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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"Mechanical Mike" (The Evolution of the Modern Airplane Autopilot)

Wiley Post's Lockheed Vega aircraft, which was called the Winnie Mae, made 1931 a world record for flying around the world in 8 days with Harold Gatty as the navigator. ( Most references /1/ )

Wiley Post and Harold Gatty.

A key item of equipment in the Winnie Mae airplane was at the time new Sperry automatic pilot that Wiley Post referred to as "Mechanical Mike" and which he had tested for about 85 flight hours.

Mechanical Mike was located in the cockpit of Winnie Mae.

Developed by Elmer and Lawrence Sperry in the early 1920s, the "robot pilot" used two gyroscopes and was only 9 by 10 by 15 inches in size. One of these was an azimuth (directional) gyroscope, which provided a datum for the heading control of the airplane. The other was a horizontal gyro, providing a datum for longitudinal and lateral control of the airplane.

Lawrence and Elmer Sperry.

The two gyros were air driven and ran at a speed of 15,000 revolutions per minute. The human pilot could shift from automatic pilot to manual control at will, but he could also change course, climb, or dive by turning remote controls to the autopilot, thus effecting flight changes without disengaging the autopilot. The apparatus was provided with a hand-operated clutch for disengaging the autopilot for takeoffs and landings.

Tthe "robot pilot" used two gyroscopes.

A mechanism was incorporated so that when the airplane banked about its longitudinal axis, the stabilized gyroscope resisted the bank, and an airjet, activated valve opened, which, through an oil-operated servo mechanism, moved the control cables to remove the resistance by adjusting the ailerons to roll the airplane wings level.

A mechanism  rolled the airplane wings level. Attitude changes of the aircraft, produced pressure differentials in airflow in the ducts ending near the discs. The result was a proportional change in air valves that was transmitted to the appropriate hydraulic system which, in turn, moved the proper-control surface (elevator for pitch, aileron for bank, and rudder for yaw). The autopilot, which Post called "Mechanical Mike," required no electrical power and functioned very well.

A similar mechanism on the same gyroscope controlled airplane pitch about the lateral axis (nose up, nose level, or nose down) by moving the control cables forward and backward as necessary to adjust the elevators. The other gyroscope controlled the directional gyro (which was set periodically in accordance with the magnetic compass) and through its oil-operated servomechanism made small adjustments on the rudder to yaw the aircraft back to maintain a predetermined heading.

"Mechanical Mike", the Sperry Autopilot. It was a "three-axis" autopilot, that is, one gyroscope sensed pitch changes and roll (bank) changes by the aircraft, and the other gyroscope sensed heading changes (yaw). The air compressor and the hydraulic pump were engine powered. The gyroscopes were driven by compressed air. Compressed air was also used to correct changes of aircraft attitudes in relation to pitch, bank, or yaw, through a system of small air ducts and semilunar plates.

The entire apparatus weighed 70 pounds and it deserves appreciation that such devices had not received extensive trials at the time Post was planning his world flight!

The Evolution of the Sperry autopilot or gyropilot (Mechanical Mike): It got smaller during 1930's.

In 1933, Post repeated his round-the-world flight, but this time did it solo, with the aid of the auto-pilot and radio compass.

The Sperry automatic pilot used by Post for the 1933 flight differed from previous autopilots in an important manner. Other automatic pilots of the time used electrical "pick-offs" to determine the relative motions of the aircraft and the "fixed" axes of the spinning gyroscopes, and utilized a slip-stream, "wind-powered" spinning gear device as the motive power to enable the autopilot to move the control surfaces. The Sperry autopilot was pneumatic. For electrical autopilots an externally placed small propeller blade was necessary to achieve this motive power. This type of device created drag and was relatively inefficient as a source of power.

Post's Sperry autopilot used airjet pick-offs closely associated with the gyroscopic platforms, a system less liable to mechanical troubles that were common in other autopilots which used varying electrical contacts. The airjet pick-off mechanism used in the Winnie Mae was the first autopilot with this new mechanism. The autopilot was completely mechanical (pneumatic) and did not use electrical power. The diagram of the autopilot (above) illustrates the relationships of the various mechanisms constituting the autopilot.

Hydraulic cylinders were located directly under the unit and connected to the control cables.

The horizontal gyroscopic air pick-offs consisted of two hemispherical discs that were fixed to the gyroscopic platform, and, with the airplane flying straight and level, the edges of the discs received equal pressure from the air jets. If the airplane nose up, and/or banked, as by a gust, one or more of the jets of air would not encounter a portion of the disc, allowing more air to flow out of the respective jet. The difference in airflow activated the associated air valve which then opened a hydraulic valve and caused pistons to be moved which were attached to the respective aileron or elevator surfaces. The proper surfaces moved to return the aircraft to the position where the airflow pickoffs were equal.

The heading gyroscope, which controlled the rudder, was arranged in such a way that the mounting of the hemispherical disc caused die airjets to move with the airplane rather than with the gyroscope. This enabled the pilot to fix the airplane on various headings in the 360° azimuth from North. Otherwise, the mechanism operated identically to that of the horizontal gyroscope.

