A segment is a single circuit with power supply, fieldbus junction boxes, and instruments. The segment connects the control system to multiple instruments. The topology design determines the physical location and connection of power supplies, fieldbus junction boxes, and instruments.
The typical topology for a segment is the trunk-and-spur topology with a main line or master cable (trunk). The trunk connects the power supplies and communication interfaces located in the control room with the field. One or more fieldbus junction boxes mounted where they are easily accessible provides connections for field devices. The connection cable with a length of up to 120 m is called a spur, as it is typically much shorter in comparison to the trunk. This topology fits practically all process plants.
With its many similarities to the topologies known from multi-core trunk cables leading to a fieldbus junction box installed in the field, the trunk-and-spur topology is easy to understand, engineer, and maintain.