WAGO-I/O-SYSTEM 750
Connect Devices
43
750-652 Serial Interface RS-232 / RS-485
Manual
Version 1.4.0
7.2.2
RS-485 Operating Mode
Figure 13: Bus Connection RS-485 Operating Mode, Half-Duplex
In the RS-485 operating mode (bus connection), a twisted-pair cable should be
used at least for larger cable lengths or higher transmission rates. The ends of the
cable should be terminated with a terminal resistor.
Generally, a passive resistance is used by connecting the signal lines via one
100 … 120 Ω resistor apiece to both bus ends.
For communication between 2 or more 750-652interface modules at low baud
rates or with shorter line lengths, it is not necessary to use the terminal resistors.
The receiver built into the I/O module has a defined output state if both inputs are
not connected and the data line has a short circuit or no transmitter is active (idle
line).
If the 750-652 serial interface module works on a network with other
communication partners, the wiring of a resistance-biasing network (known as a
fail-safe network) to one line end can be necessary.
Figure 14: Bus Connection RS-485 Operating Mode with Biasing Network, Half-Duplex
With the use of the biasing resistors, it is guaranteed in this case that all receiver
inputs A have the voltage difference of more than 200 mV to the inputs B if no
transmitter is active. The oscillations of the receiver that can cause defective
reception are thus prevented.
