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02.06.2016

General information on the CAN bus part 1.

General information about the CAN bus part 1.

This section will cover the following questions:

  • CAN bus history
  • CAN bus technology
  • CAN bus and immunity

CAN bus history

The CAN protocol regulates the messages by which devices on the network communicate. It was originally developed for use in the automotive industry.

The development of the CAN bus was launched in the early 80's. In 1986, the Controller Area Network (CAN) serial bus was introduced at the Congress of the Association of Automotive Engineers (SAE), which marked the emergence of one of the most successful network protocols ever created.

Today, the CAN protocol, which allows the integration of various control units and control systems into a single unit, is used in all modern vehicles and other vehicles, from trains to ships, as well as in industrial control systems (where it is called DeviceNet).

The CAN bus, adapted for marine applications, is known as the NMEA 2000 network .

The CAN protocol is today one of the dominant and possibly even the world's major serial bus instrument used to connect all kinds of devices and equipment in one network.

Technology

CAN is a broadcast serial bus standard for connecting several leading electronic devices. This type of connection is known as multi-master, which means that there is no central control node.

Each bus node (electronic device) can send and receive messages, but not simultaneously. The message consists primarily of an identifier (ID), which is usually chosen to determine the type of message or sender and includes up to eight data bytes. It is transmitted to the bus in series. All nodes (devices) have a node processor and a CAN interface through which they connect to the bus. If the bus is idle, any node can start the transfer. If two or more nodes start sending messages at the same time, a message with a more dominant ID (which has more dominant bits, i.e., zeros) overwrites the less dominant IDs of the other nodes. Thus, in the long run (after ID-arbitration), only the dominant message remains, which is obtained by all nodes. Receiving units are then determined,

After the completion of the transfer of the dominant message, all "losers" in the arbitration process try to send their messages again. This is the so-called nondestructive arbitration, in which the message with the highest priority is not destroyed.

The CAN protocol is event-driven as opposed to a time-initiated protocol. The bus architecture does not impose restrictions on when nodes can send messages to the network.

Interference immunity

The information is transmitted on the bus as a potential difference between the two signal lines CAN_H and CAN_L. If both lines have the same voltage, the signal is a recessive bit. If the potential of the CAN_H line is higher than that of the CAN_L line by 0.9 V, the line signal is the dominant bit. There is no independent ground reference point for these two lines. Thus, the bus is immune to any background noise.

The signals of the two CAN lines are subjected to the same electromagnetic interference, therefore, the potential difference between the two lines will remain unchanged. Therefore, the bus is immune to electromagnetic interference.

NMEA 2000 Network

This section will cover the following questions:

  • Topology of the NMEA 2000 network
  • NMEA 2000 network cables and their length
  • MNEA 2000 network connectors and terminal matching devices
  • Power and Ground of the NMEA 2000 Network

Network topology

The NMEA 2000 cable system uses a trunk topology with taps and drop lines. 

Cables

In order for the network cables to be used as a data line and for DC power, they must meet certain requirements regarding wave impedance, propagation delay and wire section.

In NMEA 2000 networks, a separate waterproof cable should be used, including one twisted pair of signal wires, one twisted pair of power wires and a ground wire. The ground wire shields the signal wires and power wires from external RF interference, and also reduces the radio's own radio emission.

Three types of cables are allowed: a mini cable, which is commonly used for a network backbone due to its ability to withstand currents of up to 8 A, a medium cable (normally used as a backbone in small networks, and a micro cable ( micro cable), commonly used as cable taps to connect devices to the backbone (both last cables withstand currents up to 4 A). The thinner the cable, the more flexibility it has when installing.

Length of network cables

Since all devices on the network must receive the same data bits at the same time intervals, the maximum length of the NMEA 2000 network between the two endpoints should not exceed 200 m, which theoretically corresponds to a maximum transfer rate of 250 kbps.

The limitations of the current supply wires can, however, further reduce the maximum cable length.

Since it is not necessary to install terminal resistors on devices connected to the network, the length of the drop line should not exceed 6 m so as not to cause reflection of the signals to the network. Up to 50 devices can be connected to the network, but the total length of the drop lines in the network can not exceed 78 m.
To connect devices to the trunk, connectors are used.
Uniting devices in a daisy chain is not allowed!

Connectors

To connect cables to devices or other components of the NMEA2000 network, 5-pin industrial connectors are used to turn the network into a plug-and-play system.

The pinout of the connector's connectors and the color coding of the wires are shown below.

Colour Name Appointment
Model FCV-295 CVS-1410
White CAN_H Signal
Blue CAN_L Signal
Bared SHIELD Screen / Ground
The black Power-C Common power cable
Red Power-S Food

Networking terminal device

To reduce signal reflections in the network, it is necessary to establish a consistent load
at each end of the trunk. If the network is not equipped with terminal resistors, it will not work properly.

The terminating resistor is usually connected to the last T-connector of the trunk as an internal line resistor, that is, the resistor is built into the connector and connected directly to the tee.
The terminating resistor can also be installed in the device connected to the last trunk of the trunk.

Power connection

The operating voltage range for compatible nodes in the NMEA 2000 network is 9-16 VDC.

Connecting the power to the network is usually done through the power outlet. If the network is fed in the middle (or at any point other than the end), the cable system can withstand a load exceeding the maximum allowable value for the cable, provided that the maximum current is not exceeded in any segment of the trunk.

Networks with a terminal power supply are usually used when there are a small number of devices in them.
The power connection in the middle is applied when the number of connected devices requires a higher current than in the configuration with the end power.

Grounding

The NMEA 2000 network must be grounded in only one location to avoid ground loops that can cause communication problems on the network. The grounding wire / shield must only be connected to the ground of the power source.

General information about the CAN bus part 2.



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