Advantech ADAM-5511 Manual page 320

Each discontinuity in impedance causes reflections and distortion.
When a impedance discontinuity occurs in the transmission line the
immediate effect is signal reflection. This will lead to signal distortion.
Specially at line ends this mismatch causes problems. To eliminate this
discontinuity, terminate the line with a resistor.
The value of the resistor should be a close as possible to the charac-
teristic impedance of the line. Although receiver devices add some
resistance to the whole of the transmission line, normally it is sufficient
to the resistor impedance should equal the characteristic impedance of
the line.
Each input of the receivers has a nominal input impedance of 18 kΩ
feeding into a diode transistor- resistor biasing network that is
equivalent to an 18 kΩ input resistor tied to a common mode voltage of
2.4 V. It is this configuration which provides the large common range of
the receiver required for RS-485 systems! (See Figure E-5 below).