The serial RS-485 interface is not electrically isolated.
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.
The pull-up, pull-down and termination resistors are not included inside the pro-
cessor module and must be connected externally.
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
NOTICE!
Risk of EMC disturbances!
Unshielded cables may cause EMC disturbances.
Always use shielded cables and connect the shield at every device.
System Assembly, Construction and Connection
AC500-eCo > Connection and Wiring
2019/04/17 3ADR010121, 13, en_US 1211
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