A common electrical specification for the signals on a serial line. When no data is being transmitted, the serial line is at a logical zero (+3 to +15 Volts). A logical one is represented as a signal level of -15 to -3 Volts. In practise, one often finds signals which switch between nominally +4.5 and +0.5 Volts. Such signals are large by modern standards, and because the impedance of the circuits is relatively high, the allowable bit rate is modest.
The data is preceded by a start bit which is always a logical one. There may be seven or eight bits of data, possibly followed by an even or odd parity bit and one or two stop bits.
A "break" condition is a continuous logical one on the line which is what would be observed if nothing was connected.
RS-232 specifies the physical connector normally used for RS-423 lines.
(02 Feb 1995)