RS-422 interface basics

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

120Ohm resistor acts as a terminal resistor that removes reflections, that may occur during transmission over long distances. Voltage levels between twisted pair lines are 4V and between transmission lines is 12V. RS-422 can be compatible with the RS-232 interface by simply connecting the negative wire of the twisted pairs to the ground.

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

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

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