RS-485 is a telecommunications standard for binary serial communications between devices. It is the protocol or specifications that need to be followed to allow devices that implement this standard to speak to each other. This protocol is an updated version of the original serial protocol known as RS-232. While the original RS-232 standard allowed for the connection of two devices through a serial link, RS-485 allows for serial connections between more than 2 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 (2-wire) bus. New technology has since introduced "automatic" repeaters and high-impedance drivers and receivers such that 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 2 wire system or a 4 wire system. If a RS-485 network is constructed as a 2 wire system, then all of the nodes will have equal ranking. A RS-485 network constructed as a 4 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 never 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 what signaling voltages must be used, what connectors are to be used, what pins on those connectors are to be used for each function, and also recommends maximum distances over which this technology can be reliably used.

Standards are very important in industry as it allows the consumer to purchase a different devices from different manufacturers and expect them to work together. Standards are also always changing as limitations are encountered and new solutions are proposed. If the changes become too drastic then a new standard evolves. It is because of this that the RS-485 standard evolved from the original RS-232 standard.

More information on the RS-485 standard can be found through the Electronic Industries Alliance or the Telecommunications Industry Association.