Beckhoff CX1500-M510 Manual page 160

Assembly and connecting
An additional shield surrounding the pair of signal wires can help to eliminate the disturbing influence of EMI.
Bus length
Bus length
The maximum length of a CAN bus is primarily limited by the signal transit time. The multi-master bus access
procedure (arbitration) requires signals to reach all the nodes at effectively the same time (before the
sampling within a bit period). Since the signal transit times in the CAN connecting equipment (transceivers,
opto-couplers, CAN controllers) are almost constant, the line length must be chosen in accordance with the
baud rate:
Baud Rate
1 Mbit/s
500 kbit/s
250 kbit/s
125 kbit/s
50 kbit/s
20 kbit/s
10 kbit/s
*) A figure of 40m at 1 Mbit/s is often found in the CAN literature. This does not, however, apply to networks
with optically isolated 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
Drop lines must always be avoided as far as possible, since they inevitably cause 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:
160
Bus length
< 20 m*
< 100 m
< 250 m
< 500 m
< 1000 m
< 2500 m
< 5000 m
Version: 1.0 CX1500-M510, CX1500-B510