Wiring Topology; Star Connection; Linear Bus Connection - Crowcon IRmax Installations-, Bedienungs- Und Wartungsanleitung

7. RS485 Modbus configuration 7. RS485 Modbus configuration

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
18 18