User's Manual

SATELLINE-3AS
User Guide, Version 2.2
20
2.4 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 shorts etc. from harming the devices themselves. (See also Chapter 10.2.3 for
more information on RS-485 wiring).
2.5 Termination of RS422/485 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 termination resistor, between the wires and at both
ends of the transmission line. Even when there are more than two devices on the same
transmission line, the termination resistors are needed only at the ends of the transmission line.
The termination resistor must be selected so that its resistance matches the characteristic
impedance of the transmission line as close as possible (typical values range from 100 to 120
Ω). When using an RS-422 type of connection the termination resistor is connected only at each
of the receiving ends. Termination resistors are particularly important when using long
transmission lines and/or high data transfer speeds.
R
T
120
Ω
Radio modem
Cable
Terminal
Cable
R
T
120
Ω
R
T
B
A
B'
A'
T
R
B'
A'
B
A