Mounting and cabling
EL6752
20
Version: 2.1
Fig. 14: Low interference through difference levels
4.5.2
Bus length
The maximum length of a CAN bus is primarily limited by the signal propagation delay. 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 propagation delays 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
Bus length
500 kbit/s
< 100 m
250 kbit/s
< 250 m
125 kbit/s
< 500 m
4.5.3
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:
Baud rate
Drop line length
Total length of all drop lines
500 kbit/s
< 6 m
< 39 m
250 kbit/s
< 6 m
< 78 m
125 kbit/s
< 6 m
< 156 m
Drop lines must not be furnished with termination resistors (Fig.
Drop line topology
).
Fig. 15: Drop line topology
