ABB FSCA-01 User Manual page 20

20 Overview of the RS-485 network and the FSCA-01 module
The RS-485 transmission line consists of two wires, A and B
(balanced pair). The signal transmission is based on the voltage
difference between the wires. The minimum detected voltage
difference is 200 mV. The potential difference between the two
wires determines the logic state bit: when B is at a higher voltage
than A, the state is defined as bit 1 (data high) and when A is at a
higher voltage than B, the state is defined as bit 0 (data low).
The maximum common mode voltage between RS-485 network
devices is limited to -7...+12 V. A ground wire and cable shield
should be connected to prevent the common mode voltage
between the network devices from drifting outside the allowable
limits.
The RS-485 bus cable should be terminated with a 120 ohm
resistor at both ends to prevent signal reflection. When no device
on the network is transmitting, noise can be falsely interpreted as
communication data. To avoid this, a termination can be included
with a fail-safe circuit (pull-up and pull-down resistors). The circuit
forces the bus into a known idle state when no device is
transmitting.