EIA-423
<communications, standard> (Formerly "RS-423") An
EIA serial line standard 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.
Comparing EIA-422, 423, 449 to RS-232-C (http://www.rad.com/networks/1995/rs232/rs449.htm).
Details on RS-232, 422, 423 and 485 (http://www.rs485.com/rs485spec.html).
