ICS ELECTRONICS 4809A, Инструкция по эксплуатации

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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 termina-
tors 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.