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In 1983, the Electronics Industries Association (EIA) approved a new balanced transmission standard called RS-485. Finding widespread acceptance and usage in industrial, medical, and consumer applications, RS-485 has become the industry’s interface workhorse.
This application report presents design guidelines for engineers new to the RS-485 standard that can help them accomplish a robust and reliable data transmission design in the shortest time possible.
This application report presents design guidelines for engineers new to the RS-485 standard that can help them accomplish a robust and reliable data transmission design in the shortest time possible.
RS-485 is an electrical-only standard. In contrast to complete interface standards, which define the functional, mechanical, and electrical specifications, RS-485 only defines the electrical characteristics of drivers and receivers that could be used to implement a balanced multipoint transmission line.
This standard, however, is intended to be referenced by higher level standards, such as DL/T645, for example, which defines the communication protocol for electronic energy-meters in China, specifying RS-485 as the physical layer standard.
Key features of RS-485 are:
The RS-485 standards suggests that its nodes be networked in a daisy-chain, also known as party line or bus topology (see Figure 3-1. In this topology, the participating drivers, receivers, and transceivers connect to a main cable trunk via short network stubs. The interface bus can be designed for full-duplex or half-duplex transmission (see Figure 3-2).
The full-duplex implementation requires two signal pairs, (four wires), and full-duplex transceivers with separate bus access lines for transmitter and receiver. Full-duplex allows a node to simultaneously transmit data on one pair while receiving data on the other pair.
In half-duplex, only one signal pair is used, requiring the driving and receiving of data to occur at different times. Both implementations necessitate the controlled operation of all nodes via direction control signals, such as Driver/Receiver Enable signals, to ensure that only one driver is active on the bus at any time. Having more than one driver accessing the bus at the same time leads to bus contention, which, at all times, must be avoided through software control.