RS-485 interfaces

AN1702

4/11

1

RS-485 interfaces

With RS-485 two kinds of communication lines can be built: half or full duplex. A full duplex

line requires four wires, and each node can simultaneously send data on two wires, while

receiving from the other pair of wires. A half duplex line uses only one pair of wires for both

sending and receiving, and in this case only one node at any one time can send data on the

line (see Figure 1 and Figure 2). This means that two or more nodes, sending

contemporaneously data to the bus, cause errors in data transmission. It is the application

engineer’s responsibility to choose the proper network management method in order to

avoid this problem. This choice may be constrained by the need to use a pre-defined

protocol on the network, or by the limitation of the calculated power of the network controller.

Figure 1.

RS-485 half-duplex network

Figure 2.

RS-485 full-duplex network