In telecommunications, RS-232 or Recommended Standard 232 is a standard introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a DTE (data terminal equipment) such as a computer terminal or PC, and a DCE (data circuit-terminating equipment or data communication equipment), such as a modem. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors. The current version of the standard is TIA-232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997. The RS-232 standard is commonly used with serial ports and serial cables. It is widely used in industrial communication devices. A serial port complying with the RS-232 standard was once a standard feature of many types of computers. Personal computers used them for connections not only to modems, but also to printers, computer mice, data storage, uninterruptible power supplies, and other peripheral devices. Compared with later interfaces such as RS-422, RS-485 and Ethernet, RS-232 has lower transmission speed, shorter maximum cable length, larger voltage swing, larger standard connectors, no multipoint capability and limited multidrop capability. In modern personal computers, USB has displaced RS-232 from most of its peripheral interface roles. Thanks to their simplicity and past ubiquity, however, RS-232 interfaces are still used—particularly in industrial CNC machines, networking equipment and scientific instruments where a short-range, point-to-point, low-speed wired data connection is fully adequate.
Scope of the standard The Electronic Industries Association (EIA) standard RS-232-C as of 1969 defines:
Electrical signal characteristics such as logic levels, baud rate, timing, and slew rate of signals, voltage withstand level, short-circuit behavior, and maximum load capacitance. Interface mechanical characteristics, pluggable connectors and pin identification. Functions of each circuit in the interface connector. Standard subsets of interface circuits for selected telecom applications. The standard does not define such elements as the character encoding (i.e. ASCII, EBCDIC, or others), the framing of characters (start or stop bits, etc.), transmission order of bits, or error detection protocols. The character format and transmission bit rate are set by the serial port hardware, typically a UART, which may also contain circuits to convert the internal logic levels to RS-232 compatible signal levels. The standard does not define bit rates for transmission, except that it says it is intended for bit rates lower than 20,000 bits per second.
History 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.
Revisions A through C Throughout the 1960s, the 232 Standard underwent a few iterations following the major innovations in computer and networking technology. In October 1963, the EIA published its first revision, EIA RS-232-A. This revision introduced iterative improvements including different connector types and different voltage ranges. In October 1965, the EIA published EIA RS-232-B. This revision increased the capacitance specifications to allow longer cable lengths and increase signal timing. It also dropped the voltage from 25 Vpp to 15 Vpp. In August of 1969, the EIA published EIA RS-232-C: Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange. This revision dropped the voltage down to 12 Vpp and introduced the use of Data Communication Equipment (DCE) modems with the standard.
RS-232 Modernization and Revision D As technology progressed, the RS-232 standard was starting to become obsolete. In 1975, modifications to the RS-232 standard resulted in the creation of its supposed successor, the EIA RS-422 standard. However, as the RS-422 standard was being developed, the RS-232 standard was gaining popularity for use in computing, so the standard was updated to accommodate legacy systems and its continued usage. In 1981, the EIA dropped the Recommended Standard (RS) nomenclature for all of their published standards and republished the 232 standard as EIA-232-C. In 1986, the EIA published ANSI/EIA-232-D. The revision included major changes including incorporating the DB-25 connector as part of the standard (it was only referenced in the appendix of RS-232-C), and setting the circuit capacitance limit to 2.5 nF.
Telecommunications Industry Association and Revision E In 1988, the Telecommunications Industry Association (TIA) was founded, as part of a merger of several organizations under the EIA. In 1991, the TIA and the EIA, together, released ANSI/EIA/TIA-232-E-1991: Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange, adding the smaller standard D-shell 26-pin "Alt A" connector, and made other changes to improve compatibility with ITU-T/CCITT V.24 (circuit identification), ITU-T/CCITT V.28 (signal voltage and timing characteristics) and ISO 2110.
EIA Name Change and Revision F In 1997, the Electronic Industries Association reorganized under the Electronics Industries Alliance, with the Telecommunications Industry Association serving as its subsidiary. That October, the TIA published TIA ANSI/TIA/EIA-232-F: Interface between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange. In 2002, the EIA delegated the standard entirely to the TIA, and Revision F was republished under TIA ANSI/TIA-232-F (R 2002), by the TR-30 Multi-Media Access, Protocols and Interfaces committee.
Post EIA Dissolution Following the dissolution of the Electronic Industries Alliance, the TIA reaffirmed the standard as TIA TIA-232-F (R 2012) in 2012 with no official changes or revisions. No official changes have been made to the standard since.
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