Conditions define some state in a system that requires interaction with an operator or users. An example would be a Dialog that is displayed when something occurs that an operator needs to respond to, such as mounting archive device. Alarms are specific conditions that represent abnormal states of a process or system. This includes unsolicited notifications when the Alarm state changes. Some common examples are:
- a temperature exceeding a configured limit
- infringing safety limits of equipment
- a device needing maintenance
- a batch process that requires a user to confirm some step in the process before proceeding
State-driven notifications have great advantages when dealing with large systems that have hundreds to millions of conditions. Alarm clients can subscribe for alarm notifications for specific areas in a plant. They will receive notifications whenever the state of a condition changes and where each notification includes enough information to identify the type and source. Such notifications will typically be used to help identify faulty equipment, create maintenance work orders and improve on operator's effectiveness.
Some state changes require actions like acknowledging the state change. An example is an Alarm becoming active when a measurement device exceeds a configured limit.
Note – Alarms & Conditions is specified in OPC UA Part 9