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RS-485 is a differential requiring no ground wire between signal source and sink. The interface uses two conductors for transmitting and receiving data, A and B. Because RS-485 drivers can be tristated, the interface can be used for multipoint data links. The differential bus voltage, Vod = Va - Vb, must be ≥ ±1.5 V. Positive Vod represents a logic high, negative Vod a logic low. While the bus voltage can ride on large common-mode voltages ranging from Vcm = -7 V to +12 V, the common-mode component is rejected by the differential RS-485 receiver. RS-485 networks can be designed for full- or half-duplex operation. Full-duplex requires a conductor pair for transmit and another one for receive data. Half-duplex is bidirectional, using only two conductors, but limiting data traffic to one direction at a time: transmit or receive. Maximum data rates range from 100 kbps @ 1200 m (4000 ft.) up to 100 Mbps @ 10 m (30 ft.) cable length. Because of its high reliability in electrical noisy environments, RS-485 has become the interface work horse of the industry.

PTM Published on: 2017-10-12