ABB AC500 PLC Hardware Manual page 1279

Terminal
block
removed
RS-485 bus 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)
● 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 activate both the pull-up and the pull-down resistors, which only are necessary
once on every bus line, at the bus master. For this reason, these two resistors are already inte-
grated within the COM1 interface of the AC500 terminal bases. They can be activated by con-
necting the terminals 1-2 and 3-4 of COM1.
The following drawing shows an RS-485 bus with the bus master at the 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, must be activated (see the following drawing).
2019/04/17
Pin
Terminal
4
block
5
inserted
6
7
8
9
3ADR010121, 13, en_US
System Assembly, Construction and Connection
AC500 (Standard) > Connection and Wiring
Signal Interface
Terminator N RS-485
RTS RS-232
TxD RS-232
SGND Signal Ground
RxD RS-232
CTS RS-232
Description
Terminator N
Request to send
(output)
Transmit data
(output)
Receive data
(input)
Clear to send
(input)
1279