CAN bus / Overview
CAN bus basic principles

Topology

The CAN bus must be laid out like a cable harness. This means that the data cable must be routed from one device to another without branching. All other routing methods are not permitted! The F-BUS (Wurm CAN field bus) and C-BUS (Wurm CAN communication bus) must be physically separated.

 

CAN bus length

The maximum length of the CAN bus depends on the type of cable used, the number of CAN bus participants, how the participants are distributed in the system, and the set transfer speed (e.g. for 20kBit = 400m; longer distances on request).

 

Cable selection

All data lines (e.g. signal, measurement or CAN bus lines) must be implemented as shielded lines. The cable type to be used is LiYCY(TP) 2x2x0.75mm (twisted pair, with copper braid; wire ident-code in accordance with DIN47100). The L and H connections must be connected to a twisted pair cable.

 

Shielding

One end of the cable shielding should generally be connected in the control cabinet, and the insulation should be stripped as near as possible to the connection terminals. Incorrect connection of the shield can lead to electromagnetic interference, data faults, and in a few cases to destruction of the CAN bus drivers.

 

CAN bus terminating resistors

To avoid reflections at the cable ends, a 120Ω CAN bus terminating resistor must be connected at each end of the CAN bus cable between CAN bus lines L and H. The terminating resistors are available as individual terminating resistors or integrated into special 5-pin terminals or RJ45 connectors. There are also devices which have an integrated and switchable terminating resistor:

  • Gateway Multigate

  • Gateway CMD300

  • GTW-LAN

  • GTW-LANplus

  • CAN-PC-Converter C2C

  • Master module FRIGOLINK

 

The CAN bus is generally implemented as a 3-wire bus with the following nomenclature

  • 0V

  • L / BUS_L / CAN_L

  • H / BUS_H / CAN_H

 

Measuring the CAN bus level

The voltage level on the CAN bus can be checked with a multimeter. This can be used to detect reversed polarity on the CAN bus or a missing CAN bus terminating resistor.

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