The RS-232-C was originally set to standardize the interconnections of terminals and host computers through public telephone networks. Modems were used to translate the digital data signals from the computer equipment to analog audio signals suitable for transmission on the telephone network, and back to digital signals at the receiving end.

In the mid- to late 1960's, nearly all serial links for remote access to computers were through a telephone line. Remote access to the large mainframes of the time was accomplished almost exclusively by using the telephone network.

At that time, each manufacturer of equipment used a different configuration for interfacing a DTE (Data Terminal Equipment) with a DCE (Data Communications Equipment). Cables, connectors and voltage levels were different and incompatible, thus the interconnection of two pieces of equipment made by two different companies required the use of voltage level converters, and the manufacturing of special cables and connectors.

In 1969, EIA with Bell Laboratories and other parties established a recommended standard for interfacing terminals and data communications equipment. The object of this standard was to simplify the interconnection of equipment manufactured by different firms.

The standard defines electrical, mechanical, and functional characteristics. The electrical characteristics include parameters such as voltage levels and cable impedance. The mechanical section describes the pin number assignments and plug. The connector itself, however, is not specified. The functional description defines the functions of the different electrical signals to be used.

This standard shortly became RS-232-C (Recommended Standard number 232, revision C from the Electronic Industry Association), and a similar standard was available in Europe, developed by the CCITT (Comite Consultatif Internatinale de Telegraphie et Telephonie), and known as V.24 (functional description) and V.28 (electrical specifications). RS-232-C was widely adopted by manufacturers of terminals and computer equipment.

In the 1980's, the rapidly growing microcomputer industry found the RS-232-C standard cheap (compared to parallel connections) and suitable for connecting peripheral equipment to microcomputers. RS-232-C quickly became a standard for connecting microcomputers to printers, plotters, backup tape devices, terminals, programmed equipment and other microcomputers.

Since the standard only supported transfer rates up to 20 kbps (Kilobits per second), and distances of up to around 16 meters, new standards were adopted by EIA. The RS449 (mechanical) and RS423 (electrical) is upward-compatible with RS-232-C and can operate at data rates up to 10Mbps and distances of up to 1200 meters. Changing to a new standard, though, is a costly and long process. The RS-232-C is so widely available that it is certain to stay with us for some time to come.