EIA-485 (formerly RS-485 or RS485) is a specification for the physical layer of a network that uses the difference in voltages between two wires (Three wire) to conveys data.
One polarity of voltage indicates a logic 1, 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 does not specify or recommend any data protocol is only a specification for the transmission of the ones and zeros. It is up to the receiver to convert the ones and zeros to a specific data protocol such as BACnet MSTP, Metasys N2 by JCI, ect
Since EIA-485 uses a voltage differential it can be used across large distances. (4000 feet or 1200 meters) and it resists electromagnetic interference from motors and welding equipment. When using a network across large distances the transmitter will use the max potential differences (+12 V and -7 volts). The resistance of the twisted pair cable will drop the voltage difference as it travels down the wire. When the receiver reads the voltage on the other end there could be as little as 0.2 volts difference between the two wires. 0.2 is the minimum recommended voltage difference that receivers use to detect a change in polarity.
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. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors are required. The value of each termination resistor should be equal to the cable impedance (typically, 120 ohms for twisted pairs).
For more information on RS485
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Metasys® as used in this document is a trademark of Johnson Controls, Inc.