Difference between RS232 RS422 and RS485

RS-232 was first introduced in 1960  by the Electronic Industries Association (EIA) as a Recommended Standard. The original DTEs were electromechanical teletypewriters, and the original DCEs were (usually) modems. When electronic terminals  (smart and dumb) began to be used, they were often designed to be interchangeable with teletypewriters, and so supported RS-232.

RS-422 is the common short form title of American National Standards Institute  (ANSI) standard ANSI/TIA/EIA-422-B Electrical Characteristics of Balanced Voltage Differential Interface Circuits and its international equivalent ITU-T   Recommendation T-REC-V.11, also known as X.27.

RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. It is generally accepted that RS-485  can be used with data rates up to 10 Mbit/s or, at lower speeds, distances up to 1,200 m (4,000 ft). As a rule of thumb, the speed in bit/s multiplied by the length in meters should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s.

Other serial signaling standards may not interoperate with standard-compliant RS-232  ports. For example, using the TTL levels of near +5 and 0 V puts the mark level in the undefined area of the standard. Such levels are sometimes used with  NMEA 0183 -compliant GPS receivers and depth finders.

A 20 mA current loop uses the absence of 20 mA current for high, and the presence of current in the loop for low; this signaling method is often used for long-distance and optically isolated links. Connection of a current-loop device to a compliant RS-232  port requires a level translator. Current-loop devices can supply voltages in excess of the withstand voltage limits of a compliant device. The original IBM  PC  serial port card implemented a 20 mA current-loop interface, which was never emulated by other suppliers of plug-compatible equipment.

Other serial interfaces similar to RS-232  :

RS-422 – a high-speed system similar to RS-232  but with differential signaling 

RS-485 – a descendant of RS-422  that can be used as a bus in multidrop configurations

RS232, RS422, and RS485  are all essentially physical layer protocols. They are all serial communication protocols and are ubiquitous device interfaces. Serial interfaces are widely used in electronics and embedded systems, often for remote acquisition of device data or remote control. They change the way the signal is transmitted, whether it is a PC, microcontroller, or serial communication using serial communication protocols or TTL level transmission. They all need to be converted to RS232, RS422.  or RS485  signals by a communication converter chip.