What is RS-422?
RS-422 is a telecommunications standard for binary serial communications between devices. It is the protocol or specifications that must be followed to allow two devices that implement this standard to speak to each other. RS-422 is an updated version of the original serial protocol known as RS-232. One device will be known as the data terminal equipment (DTE) and the other device is known as data communications equipment (DCE). For instance, in a typical example of a serial link between the computer and printer, the computer is the DTE device and the printer is the DCE device.
The RS represents the recommended standard which allows manufacturers some room for interpretation of the standard though the differences are usually small. However, to prevent problems the standard has been formalized by the Electronic Industries Alliance and the International Telecommunications Industry Association and is now referred to as the EIA/TIA-422 standard.
RS-422 is a balanced four wire system. The signal sent from the DTE device is transmitted to the DCE device through two wires and the signal sent from the DEC device to the DTE device is transmitted through the other two wires. The signals on each pair of wires are the mirror opposite of each other, i.e., a "1" datum is transmitted as a plus 2 volt reference on one wire and a minus 2 volt reference on the other wire. To send a "0" datum, a minus 2 volt reference is transmitted through one wire and a plus 2 volt reference on the other wire. That is the opposite of what was done to transmit a \'1\' datum. This balanced differential approach allows for much longer distances between the DCE device and the DTE device than was possible with the earlier 3 wire RS-232 communication standard.
The RS-422 standard states which signaling voltages must be used, what connectors are to be used, what pins on those connectors are to be used for each function, and also recommends maximum distances over which this technology can be reliably used.
Standards are very important in industry as it allows the consumer to purchase a computer (DTE device) from one manufacturer and a printer (DCE device) from a different manufacturer and expect them to work together. Standards are also always changing as limitations are encountered and new solutions are proposed. If the changes become too drastic then a new standard evolves. In this way the RS-422 standard has evolved from, and in many cases replaced, the original RS-232 standard. In fact the RS-422 standard has now been superseded by the RS-485 standard which adds additional features to the RS-422 standard.
For more information on this standard visit either the Electronic Industries Alliance or the Telecommunications Industry Association.
A thumb-nail sketch of the physical characteristics, but lacking the functional description of messages.
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