This article explains the difference between active and passive termination for serial connections using RS485 physical connections.
All Anybus products with RS485 serial physical interfaces
Termination resistors should in general be added to both ends of a serial bus in order to minimize signal reflections. The resistor value should match your cable impedance (typically 120 Ohm) in order to minimize signal reflections. By doing so, the line appears to have an infinite length for the signal, thanks to the absence of impedance mismatch.
In addition to passive termination, it is in some cases advisable to add an active termination, especially in environments with a high level of EMC emissions. It will not only prevent signal reflections, but by introducing a resistor network on one end, a defined voltage baseline is created for your RS485 idle signal.
In detail, you connect a resistor between RS485+ and the 5 V rail and another one between RS485- and signal ground. Additionally, the cable shield should be tied to functional earth (FE).
The differential signal between RS485+ and RS485- will have an idle voltage of about 560 mV. This way, external disturbance caused by EMC has a lower chance of resulting in spurious bits in the communication.
The resistor network can be easily simulated as follows.
Link to the above circuit simulation
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