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National Instruments Corporation
C-1
PCI Serial for Windows 95
C
Serial Port Information
This appendix discusses the RS-232, RS-422, and RS-485 standards and
explains some of the different issues involved with these types of serial
communication.
RS-232
RS-232, as specified in the ANSI/EIA-232-D Standard, Interface Between
Data Terminal Equipment and Data Circuit-Terminating Equipment
Employing Serial Binary Data Interchange, standardizes serial
communication between computers and between computer terminals and
modems. Most applications use the RS-232 standard for interfacing
peripherals to personal computers. RS-232 uses transmission lines in which
the state of each signal is represented by referencing the voltage level of a
single line to ground. RS-232 was designed for serial communication up to
distances of 50 ft and with data rates up to 20 kb/s. However, because of
improvements in line drivers and cabling, you can often increase the actual
performance of the bus past the limitations on speed and distance
recommended in the specification.
RS-422
RS-422, as specified in the EIA RS-422-A Standard, Electrical
Characteristics of Balanced Voltage Digital Interface Circuits, 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 much more immune to noise as well as voltage loss due to transmission
line effects. Thus, you can use RS-422 for much longer distances
