Designing Embedded Hardware, 2nd Edition

RS-485

RS-485 is a variation on RS-422 that is commonly used for low-cost networking and in many industrial applications. It is one of the simplest and easiest networks to implement. It allows multiple systems (nodes) to exchange data over a single twisted pair (Figure 9-12).

Figure 9-12. RS-485 network

RS-485 is based on a master-slave architecture. All transactions are initiated by the master, and a slave will transmit only when specifically instructed to do so. There are many different protocols that run over RS-485, and often people will do their own thing and create a protocol specific to the application at hand.

The interface to the RS-485 network is provided by a transceiver, such as a Maxim MAX3483 (Figure 9-13).

Figure 9-13. RS-485 transceiver

The MAX3483 is just an RS-422 transceiver with enable inputs, and using it in a design is straightforward. On the network side, the MAX3483 has two signal lines, A and B. This is the twisted pair (network cable) attachment point. The MAX3483 also has Data In (DI) and Receiver Out (RO). These are connected to the Tx and Rx signals of the UART (or microcontroller), respectively.

Since it is connected to a common network on which it must both listen and transmit, it has two control inputs, Data Enable (DE) and Receiver Enable (). A high ...