NMEA 2000 Explained

["What this doesn't address is the fact that just becasue the NMEA 2000 network is powered you still will have to run the power cable for chartplotter or a GPS combo unit as the NMEA backbone, as this video states, doesn't have much power. When I installed my NMEA 2000 network I was under the assumption that the NMEA network would power all my stuff and there would be no need for the original power supply to be ran to back of chartpotter and other bigger units. great video but addressing what was said above would be great." -- "You can only give power to the little displays with the nmea alone." - /YouTube comments/]
 
"NMEA 2000 (IEC 61162-3) is a low cost, moderate capacity (250 K- bits/second), bi-directional multi-transmitter/multi-receiver instrument network to interconnect marine electronic devices. /1/

Figure 1. NMEA 2000 network.

The NMEA 2000 standard contains the requirements for the minimum implementation of a serial-data communications network to interconnect marine electronic equipment onboard vessels. Equipment designed to this standard will have the ability to share data, including commands and status, with other compatible equipment over a single signaling channel.

Figure 2. Networking a ship.

Increasingly, modern marine electronic equipment requires data from multiple sources to enable the host of features and function that can be available to the mariner. Without a network standard to provide this data integration, equipment designers must provide multiple data inputs, which involve expense and additional wiring, or use devices that “merge” data onto a single channel. Individual systems on a vessel, such as engine machinery or navigation systems, perform relatively dedicated functions, often have real- time requirements measured in milli-seconds, and need fewer connected nodes. These systems tend to be smaller and more self-contained when compared to other vessel networks, and carry less data volume. Because this network application integrates inexpensive sensors and actuators into larger systems, the cost per node must be far less than in other shipboard applications. This network application is addressed by NMEA 2000 (IEC 61162-3).

YouTube video: "How NMEA 2000 Network Works"

Figure 3. A basic NMEA 2000 network.

NMEA 2000

Architecture:

• Bus (parallel) wiring configuration using 4-conductor twisted- pair wire to carry power to operate the interface and data signals.
•  Linear network with end terminations and multiple short-length drop cables connecting the backbone cable to individual nodes.

Operation:

• Network access: Carrier Sense/Multiple Access/Collision Arbitration using CAN (Controller Area Network).
• Multi-master network operation (no central control node).

 Self-configuring.

• Special network tools, desirable for diagnostic purposes, are not necessary for operation.

Size:

• •Physical nodes: Up to 50 connections. 
• Functional nodes: Up to 254 network addresses. 
• Length: Up to 200 meters (at 250kbits/second bit rate).

At the basic level, and in wide use, the older NMEA 0183 (IEC 61162-1) provides serial data distribution from a single transmitter to multiple receivers. Operating at 4800 bits/second this protocol has the capability of delivering approximately ten messages, or sentences, per second. This has generally proven adequate when a single device is broadcasting data for use by other equipment. But it quickly reaches a limit when systems start to combine data. However, its use is expected to continue well into the future for simpler applications, redundant or backup data connections, and when direct device-to-device connections are needed.

YouTube video: "NMEW 2000 Network Guide Video"


 
THE PHYSICAL LAYER

This layer defines the electrical and mechanical aspects of the physical link between network connections, and references characteristics of the CAN devices and network interfaces to be used in NMEA 2000. 

Figure 4. Shipboard networks and interfaces.

The electrical characteristics of the physical layer are dictated by the following:

• Media access uses CAN as defined by ISO 11898, Road Vehicles - Interchange of Digital Information, Controller Area Network (CAN) for High-speed Communication. 
• CAN utilizes dominant/recessive bit transmission.
• Time delays and network loading limit bit rate and network length.
• Differential signaling improves noise immunity.
• Network single-point common signal reference controls ground voltage levels and reduces RFI.
  

Differential signaling indicates that powered interface circuits and a signal-reference common to all nodes on the network is required. A single-point common reference is specified in order to avoid radio-interference caused by ground loops and to maintain control of ground-voltage levels between nodes such that they remain within the common-mode range (approximately +/-2.5 Volts) of the network transceiver circuits. An important change from previous draft versions allow the use of the vessel’s 12-Volt battery to power the network, if the length of the backbone cable and the number of nodes are small enough, instead of the use of a more expensive regulated power supply that was previously required. 

Figure 5. Typical ground loop problem solved with optically isolated network.

Single-point power and common may be distributed via the network backbone cable as previously required, or for heavier current, by dedicated twisted-pair wires to individual devices. This feature allows equipment to draw additional operating current from the network power source and to be built with minimum interface complexity. In all cases the power and common for the interface circuits must not connect to other power or ground in a network device. This isolation may be achieved in a number of ways. One is by use of isolation circuits (e.g., optoisolators) within the device, either at the interface or at specific places where the equipment connects to other devices. Another way is by assuring that no power or ground connections, other than the network power and network common, connect to the device. The latter method is suitable for equipment such as displays or sensors that have no interfaces other than with the NMEA 2000 network, can draw all of their operating current from the network source, and have isolated packaging and mounting designs.

The figure below illustrates a typical physical layer interface circuit using available transceiver integrated circuits meeting the requirements of ISO 11898. Ground isolation, illustrated with optoisolators, is shown between the network and the CAN controller and other device circuits (e.g., microprocessor and other circuits). However, as pointed out above, isolation from other circuits may be accomplished by other means. 

Figure 6. Typical optically isolated network interface.

