BECKHOFF FC520x - Manual (Page 17)

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FC520x / CX1500-x520 16
BECKHOFF FC520x / CX1500-x520
3.2.2 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 (trans-
ceivers, opto-couplers, CAN controllers) are almost constant, the line length must be chosen in accor-
dance with the baud rate:
For the DeviceNet use of the CAN Bus following Baud Rates and Bus Lengths are specified:
BaudRate Bus Length
500 kbit/s < 100 m
250 kbit/s < 250 m
125 kbit/s < 500 m
3.2.3 Drop-Lines
Drop lines must always be avoided as far as possible, since they inevitably cause reflections. The reflec-
tions 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 (according to DeviceNet Speci-
fication):
BaudRate 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 have terminating resistors.
Star Hub (Multiport Tap)
Shorter drop line lengths must be maintained with 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):
Note: These values are recommended by Beckhoff
BaudRate Drop line length with multi-
port topology Trunk line length (without drop
lines)
500 kbit/s < 1,2 m < 66 m
250 kbit/s < 2,4 m < 120 m
125 kbit/s < 4.8 m < 310 m