National Instruments ENET-232 Series User Manual page 78

Appendix D Serial Port Information
RS-422
RS-485
ENET-232 and ENET-485 Series User Manual
distances of 50 ft. and with data rates up to 20 kbits/s. However, because of
improvements in line drivers and cabling, you can usually increase the
actual performance of the bus past the limitations on speed and distance
recommended in the specification.
As specified in the EIA/RS-422-A Standard, Electrical Characteristics
of Balanced Voltage Digital Interface Circuits, RS-422 defines a
serial interface much like RS-232. However, RS-422 uses balanced
(or differential) transmission lines. Balanced transmission lines use two
transmission lines for each signal. The state of each signal is represented,
not by a voltage level on one line referenced to ground as in RS-232, but
rather by the relative voltage of the two lines to each other. For example,
the TX signal is carried on two wires, wire A and wire B. A logical 1 is
represented by the voltage on line A being greater than the voltage on
line B. A logical 0 is represented by the voltage on line A being less than
the voltage on line B. Differential voltage transmission creates a signal that
is more immune to noise as well as voltage loss due to transmission line
effects. Thus, you can use RS-422 for longer distances (up to 4,000 ft.) and
greater transmission speeds (up to 10 Mbits/s) than RS-232.
As specified in the EIA-485 Standard, Standard for Electrical
Characteristics of Generators and Receivers for Use in Balanced Digital
Multipoint Systems, RS-485 expands on the RS-422 standard by increasing
the number of devices you can use from 10 to 32 and by working with
half-duplex bus architectures. Unlike the RS-422 standard, RS-485
addresses the issue of using multiple transmitters on the same line. RS-485
defines the electrical characteristics necessary to ensure adequate signal
voltages under maximum load and short-circuit protection. RS-485 can
also withstand multiple drivers driving conflicting signals at the same time.
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