Cabling
- Updated2023-02-21
- 3 minute(s) read
Cabling
Cabling Requirements
Cables must meet the physical medium requirements specified in ISO 11898, shown in the following table.
Belden cable (3084A) meets all these requirements and is suitable for most applications.
| Characteristic | Value |
|---|---|
| Impedance | 108 Ω minimum, 120 Ω nominal, 132 Ω maximum |
| Length-related resistance | 70 mΩ /m nominal |
| Specific line delay | 5 ns/m nominal |
Cable Length
The cabling characteristics and desired bit transmission rate affect the allowable cable length. Detailed cable length recommendations are in the ISO 11898 and CiA DS 102 specifications. ISO 11898 specifies 40 m total cable length with a maximum stub length of 0.3 m for a bit rate of 1 Mbps. The ISO 11898 specification says that significantly longer cable lengths may be allowed at lower bit rates, but each node should be analyzed for signal integrity problems.
Maximum Number of Devices
The maximum number of devices that you can connect to a CANopen port depends on the electrical characteristics of the devices on the network. If all of the devices on the network meet the CANopen specifications, 64 devices may be connected to the network.
Connector
9-Pin D-Sub Connector
The CANopen hardware product has a D-Sub terminal connector. The following figure and table show the pinout for this connector.
| Pin | Signal | Description |
|---|---|---|
| 1 | NC | No connection. |
| 2 | CAN_L | CAN_L bus line. |
| 3 | V— | CAN reference ground. |
| 4 | NC | No connection. |
| 5 | (SHLD) | Optional CAN shield. |
| 6 | (COM) | Optional CAN reference ground. |
| 7 | CAN_H | CAN_H bus line. |
| 8 | NC | No connection. |
| 9 | (V+) | Also referred to as Ext_Vbat, this is an optional CAN power supply if bus power/external VBAT is required. |
5-Pin Combicon Connector
Some CANopen devices may use a Combicon-style pluggable screw terminal connector. The 5-pin Combicon-style pluggable screw terminal follows the pinout required by the CANopen Specification. The following figure shows the pinout for this connector.
|
||
| 1 V— | 3 GND | 5 V+ |
| 2 CAN_L | 4 CAN_H | |
Bus Power Requirements
The physical layer on the PXI and PCI CANopen interfaces is internally powered. As such, there is no need to supply bus power. The COM pin serves as the reference ground for the bus signals.
The physical layer on C-Series NI 9881 requires external power supply of +9 to +30 V to operate. Connect the external power supply to the Vsup pin on the module. The COM pins are for reference ground.
Termination
The pair of signal wires (CAN_H and CAN_L) constitutes a transmission line. If the transmission line is not terminated, each signal change on the line causes reflections that may cause communication failures.
Because communication flows both ways on the CANopen bus, CANopen requires that both ends of the cable be terminated. However, this requirement does not mean that every device should have a termination resistor. If multiple devices are placed along the cable, only the devices on the ends of the cable should have termination resistors.
The termination resistors on a cable should match the nominal impedance of the cable. CANopen requires a cable with a nominal impedance of 120 Ω; therefore, a 120 Ω resistor should be used at each end of the cable. Each termination resistor should be capable of dissipating at least 0.25 W of power.
Cabling Examples
The following figure shows an example of a cable connecting two CANopen devices.
The following graphic shows an example of where termination resistors should be placed in a system with more than two devices.
