RS-232


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 such as a computer terminal or PC, and a DCE, 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 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, the framing of characters, 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 as a Recommended Standard. The original DTEs were electromechanical teletypewriters, and the original DCEs were modems. When electronic terminals 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 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 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, and setting the circuit capacitance limit to 2.5 nF.

Telecommunications Industry Association and Revision E

In 1988, the Telecommunications Industry Association 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 , 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, 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 in 2012 with no official changes or revisions. No official changes have been made to the standard since.

Limitations of the standard

Because RS-232 is used beyond the original purpose of interconnecting a terminal with a modem, successor standards have been developed to address the limitations. Issues with the RS-232 standard include:
  • The large voltage swings and requirement for positive and negative supplies increases power consumption of the interface and complicates power supply design. The voltage swing requirement also limits the upper speed of a compatible interface.
  • Single-ended signaling referred to a common signal ground limits the noise immunity and transmission distance.
  • Multi-drop connection among more than two devices is not defined. While multi-drop "work-arounds" have been devised, they have limitations in speed and compatibility.
  • The standard does not address the possibility of connecting a DTE directly to a DTE, or a DCE to a DCE. Null modem cables can be used to achieve these connections, but these are not defined by the standard, and some such cables use different connections than others.
  • The definitions of the two ends of the link are asymmetric. This makes the assignment of the role of a newly developed device problematic; the designer must decide on either a DTE-like or DCE-like interface and which connector pin assignments to use.
  • The handshaking and control lines of the interface are intended for the setup and teardown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices.
  • No method is specified for sending power to a device. While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low-power devices such as mice.
Because the standard did not foresee the requirements of devices such as computers, printers, test instruments, POS terminals, and so on, designers implementing an RS-232 compatible interface on their equipment often interpreted the standard idiosyncratically. The resulting common problems were non-standard pin assignment of circuits on connectors, and incorrect or missing control signals. The lack of adherence to the standards produced a thriving industry of breakout boxes, patch boxes, test equipment, books, and other aids for the connection of disparate equipment. A common deviation from the standard was to drive the signals at a reduced voltage. Some manufacturers therefore built transmitters that supplied +5 V and −5 V and labeled them as "RS-232 compatible".

Role in modern personal computers

In the book PC 97 Hardware Design Guide, Microsoft deprecated support for the RS-232 compatible serial port of the original IBM PC design. Today, RS-232 has mostly been replaced in personal computers by USB for local communications. Advantages compared to RS-232 are that USB is faster, uses lower voltages, and has connectors that are simpler to connect and use. Disadvantages of USB compared to RS-232 are that USB is more susceptible to electromagnetic interference and that maximum cable length defined by standards is much shorter. RS-232 cable lengths of 2000 meters are possible with appropriate line drivers.
In fields such as laboratory automation or surveying, RS-232 devices continue to be used. Some types of programmable logic controllers, variable-frequency drives, servo drives, and computerized numerical control equipment are programmable via RS-232. Computer manufacturers have responded to this demand by re-introducing the DE-9M connector on their computers or by making adapters available.
RS-232 ports are also commonly used to communicate to headless systems such as servers, where no monitor or keyboard is installed, during boot when an operating system is not yet running and therefore no network connection is possible. A computer with an RS-232 serial port can communicate with the serial port of an embedded system as an alternative to monitoring over Ethernet.
Personal computers also made use of the standard so that they could connect to existing equipment. For many years, an RS-232-compatible port was a standard feature for serial communications, such as modem connections, on many computers. It remained in widespread use into the late 1990s. In personal computer peripherals, it has largely been supplanted by other interface standards, such as USB. RS-232 is still used to connect older designs of peripherals, industrial equipment, console ports, and special purpose equipment.