A-24
A2
A2.3
RS-422 AND RS-485 STANDARDS
In 1978, the EIA adopted the RS-422 standard to overcome the noise
and distance problems associated with the single-ended RS-232
signals. The RS-422 standard specified a differential signal that used
two lines per signal.
The RS-422 differential signals have the advantage of higher speed
(up to 2Mbs) and longer distance capability (up to 1200M) over the
single-ended RS-232 signals. The RS-422 differential signals require
a differential receiver and are not referenced to Signal Ground.
Differential transmitted signals applied to the interconnecting cable
are +2 to +6V or -2 to -6V. Receivers are specified to have a
±
0.2V
sensitivity, 4Kohm minimum input impedance and be capable of
withstanding a maximum input of
+
10V. Cable terminators and
transmitter wave shaping may be required to minimize cross talk.
Logic levels are:
+2 to +6V
-2 to -6V
Data A/B
0
1
Control A/B
1 (On)
0 (Off)
The differential transmitter output terminal that is positive with
respect to the other terminal for the Control On Signal is designated
the A terminal. The negative terminal is designated the B terminal.
All voltage measurements are made by connecting a voltmeter between
the A and B terminals.
RS-485 signals are similar to RS-422 signals except their transmitters
are capable of driving up to 32 receivers and their protocol addresses
individual devices.
