EIA-485

EIA-485 (formerly RS-485 or RS485) is an electrical specification of a two-wire, half-duplex, multipoint serial connection. It enables the configuration of inexpensive local networks and multidrop communications links. It offers high data transmission speeds (35Mbit/s up to 10m and 100kbit/s at 1200m), and since it uses a differential balanced line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). EIA-485 only specifies electrical characteristics of the driver and the receiver. It does not specify or recommend any data protocol.

In contrast to EIA-422, which has a single driver circuit which cannot be switched off, EIA-485 drives need to be put in transmit mode explicitly by asserting a signal to the driver. This allows EIA-485 to implement star network topologies using only two lines.

EIA-485, like EIA-422 can be made full-duplex by using four wires, however, since EIA-485 is a multi-point specification, this is not necessary in many cases.

It is worthwhile to note that SCSI-2 and SCSI-3 (for instance) use this specification to implement the physical layer.

External link

• What is RS485?