Figure 2.27:Resistor Selection - Advantech PCI-COMM Series User Manual

Terminator resistor setup (for PCI-1601/1602/1611/1612/1622)
You can install terminator resistors if necessary to match impedance.
Each signal line (Tx, Rx) has a separate resistor.
Especially in fields with serious electric noise, installing terminal resis-
tors is helpful to stabilize communications. Make sure that both sides of
the RS-485 communication ports are installed on BUS. See details in
Chapter 5.2.2 and 5.2.3
Figure 2.27: Resistor Selection
PCI-COMMUNICATION User Manual 34