RS-232 is an EIA standard for serial digital communication ("Interface Between Data Terminal Equipment and Data Communication Equipment Employing Serial Binary Data Interchange"), originally issued in 1960.
The 1969 version RS-232C introduced a standard 25-pin D-sub connector (albeit only in an appendix, formally added to the next version RS-232D in 1986).
Related ITU-T standards include V.24 (circuit identification) and V.28 (signal voltage and timing characteristics).
The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels for the data transmission and the control signal lines. Valid signals are either in the range of +3 to +15 volts or the range −3 to −15 volts with respect to the "Common Ground" (GND) pin; consequently, the range between −3 to +3 volts is not a valid RS-232 level. For data transmission lines (TxD, RxD, and their secondary channel equivalents), logic one is defined as a negative voltage, the signal condition is called "mark". Logic zero is positive and the signal condition is termed "space". Control signals have the opposite polarity: the asserted or active state is positive voltage and the deasserted or inactive state is negative voltage. Examples of control lines include request to send (RTS), clear to send (CTS), data terminal ready (DTR), and data set ready (DSR).
|Data circuits||Control circuits||Voltage|
|0 (space)||Asserted||+3 to +15 V|
|1 (mark)||Deasserted||−15 to −3 V|
(Incorporates text from https://en.wikipedia.org/wiki/RS-232)