User's Manual

SATELLINE-M3-TR1
User Guide, Version 3.0
17
RS-422 to RS-485 using cable conversion.
3.4.2 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 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.
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.
To enable RS-422 function the modem must be ordered with RS-232/RS-422 Interface. When
RS-422 is used RS-422 of the Port 2 must be ON.
Radio modem set to
422-mode
T
R
B
'
A'
B
A
R
T
120
Ω
Radio modem
Cable
Terminal
Cable
R
T
120
Ω
R
T
B
A
B`
A`
5
4
2
3
Pin
R
T
120
Ω
R
T
R
T 120
Ω
R
T
B B'
A A'
B' B
A' A
Radio modem
Cable
Terminal
Cable