Hardware PropertiesInterfacing devices
Types of UARTs
Loop Back Plug
In 1969 the Standards Committee, today known as Electronic Industries Association, worked out a general standard for data transmission devices. Those days data transmission meant digital data exchange between the main computer and a remote terminal or between two terminals without using a computer. The connection was established in such a way that terminals were connected via simple telephone lines and hence installed modems were needed on each end of the line to carry out data transmission. Without using modems digital data transmission via an analog channel had its shortcomings, mainly possible transmission errors. To get rid of these shortcomings the project had to be rather complicated.
Thus there emerged a task of producing a new standard which could guarantee reliable connection and simplicity of decisions, at the same time this standard could be used by various communication equipment manufacturers without problems of compatibility with devices by other manufacturers.Thus usage of such a solution could turn out profitable at repetition work.
The RS-232 standard was created according to these ideas. The demands to this standard defined signal levels, signal timing, signal function, mechanical plugs and information exchange protocols.
For more than 30 years from creation of EIA interface three standard modifications have been worked out:
Most equipment using RS-232 serial port has DB-25 plug, though standard documents didn't define a specific plug, nowadays most computers use the DB-9 plug for asynchronous data exchange.
Further development of the standard was oriented to elimination of defects of initial specifications of RS–232-C interface. Also some new standards have been worked out, amongst them we should distinguish RS–422 (balance system allowing line impedance up to 50 ohm), RS–423 (imbalance system with min line impedance of 450 ohm) and RS–449 (a standard with high baud rates having changed cirquit functions and using 37 pin D-type plug).
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