CANopen: Connection and wiring
Notice | |
Improper wiring On account of the lack of electrical isolation, the CAN driver can be destroyed or damaged due to incorrect cabling. Always carry out the cabling in the switched-off condition. First connect the power supply and then the CAN. |
CAN is a 2-wire bus system, to which all devices are connected in parallel (i.e. using short drop lines). The bus must be terminated at each end with a 120 (or 121) ohm termination resistor to prevent reflections. This is also necessary even if the cable lengths are very short!
Since the CAN signals are represented as differential levels on the bus, the CAN line is comparatively insensitive to interference (EMI). Both lines are affected, so the interference hardly changes the differential level.
Additional shielding of the twisted wires can be used to further reduce EMI interference.
Bus length
The maximum length of a CAN bus is primarily limited by the signal propagation time. The multi-master bus access method (arbitration) requires that the signals are present quasi-simultaneously (before sampling within a bit time) at all nodes. Since the signal propagation time in the CAN connections (transceiver, optocoupler, CAN controller) are almost constant, the cable length must be adapted to the baud rate.
Baud rate | Bus length |
|---|---|
1 Mbit/s | < 20 m* |
500 kbit/s | < 100 m |
250 kbit/s | < 250 m |
125 kbit/s | < 500 m |
50 kbit/s | < 1000 m |
20 kbit/s | < 2500 m |
10 kbit/s | < 5000 m |
*) Often you can find the specification 40 m at 1 Mbit/s in the literature for CAN. However, this does not apply to networks with opto-decoupled CAN controllers. The worst case calculation for opto-couplers yields a figure 5 m at 1 Mbit/s - in practice, however, 20 m can be reached without difficulty.
It may be necessary to use repeaters for bus lengths greater than 1000 m.
Drop lines
Drop lines must always be avoided as far as possible, since they inevitably cause signal reflections. The reflections caused by drop lines are not however usually critical, provided they have decayed fully before the sampling time.
In the case of the bit timing settings selected in the bus couplers it can be assumed that this is the case, provided the following drop line lengths are not exceeded:
Baud rate | Drop line length | Total length of all drop lines |
|---|---|---|
1 Mbit/s | < 1m | < 5 m |
500 kbit/s | < 5 m | < 25 m |
250 kbit/s | < 10 m | < 50 m |
125 kbit/s | < 20m | < 100 m |
50 kbit/s | < 50m | < 250 m |
Drop lines must not have termination resistors.
Star Hub (Multiport Tap)
When using passive distributors ("Multiport Taps"), e.g. the BECKHOFF distribution box ZS5052-4500, shorter drop line lengths must be maintained.
The following table indicates the maximum drop line lengths and the maximum length of the trunk line (without the drop lines):
Baud rate | Drop line length with multiport topology | Trunk line length (without drop lines) |
|---|---|---|
1 Mbit/s | < 0.3 m | < 25 m |
500 kbit/s | < 1.2 m | < 66 m |
250 kbit/s | < 2.4 m | < 120 m |
125 kbit/s | < 4.8 m | < 310 m |