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Bus Termination
2 BusTermination
2.1 BusTermination(RS485orRS485/422)
There are many causes of communication interference in a RS485 bus; one of them is bus reflexion. A
reflexion occurs when the transmitter signal is not completely absorbed by the load. That is why the source
impedance should correspond to the load impedance and line surge impedance since this allows the full
signal power to be reached with only minimal reflexion. The serial communication of the RS485 interface
operates most efficiently when the source and load impedance is at 120 ohms. Consequently, the RS485
standard recommends a bus cable with a line surge impedance of Z0 = 120 ohms. To avoid reflexion in the
bus, the beginning and end of the bus cable has to be terminated according to the line surge impedance.
Here are two bus topology examples:
Fig.: Example 1 of bus topology
Fig.: Example 2 of bus topology
Note
Other bus topologies (e.g. star topologies) are outside the specifications and should be avoided.
=> In addition, it is important to note: The slower the baud rate, the more unsusceptible the bus is
regarding the connection and the topology.
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