Some field controllers are assigned a higher priority where they are polled more frequently than other field controllers. For example, critical field controllers may be polled twice a second, while lower priority devices are polled once per minute.
Polling rates also depend on the information being reported. The status of regional high voltage transmission lines will be checked more frequently than ambient temperature because the line status can change quickly compared to relatively slower temperature changes.
This image shows several types of relationships between the master and the field controllers. For instance, a master can communicate directly with a single field controller or with multiple field controllers. Multiple masters can also communicate with a common field controller. Most modern control system networks are made up of a blend of communications methods, topologies, architectures, and protocols, which introduces complexity when creating security mitigation strategies.
