RS-232/EIA-432/V.24 Unbalanced Transmission

In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended (unbalanced) data and control signals connecting between a data terminal equipment (e.g. computer, terminal, or device) and a data circuit-terminating equipment (e.g. modem). It is commonly used for computer serial ports. It defines the electrical characteristics and timing of the signals, the meaning of the signals, and the physical size and pinout of connectors.

RS-232 was defined by the US Electronics Industries Association (EIA) in 1962 and is one of a number of standards that are mostly compatible, including the international V.24 standard and the newer RS-432. Many modern personal computers have no RS-232 ports and must use either an external USB-to-RS-232 converter or an internal expansion card with one or more serial ports to connect to RS-232 peripherals. RS-232 devices are still found, especially in industrial machines, the configuration interface of networking equipment, and scientific instruments.

The RS-232 standard defines two voltage levels that correspond to a logical one and a logical zero. Valid signals are either in the range of +3 to +15 volts or the range −3 to −15 volts with respect to the ground/common pin. Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero. This is known as "single-ended" and requires sender and receiver to have a common signal ground. This does not provide significant noise immunity and requires a higher signalling voltage, limiting the rate and fixing the maximum transmission distance.

The specification defined transmission up to 20,000 baud. The baud is the unit of transmission speed, measured in symbols/second. The baud rate differs from the bit rate, since it in includes overhead, such as the start, stop, and parity bits used in asynchronous communications. Later equipment supported rates unto 100,000 baud. Most cables were less than 15m, but low-capacitance cables, could allow full speed communication over distances up to about 300m.

Multi-drop connection among more than two devices is not supported, so there is only one receiver.

The 25-way connector recommended in the standard is large compared to current practice. A minimal "3-wire" RS-232 connection is more often used, this consists only of the transmit data, receive data, and ground. A two-wire connection (data and ground) can be used if the data flow is one way (for example, a digital postal scale that periodically sends a weight reading, or a GPS receiver that periodically sends position information).

Note: The term "baud" is named after Jean-Maurice-Émile Baudot, who was a French telegraph engineer, born September 11, 1845.

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Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2014)