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2.7 Baud rate and wire length of CAN bus
The CAN bus is a balanced (differential) 2-wire interface running over
either a Shielded Twisted Pair (STP), Un-shielded Twisted Pair (UTP), or
Ribbon cable. However, the baud rate of communication is inversely
proportional to the wire length. When the baud rate is lower, the ideal CAN bus
wire length that can be used is longer, as shown in the following table.
(Note: The ideal CAN bus wire length means the sum of the wire lengths of
each CAN bus in a single CAN network.)
Table2. Baud rate and ideal CAN bus wire lehgth
In addition, the I-7531-FD can isolate and re-amplify the CAN bus signal,
so the user will think that connecting multiple I-7531-FDs in series on the
same CAN bus, the CAN bus signal will always re-amplify and then extend
the wire length of CAN bus, but the CAN bus signal passing through the I-
7531-FD will reduce the ideal CAN wire length due to the time delay at the
physical layer in fact.
In other words, when the user add one I-7531-FD to the CAN bus, the
CAN signal passing through the I-7531-FD will be delayed by 175ns, and the
ideal CAN bus wire length will be reduced by 35 meters.
For example, the baud rate used by the user is 500Kbps, and the ideal
CAN bus wire length is 100 meters, when one I-7531-FD is connected in
series to the CAN bus, the ideal CAN bus wire length will be reduced by 35
meters that is reduced from 100 meters to 65 meters (100 – 35 x 1 = 65).
By analogy, if two I-7531-FDs are connected in series on the CAN bus, it
will reduced by 35 x 2 = 70 meters as shown in the below figure.
I-7531-FD CAN Repeater User Manual (ver. 1.0, 2021/10/21) ------12
Baud rate
Ideal CAN bus wire length
[bit/sec]
800K
500K
250K
125K
50K
20K
10K
[meters]
50
100
250
500
1000
2500
5000
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