BECKHOFF FC5101 - Manual (Page 15)

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couplers, CAN controllers) are almost constant, the line length must be chosen in accordance with 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
*) 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 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
1 Mbit/s < 1m < 5 m
500 kbit/s < 5 m < 25 m
250 kbit/s < 10m < 50 m
125 kbit/s < 20m < 100 m
50 kbit/s < 50m < 250 m
Drop lines must not have terminating resistors.
Star Hub (Multiport Tap)
Shorter drop line lengths must be maintained when passive distributors ("multiport taps"), such as the Beckhoff
ZS5052-4500 Distributor Box. 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