WAGO-I/O-SYSTEM 750 Connect Devices 43750-652 Serial Interface RS-232 / RS-485ManualVersion 1.4.07.2.2 RS-485 Operating ModeFigure 13: Bus Connection RS-485 Operating Mode, Half-DuplexIn the RS-485 operating mode (bus connection), a twisted-pair cable should beused at least for larger cable lengths or higher transmission rates. The ends of thecable should be terminated with a terminal resistor.Generally, a passive resistance is used by connecting the signal lines via one100 … 120 Ω resistor apiece to both bus ends.For communication between 2 or more 750-652interface modules at low baudrates 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 arenot connected and the data line has a short circuit or no transmitter is active (idleline).If the 750-652 serial interface module works on a network with othercommunication partners, the wiring of a resistance-biasing network (known as afail-safe network) to one line end can be necessary.Figure 14: Bus Connection RS-485 Operating Mode with Biasing Network, Half-DuplexWith the use of the biasing resistors, it is guaranteed in this case that all receiverinputs A have the voltage difference of more than 200 mV to the inputs B if notransmitter is active. The oscillations of the receiver that can cause defectivereception are thus prevented.
