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

In telecommunications, there are a series of standards for sending serial binary data and control signals between a data terminal equipment (e.g. computer, terminal, or device) and a data circuit-terminating equipment (e.g., modem) using a cable with unbalanced conductors and a ground. It defines the electrical characteristics and timing of the signals, the meaning of the signals, and the physical size and pinout of connectors. The specification is ground-referenced. That is, all voltages at the transmitter and receiver are referenced to a common ground (zero volts).

EIA-232 was defined by the US Electronics Industries Association (EIA) in 1962 under the former name of RS-232 and is one of a number of standards that are mostly compatible, including the international V.24 standard and the newer EIA-432 (using a 5V signal). One common,most modern personal computers have no EIA-232 ports and must use either an external converter or an internal expansion card with one or more serial ports to connect to EIA-232 peripherals. EI-232 devices are still found, especially in industrial machines, servers, the configuration interface of networking equipment, and scientific instruments.

Multi-drop connection among more than two devices is not supported, so there can only be only one receiver connected to a cable.


The EIA-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 EIA-232 line depend on the ground pin being at 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 a "single-ended" transmission and does not provide high levels of noise immunity. It typically therefore requires a higher signalling voltage, limiting the rate and fixing the maximum transmission distance.

The EIA-232 specification defined transmission up to approx. 20,000 baud. (A 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.


Each end of a serial cable has a connector, allowing the cable to an interface port.

Note 1: The term "baud" is named after Jean-Maurice-Émile Baudot, who was a French telegraph engineer, born September 11, 1845.

Note 2: RS-232 became famous amongst the growing number of people with personal computers. The hit UK TV satirical puppet show "Spitting Image" (of 1980's and 1990's) produced a notable version of Tosca providng an operatic parody describing the RS-232 Interface Lead. This explained how it couldn't solve all local networking difficulties. Alas,this is video material is now sadly hard to find.

See also:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland (2023).