National Instruments AT-232/2 Getting Started page 46

Serial Port Information
RS-232
RS-422
© National Instruments Corporation
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, 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, 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
B-1
Appendix
B
Serial for Windows NT