networks//network protocols//RS-485

RS-485 is an electrical standard for serial communication over a differential pair (A and B lines), allowing many devices on one multidrop bus over long distances with good noise immunity. It defines the physical and electrical link, not the messages. The standard allows about 32 unit loads on one segment and runs up to roughly 1,200 m at around 100 kbit/s, or 10 Mbit/s over a few metres; length and speed trade against each other.

RS-485 is an electrical standard for serial communication over a differential pair (A and B lines), allowing many devices on one multidrop bus over long distances with good noise immunity. It defines the physical and electrical link, not the messages. The standard allows about 32 unit loads on one segment and runs up to roughly 1,200 m at around 100 kbit/s, or 10 Mbit/s over a few metres; length and speed trade against each other.

A typical topology is one master and a daisy chain of devices, each with an address, on a single twisted pair. What the bytes mean is decided by the protocol above: Modbus RTU is the usual one in plants, and PROFIBUS also rides on RS-485 signalling (protocol stack). Its noise immunity comes from differential signaling, and its stability from the wiring discipline the standard assumes: one line with short stubs, a 120 ohm terminator at each end, bias resistors that hold the idle line, a shield grounded at one point. Most RS-485 faults in the field are wiring, found with a multimeter before any protocol analyser. RS-485 is the cheapest digital field link there is, and the reason a digital device never had to be an Ethernet device: a 2003 flow meter on RS-485 is fully digital and has no IP address (fieldbus).