SATELLINE-EASy / SATELLINE-EASy 869
User Guide, Version 3.0
19
4.1
RS-232 interface (Port 1, Port 2)
RS-232 standard defines the method of transferring binary single-ended data serially between
DTE and DCE. Although the standard defines the electrical characteristics, timing and meaning
of the signals, as well as the pin out of connectors, it is applied in a multitude of slightly differing
ways (e.g. different pin configurations). For this reason different computers and peripherals are
not necessarily directly compatible with each other.
RS-232 standard defines transmission lines, in which each single signal line level is referenced to
a common ground level. 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 ground (GND) wire and may lead to a
malfunction or damage the connected devices!
RS-232 has been designed for serial data transfer over short distances (usually less than 15 m).
For longer distances, RS-422 or RS-485 is better suited in order to maintain the integrity of data.
4.2
RS-422 interface (Port 2)
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.
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
