CAN Hardware

CANOpen is a serial network that uses a bus topology. The CANOpen bus always contains two signal wires, CAN+ and CAN-, which carry the differential serial data and a ground (GND). It is also common for most CANOpen nodes to provide a shield connection.

Similar to most industrial buses, the signal wires need to be terminated. CANOpen requires a 120ohm resistor at both ends of the main bus. If these resistors are not fitted, the network will not function properly. Some node suppliers build the terminating resistor into the node and provide a jumper or switch to enable it. You will need to check your nodes' datasheets for the inclusion of a terminating resistor. The XMP does not have any terminating resistors.

For pinout information, go to the XMP's CAN D-9 connector page.

A CANOpen node either has an opto-isolated or non-isolated interface. The use of optoisolation is primarily provided as an EMC countermeasure and is used to cope with potential differences in the ground. These effects are more pronounced for large machines and cable lengths. Therefore, the use of opto-couplers is recommended for bus lengths greater that 200m. The disadvantage of opto-couplers is that they reduce the maximum permissible bus length for a given bit rate.

The XMP CAN interface is available with or without opto-isolation. This option needs to be specified at the time your XMP is ordered.

Most types of nodes require a separate power supply to drive the local logic and the I/O interfaces. For nodes that use opto-isolated interfaces, a separate supply of +7 to 24V needs to be provided to power the interface circuitry. The user must also supply an external 24V to the XMP (CAN_V+) if the opto-isolated interface option is being used.

Each node on the network must have a unique node number, in the range of 1 to 127. The node number is commonly set with a bank of DIP switches on each node. If two nodes are given the same node number, network errors are generated and unpredictable problems will be encountered. The node number of the XMP can be changed from the factory default of 1 using the meiCanConfigSet function.

In order for all nodes to communicate they must all use the same bit rate. Normally the bit rate that a node uses is set by DIP switches. If all of the nodes on a CANOpen network do not use the same bit rate then the whole network or some of the nodes on the network will not work properly. The bit rate of the XMP is set via software meiCanConfigSet. See also CAN Bit Rate.