User guide

SATELLINE-3AS NMS / NMS Epic / NMS 869 / VHF
User Guide, Version 1.0
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5.3
RS-485 interface
RS-485
is an extension of the RS-422 standard and enables the connection of more than two
devices on to the same bus. Communication is half-duplex, so there is only one cable pair,
compared to two when using the RS-422. The RS-485 standard defines the electrical
characteristics of the connections in such a way as to prevent possible data contention states as
well as cable short circuits etc. from harming the devices themselves. (See also Chapter 14.2 for
more information on RS-485 wiring).
Picture 5.2. RS-485 interface
5.4
Termination of RS-422/485 lines
Each differential pair of wires is a transmission line. A transmission line must be terminated
properly to prevent, or at least minimise, harmful reflections formed between the transmitting
and receiving end of the transmission line. A common method of terminating an RS-485 type of
transmission line is to connect a so-called terminating resistor between the wires at both ends of
the transmission line. Even when there are more than two devices on the same transmission line,
the terminating resistors are needed only at transmission line ends. The terminating resistor
must be selected so that its resistance matches to the characteristic impedance of the
transmission line as close as possible. Typical value range is from 100 to 120. When using
an RS-422 type of connection the terminating resistor is connected only at both receiving end.
Terminating resistors are particularly important, when long transmission lines and/or high data
transfer speeds are used.
R T 120

Radio modem
Cable
Terminal
Cable
R T 120

R
T
B
A
B'
A'
T
R
B'
A'
B
A