RS-485 is a US-based telecommunications standard for binary serial communications between devices. It is the protocol, or set of specifications, that needs to be followed to allow devices that implement the standard to speak to each other. The protocol is an updated version of the original serial protocol known as RS-232. While the RS-232 standard allowed for the connection of two devices through a serial link, RS-485 allows for serial connections among more than two devices on a networked system.
A RS-485 compliant network is a multi-point communications network. The RS-485 standard specifies up to 32 drivers and 32 receivers on a single, two-wire bus. New technology has since introduced "automatic" repeaters and high-impedance drivers and receivers so the number of drivers and receivers can be extended to hundreds of nodes on a network. RS-485 drivers are now even able to withstand bus contention problems and bus fault conditions.
A RS-485 network can be constructed as either a balanced two-wire system or a four-wire system. If a RS-485 network is constructed as a two-wire system, all of the nodes have equal ranking. A RS-485 network constructed as a four-wire system has one node designated as the master, and the remaining nodes are designated as slaves. Communication in such a system is only between master and slaves and not between slaves. This approach simplifies the software protocol that needs to be used, at the cost of increasing the complexity of the wiring system slightly.
The RS-485 standard states which signaling voltages must be used, which connectors must be used and which pins on those connectors must be used for each function. It also recommends maximum distances over which this technology can be reliably used.
Industry standards are important because they allow consumers to purchase different devices from different manufacturers and expect them to work together. Standards also change as limitations are encountered and new solutions are proposed. If the changes become too drastic, a new standard evolves, which is how the RS-485 standard emerged from the original RS-232 standard.
More information on the RS-485 standard can be found through the Electronic Industries Alliance — a US trade association that developed this standard — or the Telecommunications Industry Association.