The illustrated transceiver circuit requires regulated +5 Volt power that is provided by the Regulator and Protection circuits. The purpose of the protective circuits is to prevent damage to the regulator and the interface circuits from overvoltage and reverse voltage. No permanent damage should result from a voltage level of +/-18.0 Volts or less applied between any two wires in the interface for an indefinite period of time or from miswiring the interface lines in any combination.


YouTube video: "Tips - Installing a NMEA 2000 Backbone on a Boat"


THE MAIN POINTS OF THE PHYSICAL LAYER

Environmental and Radio Frequency Interference

NMEA 2000 implementations must meet the Durability and Resistance to Environmental Conditions described in Section 8 of IEC 60945 and meet the Unwanted Electromagnetic Emissions and the Immunity to Electromagneic Environment conditions of Sections 9 and 10 of IEC 60945. Shielded cables are recommended, and may be necessary to meet these latter requirements.

Ground Isolation

AC and DC isolation is required between all of the terminals at the interface connector, with the network cables disconnected, and any other ship’s ground or voltage sources. As discussed above this can be accomplished with isolation devices such as opto-isolators or by wiring and packaging design. For most applications, except those with very low power needs, the isolated interface is the preferred implementation.

Network Signaling

The two signal lines carry differential signals measured with respect to the network power common. The signals on the network represent two states: Dominant state or Logic ‘0’, and Recessive state or Logic ‘1’, during the transmission of the Dominant state by one or more nodes the state of the network is Dominant. The interface must be designed so that the signal lines are in the Recessive state when node power is off.

The AC and DC voltage parameters of the network signals are specified by ISO 11898. The nominal voltage levels are:

• Dominant state:
 CAN+ = 3.5V CAN- = 1.5V V diff = CAN+ - CAN- = 2.0V

• Recessive state:
 CAN+ = 2.5V CAN- = 2.5V V diff = CAN+ - CAN- = 0.0V

• Common Mode range: Difference in network common voltage between nodes:
 -2.5 to +2.5 Volts

Network Power

The interface circuits must operate over the range of 9.0 to 16.0 Volts DC. The voltage for the interface can either be supplied from the network backbone cable or supplied by a dedicated twisted-pair power cable connected only between a single node and the network power source (the vessel’s battery or one regulated power supply). The amount of current delivered by the network cable is limited. When a dedicated power connection is used the node is allowed to draw additional current but the connections must be labeled, and physically separated and isolated from other power and ground connections. Under no condition may the node power or ground be connected to other power or ground in the equipment.

To aid in planning network installations manufacturers are required to specify the power rating for each connected device as a “load equivalency number”. The actual power source for the network can be either a single-point connection to the vessel’s battery or one or more isolated power supplies distributed along the network. The size and routing of the cables must be carefully considered. As the number of nodes with high load equivalency number increase, DC voltage loss in the cables quickly becomes the limiting factor for network length rather than the propagation time for the signals. For networks of shorter length and with a lower number of connected devices the ship’s battery may be used to power the network nodes directly. In place of the battery, electrically isolated regulated power supplies may be used if it is necessary to extend the size of the network.

Cables and Connectors

Two methods are provided for connecting to the network backbone cable: a standard connector or barrier strips. These connections are used for connecting segments of backbone cable together, for connecting terminations at the two ends of the cable, for connecting the network power source, and for connecting nodes. The drop cable, the short cable running from the backbone connection to the node equipment, may connect to the equipment anyway the manufacturer chooses. It is the connections at the backbone that are controlled by the NMEA 2000 standard. 

Figure 7. Typical "star" configuration. (The actual network is a short stub and all the devices connect to it with longer cables.)

Barrier strips are only recommended when the connections are made in a protected location, or when they are installed in a weatherproof enclosure, thus meeting the requirements for Resistance to Environmental Conditions for exposed equipment in IEC 60945. Barrier strips positions must be either numbered or color-coded in accordance with the definitions in the standard.

The connector selected for the NMEA 2000 backbone is a 5-pin type used in industrial networks and is available from multiple sources (including Molex, Turck Inc., Methode Components, and Daniel Woodhead Company). The connector is available as a 3-port “T” connector, cable-end connector, bulkhead-mount connector and special configurations with internal termination resistors.

YouTube video: "How to Install NMEA 2000 Boat Electronics System"

Cable specified for the network must meet both the characteristic impedance and propagation delay requirements for use as a transmission line, and also the wire-size needs of the DC power distribution function of the cable. The cable lengths on the network, the number of nodes connected, the distribution of the nodes, and the location of the power source connection(s) into the backbone cable determine the actual cable requirements in a particular installation. Two cable sizes are specified and can be used as needed in an installation. NMEA 2000 Heavy cable is 5-wire consisting of two shielded- twisted-pairs and a common shield drain wire. The wire pairs are No. 16 AWG (1.33 sq. mm) for DC power and No. 18 AWG (0.83 sq. mm) for signals. NMEA 2000 Light cable uses No. 22 (0.38 sq. mm) and No. 24 (0.24 sq. mm) respectively.

The cable specified has a defined color code, in the event that these colors are not available the substitute cable must be marked according to the standard."
 

Figure 8. Typical connectors, running power and signal.

 ["The solution to ground loop noise is to break the ground loop, or otherwise prevent the current from flowing. The diagrams show several solutions which have been used " (Read more about ground loops.] 

[Below is an interesting video about the NMEA 2000 networking problems. In this case it is about the fact that there can only be one terminating resistor in the network at each end of it (2 all together). (And also other installations problems.)]

YouTube video: "Mast Climbing Madness (Sailing Giraffe)"



RESOURCES


/1/ Cassidy, Frank - "NMEA 2000 Explained - The Latest Word" - 1999 


/2/ Wikipedia


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