User guide

SATELLINE-3AS NMS / NMS Epic / NMS 869 / VHF
User Guide, Version 1.0
31
When connecting equipment using RS-232 interface make sure that the equipment are
connected together sharing the same ground potential. Major differences in ground potentials
may result to large current flow in the GND wire of the RS-232 interface and may lead to a
malfunction or damage the connected devices!
5.2
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 interferences 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 up to 1 km are possible. (See also Chapter 14.2 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.
Picture 5.1. RS-422 interface
R
T
120

R
T
RT 120

R
T
BB'
AA'
B' B
A' A
Radio modem
Cable
Terminal
Cable