From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
(Formerly "RS-423") An EIAserial linestandard which specifies single ended communication. The mechanical connections for this interface are specified by EIA-449. Although it was originally intented as a successor of EIA-232 it is not widely used. The EIA-232 standard has its limits at 20kbps and 1.5m. EIA-423 can have a cable lenght of 1200m, and achieve a data rate of 100Kbps. 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.
