ABB AC500 PLC Hardware Manual page 1211

If the RS-485 bus is used, each interconnected bus line (each bus segment) must be electrically
terminated. The following is necessary:
● 2 resistors of 120 Ω each at both line ends (to avoid signal reflections)
● In addition, a pull-up resistor at RxD/TxD-P and a pull-down resistor at RxD/TxD-N. These 2
resistors care for a defined high level on the bus, while there is no data exchange.
It is useful to add both the pull-up and the pull-down resistors, which only are necessary once
on every bus line, at the bus master.
The following figure shows an RS-485 bus with the bus master at one line end.
1 Master at the bus line end, pull-up and pull-down activated, bus termination with 120 Ω
resistors
2 Slave within the bus line
3 Slave at the bus line end, bus termination with 120 Ω resistors
If the master is located within the bus line, it does not need a terminating resistor. The pull-up
and the pull-down resistors, however, are necessary:
1 Slave at the bus line end, bus termination with 120 Ω resistors
2 Master within the bus line, pull-up and pull-down activated
3 Slave within the bus line
4 Slave at the bus line end, bus termination with 120 Ω resistors
2019/04/17
The serial RS-485 interface is not electrically isolated.
The pull-up, pull-down and termination resistors are not included inside the pro-
cessor module and must be connected externally.
NOTICE!
Risk of EMC disturbances!
Unshielded cables may cause EMC disturbances.
Always use shielded cables and connect the shield at every device.
3ADR010121, 13, en_US
System Assembly, Construction and Connection
AC500-eCo > Connection and Wiring
1211