7. RS485 Modbus configuration
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Detecting Gas Saving Lives
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7.2 Wiring topology
The two common wiring topologies in use are the star connection and the linear bus.
7.2.1 Star connection
In a star-connected topology all detectors are wired to a central point, which is usually the
control panel. No cables are required to have more than 5 cores as all the 4-20mA signal
connections are taken directly to the control panel.
The RS485 A and B signals can then all be connected together at the star point. The bus
should then be terminated at the star point with a single 110 ohm termination resistor.
The length of each arm of the star may not exceed 750 metres.
7.2.2 Linear bus connection
In a bus-connected topology all
IRmax
are wired to a linear arrangement, usually with the
control panel at one end. A classic situation is a tunnel installation, with
IRmax
installed at
regular intervals (see Diagram 9 below).
To wire the installation, two 110 ohm-terminating resistors should be fitted: one at each
physical end of the bus.
As the 4-20mA signals cannot be shared, an additional conductor is required on each wiring
hop. If none of the 4-20mA signals are being used, then only four conductors are required.
Diagram 9
