User's Manual

SATELLINE-3AS
User Guide, Version 2.2
19
2.3 RS-422 interface
RS-422 standard defines a serial data transfer method, which is very similar to the RS-232
standard. In RS-422 however, the signal lines are balanced (or differential) transmission lines. A
balanced (or differential) transmission line is formed by using two signal wires together to convey
each single signal. Because the state of the signal is defined by the mutual voltage difference
(hence the name differential), any common mode disturbances induced into the lines will cancel
out. The effect of different signals moving in the same cable will also be smaller than in the case
of the RS-232. Transmission distance can be considerably longer than when using RS-232 type
of connection, and distances of 1 km are possible. (See also Chapter 10.2.2 for more
information on RS-422 wiring).
As an example, let’s examine the TX-signal: TX-signal will be transmitted using two lines (A and
B). A logical ”1” corresponds to a situation, where the voltage on line A is greater than the
voltage on line B. Correspondingly a logical ”0” corresponds to a situation, where the voltage
on line A is smaller than the voltage on line B.
R
T
120
Ω
R
T
R
T
120
Ω
R
T
BB'
AA'
B' B
A' A
Radio modem
Cable
Terminal
Cable