ABB AC500 PLC Hardware Manual page 1280

System Assembly, Construction and Connection
AC500 (Standard) > Connection and Wiring
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
The following photo shows a wiring example "master within the bus line", wired at the COM1
bus connector of the terminal base:
The earthing of the cable shields of the bus lines are described in the CS31 system bus
(PM57x, PM58x and PM59x)
2.6.4.7 Serial Interface COM2 of the Terminal Bases
The serial interface COM2 is connected via a 9-pole D-sub connector. It is not intended to use
COM2 to establish a CS31 system bus. It is configurable for RS-232 or RS-485 and can be
used for
● online access
● a free protocol
● Modbus RTU, master and slave
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)
● 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.
1280
If the bus is operated with several masters, the pull-up and pull-down resistors
may only be activated at one master.
Ä Chapter 2.6.4.8 "CS31 System Bus" on page 1282.
The serial interface COM2 is not available at:
– Processor modules with type designator -2ETH (e. g. PM591-2ETH)
– Processor modules PM56xx
3ADR010121, 13, en_US 2019/04/17