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RS-422 interface basics

2017-12-23 07:17  
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RS-422 is a standard for serial data transfer similar to RS-232, but it uses difference between two lines (RS-232 uses reference to ground). RS-422 uses twisted pair (difference pair) to represent logic level. Such data transmission type is so called balanced transmission as it is not referenced to ground. This is noise proof solution because same noise affect both lines which is differentiated out. This allows to carry data at much longer distances at higher data transfer rates. RS-422 can transmit data to up to 1200metters. Maximum transfer rate can be 10Mbits/s.


120Ohm resistor act as terminal resistor which removes reflections, that may occur during transmission over long distances. Voltage levels between twisted pair lines are4V and between transmission lines is?±12V. RS-422 can be compatible to RS-232 interface by simply connecting negative wire of twisted pair to ground.


There are many RS-422 interface chips available in the market. One of them is MAX3488. They can be connected to MCU UART interface like they are RS232.


The single difference between RS232 and RS422 is voltages but transfer protocol is the same and you can work with it like it was RS232, jus more noise immune and longer transmission distances.

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