EIA RS 232 Standard

- an overview, introduction or tutorial about the basics of the EIA RS 232 standard detailing its variants including RS232C, RS232D, V24, V28 and V10.

The RS 232 standard has been used as the basis for a form of serial data communications for many years. The RS 232 standard has been used in many areas, well beyond its original intended applications. As a result this has lead to uncertainty in the way some applications use the RS232 standard. However the RS 232 standard operates very reliably when correctly set up and for many years it has provided one of the main forms of serial data transmission.

Even though many other standards are available for data transmission these days, the RS 232 standard is still widely sued, and is likely to remain so for many years to come.


Development of the RS 232 standard

The RS 232 standard for data communications was devised in 1962 when the need to be able to transmit data along a variety of types of line started to grow. The idea for a standard had grown out of the realisation in the USA that a common approach was required to allow interoperability. As a result the Electrical Industries Association in the USA created a standard for serial data transfer or communication known as RS232. It defined the electrical characteristics for transmission of data between a Data Terminal Equipment (DTE) and the Data Communications Equipment (DCE). Normally the data communications equipment is the modem (modulator/demodulator) which that encodes the data into a form that can be transferred along the telephone line. A Data Terminal Equipment could be a computer.

The RS 232 standard underwent several revisions, the C issue known as RS232C was issued in 1969 to accommodate the electrical characteristics of the terminals and devices that were being used at the time.

The RS 232 standard underwent further revisions and in 1986 Revision D was released (often referred to as RS232D). This revision of the RS 232 standard was required to incorporate various timing elements and to ensure that the RS 232 standard harmonised with the CCITT standard V.24, while still ensuring interoperability with older versions of RS 232 standard.

Further updates and revisions have occurred since then and the current version is TIA-232-F issued in 1997 under the title: "Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange."

The name of the RS 232 standard has changed during its history, several times as a result of the sponsoring organisation. As a result it has variously been known as EIA RS-232, EIA 232, and most recently as TIA 232.


Variations of the RS 232 standard

There are number of different specifications and standards that relate to RS 232. A description of some of the RS 232 standards and the various names and references used is given below:

  • EIA/TIA-232:   This reference to the RS 232 standard includes the names of the first and current sponsoring organisations, the Electronic Industries Alliance (EIA) the Telecommunications Industry Alliance (TIA).
  • RS-232C:   This was the designation given to the release of RS 232 standard updated in 1969 to incorporate many of the device characteristics.
  • RS-232D:   This was the release of the RS 232 standard that occurred in 1986. It was revised to incorporate various timing elements and to ensure that the RS 232 standard harmonised with the CCITT standard V.24.
  • RS-232F:   This version of the RS 232 standard was released in 1997 to accommodate further revisions to the standard. It is also known as TIA-232-F.
  • V24:   The International Telecommunications Union (ITU) / CCITT (International Telegraph and Telephone Consultative Committee) of the ITU developed a standard known as ITU v.24, often just written as V24. This standard is compatible with RS232, and its aim was to enable manufacturers to conform to global standards and thereby allow products that would work in all countries around the world. It is entitled "List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE)."
  • V28:   V.28 is an ITU standard defining the electrical characteristics for unbalanced double current interchange circuits, i.e. a list of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE).
  • V10:   V.10 is an ITU standard or recommendation for unbalanced data communications circuits for data rates up to 100 kbps that was first released in 1976. It can inter-work with V.28 provided that the signals do not exceed 12 volts. Using a 37 pin ISO 4902 connector it is actually compatible with RS423.

RS-232 Applications

The RS-232 standard has come a long way since its initial release in 1962. Since then the standard has seen a number of revisions, but more importantly, RS232 has been used in an ever increasing number of applications. Originally it was devised as a method of connecting telephone modems to teleprinters or teletypes. This enabled messages to be sent along telephone lines - the use of computers was still some way off.

As computers started to be used, links to printers were required. The RS-232 standard provided an ideal method of connection and therefore it started to be used in a rather different way. However its use really started to take off when personal computers were first introduced. Here the RS-232 standard provided an ideal method of linking the PC to the printer.

The RS-232 standard provided an ideal method of linking many other remote items to computers and data recorders. As a result, RS-232 became an industry standard, used in a host of applications that were never conceived when it was first launched in 1962.

The RS 232 standard is very widely used and is probably the most widely used standard for serial data communications over distances. The RS 232 standard is often referred to by the other related standards and in particular V.24 which is the ITU / CCITT designation for the serial data communications standard. Whatever it is called, the RS 232 standard has stood the test of time, and being introduced in 1962 it has been in use for well over 45 years.

By Ian Poole


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