RS-485 Interface
RS-485 is a differential-signal electrical interface used for multi-point serial communication over long distances and in noisy environments. Because the information is carried by the voltage difference between two conductors, noise that couples equally onto both wires is largely cancelled at the receiver.
The interface's rules in practice are simple, but when they are skipped the line does not work.
- Topology: the line is run as a straight chain; taking long stubs off the devices leads to reflections.
- Termination: a resistor, typically 120 ohm, is placed at both ends of the line in accordance with the characteristic impedance of the cable.
- Biasing: biasing resistors are used so that the line does not stay undefined when no device is talking; otherwise the receiver may take noise for data.
- Common reference: even though it is a differential signal, receivers have a common-mode limit; if there is a large ground difference between distant points, a reference conductor or isolation is required.
- Speed-distance trade-off: at low speeds, distances in the hundreds of metres range are common; as the speed increases the permitted length shortens noticeably.
On industrial boards RS-485 is mostly used together with a protocol such as Modbus; keeping in mind that the electrical interface and the protocol are separate things makes it easier to separate the layers during fault finding.
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