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RS485 - What is RS485, EIA-485

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.


RS-485_waveform


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).


multidropped network


For more information on RS485

© Chipkin Automation Systems 2017


Metasys® as used in this document is a trademark of Johnson Controls, Inc.

  1. Randy Johnson says:

    Thank you very much for the information you put on this site, It has been a tremendous help.
    Thank you again and have a great day.

    Randy Johnson

    Reply
  2. Cosmo Di Schino says:

    I will know the price of the (TWO) wire RS485. Cosmo

    Reply
  3. […] Open submission – Write for us What is RS485, EIA-485 […]

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  4. eddie gao says:

    we have a unique technology which can use ready RS485, CAN bus to build network with both of the signal/power in two wires (total bus can provide 960 watt power and 12 watt in every terminal). to 256 terminals.

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