Pin Signal Interface Description
Terminal
block
removed
Terminal
block
inserted
4 Terminator N RS-485 Terminator N
5 RTS RS-232 Request to send
(output)
6 TxD RS-232 Transmit data
(output)
7 SGND Signal Ground
8 RxD RS-232 Receive data
(input)
9 CTS RS-232 Clear to send
(input)
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).
RS-485 bus
System Assembly, Construction and Connection
AC500 (Standard) > Connection and Wiring
2019/04/17 3ADR010121, 13, en_US 1279
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