller of the two sizes is denoted as "Micro" (or alternatively, "Thin") cable, and is rated to carry up to 3 Amperes of power supply current.

Mini cable is primarily used as a "backbone" (or "trunk") for networks of larger vessels (typically with lengths of 20 m and above), with Micro cable used for connections between the network backbone and the individual components. Networks on smaller vessels often are constructed entirely of Micro cable and connectors.

An NMEA 2000 network is not electrically compatible with an NMEA 0183 network, and so an interface device is required to send messages between devices on the different types of network. Examples include Actisense's NGW-1, the Maretron USB-100, and Simrads AT10. These devices vary in which messages they will translate between the two networks. An adapter such as the Actisense NGT-1-USB, Airmar U200 or Maretron USB100 is also required if NMEA 2000 messages are to be received by or transmitted from a PC.

NMEA-2000 cabling components incl. power-T

Message format and parameter group numbers (PGNs)

In accordance with the SAE J1939 protocol, NMEA 2000 messages are sent as packets that consist of a header followed by (typically) 8 bytes of data. The header for a message specifies the transmitting device, the device to which the message was sent (which may be all devices), the message priority, and the PGN (Parameter Group Number). The PGN indicates which message is being sent, and thus how the data bytes should be interpreted to determine the values of the data fields that the message contains.

A parameter group definition may describe a data record that consists of more data than can be contained within a single CAN frame.[2] NMEA 2000 transfer methods include transmitting single-frame parameter groups and two methods of transmitting multi-frame parameter groups. These transfer methods are compared below:

Single frame ISO 11783 multi-packet NMEA 2000 fast packet
8 bytes Up to 1,785 bytes Up to 223 bytes
Broadcast by default unless pf < 0xF0 Can be used to send any message up to 1,785 to a specific device with flow control (RTS/CTS) or broadcast (BAM) Broadcast by default unless pf < 0xF0
CAN layer assures all (connected) nodes received the message and validated its CRC With handshaking (RTS/CTS) / time based (50 ms) in case of BAM (broadcast) No handshaking
No transfer protocol delays Takes longer to send the same amount of data as fast packet Takes less time to send up to 223 bytes; no transfer protocol delays; no guarantee it is received by all nodes
Implemented by all industries Implemented by all industries NMEA 2000 and adopted by ISO for GNSS Con/Ag applications

The Multi-Packet protocol specified in ISO 11783-3 provides for the transmission of multi-frame parameter groups up to 1,785 bytes. The protocol encapsulates the parameter group in a transport protocol, either globally or to a specific address. In case of address specific transfer (RTS/CTS), the receiving device can control the data flow in accordance with the receiving device’s available resources. In both cases (RTS/CTS) verus BAM. the message being transferred is announced in the first message. In case of CTS/RTS the receiver can refuse the message. In case of a BAM the message can simply be ignored.

The Fast Packet protocol defined in NMEA 2000 provides a means to stream up to 223 bytes of data, with the advantage that each frame retains the parameter group identity and priority. The first frame transmitted uses 2 bytes to identify sequential Fast Packet parameter groups and sequential frames within a single parameter group transmission. The first byte contains a sequence counter to distinguish consecutive transmission of the same parameter groups and a frame counter set to frame zero. The second byte in the first frame identifies the total size of the parameter group to follow. Successive frames use just single data byte for the sequence counter and the frame counter. Because many of the NMEA 2000 parameter groups exceed 8 bytes but do not require the 1,785-byte capacity of multi-packet, the default method of transmitting multi-frame parameter groups in NMEA 2000 is using the Fast Packet protocol.

Regardless of which protocol is used, multi-frame parameter groups are sent on a frame-by-frame basis and may be interspersed with other higher priority parameter groups using either protocol, or even single- frame parameter groups. Each device is responsible for reassembling the parameter group once all the frames for the parameter group are transmitted.

Device certification

Devices go through a certification process overseen by the NMEA, and are permitted to display the "NMEA 2000 Certified" logo once they have completed the certification process. The certification process does not guarantee data content, that is the responsibility of the manufacturers. However, the certification process does assure that products from different manufacturers exchange data in a compatible way and that they can coexist on a network.

NMEA 2000 and proprietary networks

Several manufacturers, including Simrad, Raymarine, Stowe, the Brunswick Corporation and Mastervolt, have their own proprietary networks that are compatible with or akin to NMEA 2000. Simrad's is called SimNet, and Raymarine's is called SeaTalk NG. Stowe's is called Dataline 2000. Brunswick's is called SmartCraft. Some of these, such as SimNet and Seatalk NG, are a standard NMEA 2000 network but use non-standard connectors and cabling; adapters are available to convert to standard NMEA 2000 connectors, or the user can simply remove the connector and make a direct connection.[3]

Trademarks

The term "NMEA 2000" is a registered trademark of the National Marine Electronics Association. Devices which are not "NMEA 2000 Certified" may not legally use the NMEA 2000 trademark in their advertising.

Manufacturers

The following are some of the companies that have registered with the NMEA for the purpose of producing NMEA 2000 certified products:[4]

Notes

  1. ^ https://github.com/canboat/canboat/blob/master/README
  2. ^ Spitzer 2009, p. 19
  3. ^ N2K, cable mixing not a big woop , Panbo, Feb 2008
  4. ^ "NMEA registration list" (PDF). Retrieved 2015-02-04.
  5. ^ "NMEA 2000® Brad® Micro-Change® (M12) and Brad® Mini-Change® Cables and Connectors". Retrieved 2013-11-30.
  6. ^ "Hemisphere GNSS Marine Products". Retrieved 2015-01-26.

References

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