EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. The standard specifies a differential form of signalling. The difference between the wires? voltages is what conveys the data. One polarity of voltage indicates a logic 1 level, the reverse polarity indicates logic 0. The difference of potential must be at least 0.2 volts for valid operation, but any applied voltages between +12 V and -7 volts will allow correct operation of the receiver. EIA-485 is better described as un-balanced interface, as the balanced normallly implies that the voltages on the differential wires are balanced with respect to ground or earth potential (eg +5V and -5V), but EIA-485 is usually +5V and 0V .
EIA-485 only specifies electrical characteristics of the driver and the receiver. It does not specify or recommend any data protocol. Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). The recommended arrangement of the wires is as a connected series of point-to-point nodes, a line or bus. Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs).
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