A-20
A2
In Europe, the Comite Constultatif International Telephonique it
Telegraphiqe (CCITT) has established standards that correspond to RS-
232C. While these standards, CCITT V.24 and CCITT V.28, are very
similar to RS-232C, they are not identical. The Model 4984 conforms to
both RS-232 and CCITT V.24 standards, but does not contain or use all of
the circuits allowed for in both standards.
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.
