Implementing RS485 Communication

The interface popularly known as RS-485 is an electrical specification for multipoint systems that use balanced lines. RS-485 is similar to RS-422, but RS-422 allows just one driver with multiple receivers whereas RS-485 supports multiple drivers and receivers.

The specification document (TIA/EIA-485-A) defines the electrical characteristics of the line and its drivers and receivers. There are brief suggestions relating to terminations and wiring, but there's no discussion of connector pinouts or software protocols (as there is for RS-232).

An RS-485 network can have up to 32 unit loads, with one unit load equivalent to an input impedance of 12k. By using high-impedance receivers, you can have as many as 256 nodes.

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