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5.2.1.3 Required Conductor Cross Section

The conductor cross section depends on the conductor length and has to be selected according to the number of nodes connected.

0,75 0,75

Fig. 5-1: Conductor cross section depending on the conductor length and the number of nodes

5.2.2 Cabling

The connection of a WAGO fieldbus node to the CANopen bus cable is made by the appropriate supplied plug (5-pole plug MCS or 9-pole plug D-SUB).

For cabling with a shielded copper cable (3x0.25 mm²), the plug is assigned with the CAN_High, CAN_Low and CAN_GND connections.

CAN_High and CAN_Low are two physically different bus levels. CAN_GND is the common reference potential.

The conductor shield of the cable can be routed on the connection drain, which is terminated with 1 MΩ as against the ground or PE (carrier rail contact). A low ohmic connection of the shield to the PE can only be made externally (i.e. by means of a supply module). The aim is for a central PE contact for the entire CANopen bus conductor screening.

Note

WAGO offers the screen connection system (series 790) for an optimum connection between fieldbus cable screening and functional earth.

Each CAN node forms the differential voltage UDiff with: UDiff = UCAN_High - UCAN_Low. from the bus levels CAN_High and CAN_Low.

The different signal transmission offers the advantage of being immune to common mode interference and ground offset between nodes.

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If the bus level is in the recessive status, the voltage between CAN_Low and CAN_GND is 2.5 V and also 2.5°V between CAN_High and CAN_GND. This means that the differential voltage is 0 V.

If the bus level is in the dominant status, the voltage between CAN_Low and CAN_GND is 1.5 V and 3.5°V between CAN_High and CAN_GND.

Then differential voltage is approx. 2 V.

Note

When connecting subscribers, ensure that the data lines are not mixed up.

At its conductor ends, the bus cable must always be connected with a matching resistor of 120 Ohm to avoid reflections and, as a result, transmission problems.

This is also required for very short conductor lengths.

CAN_High

CAN_Low

R = 120 Ohm

T

Fig. 5-2: Connection principle of a WAGO fieldbus node to the CAN bus

g012402x

Before starting the buscoupler on the network, the installation should be checked. The physical connection can be checked in the CAN fieldbus with an ohmmeter at any place. You have to remove all connections to other devices except for the terminating resistors.

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ohmmeter

CAN_High

CAN_Low

GND

R = 120 Ohm

T

The CAN bus is 2-wire bus and bus error management can detect a cable break or a short-circuit by the asymmetric operation.

Further information

The CiA provides documents regarding specifications, especially cable specifications in the Internet under:

http://www.can-cia.de

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5.2.3 Network Topology

To build a simple CANopen network, you need a master (PC with a CANopen fieldbus PCB card), a connection cable and a DC 24 V power pack to ensure the power supply in addition to a CANopen fieldbus node.

The CANopen network is constructed as a line structure with matching resistors (120 Ohm).

In systems having more than two stations, all subscribers are wired in parallel. The maximum length for a conductor branch should not exceed 0.3 m.

Line, Bus

Network node

All net subscribers communicate at the same Baud rate. The bus structure permits coupling in and out without side effect of stations, or the step-by- step start-up of the system.

Later extensions have no influence on stations already in operation. The system automatically detects when a subscriber fails or is newly added to the net.

Also branches from the line-shaped bus and as such the establishment of hierarchic net structures are possible via router nodes.

Repeaters can be used to increase the maximum possible number of nodes to 110 and to enlarge the network spatial extension (bus length). Although the network spatial extension depends on the transmission rate, CAN can also be used for spatially extended networks. The data rates achievable are of the same order as with other bus systems. However, the maximum possible cable length is reduced per repeater by 20 - 30 m due to the signal delay.

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