Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 27 of 137 Manual Documentation Number: <DocNumber>
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Four-Wire Systems
Like two-wire systems, the name "four-wire" system is something of a
misnomer. Four-wire systems should use five conductors for reliable
operation in all situations.
Typically, four-wire systems implement a master-slave protocol in
which the driver of the master is connected by one twisted pair to the
receivers of all slaves. Another twisted pair connects the drivers of all
slaves to the receiver in the master. The fifth conductor is the signal
common, which connects to all nodes, ensuring that V
CM
is limited
within a safe range.
Figure 18: RS-485 Four-Wire Configuration
Termination
Termination is used to match the impedance of a communications node
to the impedance of the transmission line being used. Data signals
travel as a wave along the transmission line at a speed somewhat less
than the speed of light. When the wave reaches the end of the