Sperry autopilot (Mechanical Mike) was located in front of the pilot.

An "elevator" knob at the right of the gyro horizon on the control panel enabled Post to establish a given "pitch attitude" of the airplane to be held by the autopilot. An aileron knob, just above the gyro horizon, permitted a similar setting for bank and a rudder knob, immediately above the direction indicator, was used fo azimuth. A "directional gyro compass" display was mounted immediately to the left of the "artificial horizon," both being centered on the panel in front of the pilot's seat. The lever that shifted the autopilot from manual to automatic was located immediately under the servo-unit hydraulic cylinders, which were mounted just beneath the autopilot gyroscopes in the instrument panel. The hydraulic cylinders were in plain view.

A "caging" knob for the artificial horizon was placed just to its left, and a similar caging knob was just below the directional gyro indicator. The gyros were "caged" during periods of manual, visual flight to save wear and tear on their bearings.

Oil sump and pump and air compressor parts.

Lawrence Sperry was the son of Elmer Ambrose Sperry and his wife, Zula Augusta Goodman. Both father and son were noted inventors. His father was best known for inventing the gyroscopic compass. Lawrence invented a three-way gyrostabilizer, effectively inventing the first autopilot.

REFERENCES

/1/ Smithsonian Annuals of Flight, Number 8, Stanley R. Mohler, Bobby H. Johnson,
     "Wiley Post, His Winnie Mae, and the World's First Pressure Suit",
      Smithsonian Institution Press, City of Washington, 1971

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The Gatty Navigation Instrument (Gatty Drift Sight) 1930's

The Gatty Navigation Instrument was a ground speed and wind drift angle indicator developed by Harold Gatty, an Australian-born navigator who lived in California. There is an US patent number US1977762 about it. This instrument was also called the Gatty Drift Sight (Popular Mechanics Magazine, Sep 1931, had a story about it). This sight was widely used by airmen during the late 1930s and eventually Gatty sold it to the US Army Air Corps. Gatty also perfected a 'double drift' method of taking drift reading headings by flying on courses 90 degrees apart to get accurate ground speeds. /1/ /2/ /3/

Harold Gatty and Wiley Post standing by the Winnie May aircraft. The red arrow points the instrument position in the aircraft.

Harold Gatty invented a special combination wind drift and ground-speed  indicator to assist with his navigation duties during the 1931 world flight with Wiley Post.

The operation principe of the Gatty Navigation Instrument

The principle utilized for determining ground speed was based upon comparing the  rate of passage of terrain, as viewed through an eyepiece and two prisms, with  the rate of passage of a series of red marks placed on a film strip, moved by a  clock mechanism at a fixed speed.

The usage of the instrument

The eyepiece was raised or lowered until the two rates were optically the same.  Gatty then read off the distance above the film to which the eyepiece had been moved and consulted a table he had previously made in test flights with the  Winnie Mae at known altitudes and ground speeds.

All Gatty needed to know to use the instrument for ground speed was the altitude  of the airplane above the ground (which could be determined by flying down and  climbing to the cruise altitude or by using the altimeter that was set in  accordance with the previous takeoff point). The instrument was useful over land  or water.

The periscope case of the instrument

For wind drift, grids of the eyepiece were turned until objects on the earth's  surface flowed parallel through the lines of the grids. The angle of "crab"  across the surface could then be directly read by Gatty. The patented version of the instrument used to turn the whole periscope case.

The instrument was mounted near Gatty's seat in the Winnie Mae and the ground  viewing portion extended through the right side of the fuselage where the  external portion was encased in a teardrop fairing.

 The (none turning version) instrument's 3 View

The wind drift and ground-speed indicator was developed by Gatty especially for the around the world flight with Post. This instrument consisted of an adjustable optical periscope through which the ground was viewed. A small motor-driven constant speed device produced moving lines that was superimposed upon the ground image.

The patented version of the instrument had a turning periscope case instead of just turning the eye piece grid of the other version. This version was supposed to be used through a window.

Gatty adjusted the eye piece height until no motion was apparent on the superimposed images. Proportional to the adjustment height Gatty had a reading of the actual ground speed by virtue of previously made calibrations of the equipment. Simultaneously, a rotating grid on the eye piece (or the whole periscope) was turned until the ground image flowed parallel to the grid lines, giving the degree of ground drift occurring.

1) Adjust height until no apprent motion - 2) Rotate eye piece until ground flow is parallel to the grid lines - 3) Read the height and the "crab" angle and consult calibrated tables

The external portion of this optical device protruded beneath the second and third windows on the Winnie Mae's starboard side and was surrounded by a teardrop streamlined fairing, and its small window was enclosed within the fairing.

REFERENCES

/1/ Aero Digest - p. 57-58. - January 1932

/2/ Stanley R. Mohler and Bobby H. Johnson -
Wiley Post, His Winnie Mae, and the World's First Pressure Suit -
SMITHSONIAN INSTITUTION PRESS, City of Washington - 1971

/3/ US Patent 1977762 - 1931 , 1934

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