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4.2 Conditions ToC Previous Next

Conditions are used to represent the state of a system or one of its components. Some common examples are:

  • a temperature exceeding a configured limit
  • a device needing maintenance
  • a batch process that requires a user to confirm some step in the process before proceeding Each Condition instance is of a specific ConditionType. The ConditionType and derived types are sub-types of the BaseEventType (see OPC 10000-3 and OPC 10000-5). This part defines types that are common across many industries. It is expected that vendors or other standardisation groups will define additional ConditionTypes deriving from the common base types defined in this part. The ConditionTypes supported by a Server are exposed in the AddressSpace of the Server.

Condition instances are specific implementations of a ConditionType. It is up to the Server whether such instances are also exposed in the Server’s AddressSpace. Clause 4.10 provides additional background about Condition instances. Condition instances shall have a unique identifier to differentiate them from other instances. This is independent of whether they are exposed in the AddressSpace.

As mentioned above, Conditions represent the state of a system or one of its components. In certain cases, however, previous states that still need attention also have to be maintained. ConditionBranches are introduced to deal with this requirement and distinguish current state and previous states. Each ConditionBranch has a BranchId that differentiates it from other branches of the same Condition instance. The ConditionBranch which represents the current state of the Condition (the trunk) has a NULL BranchId. Servers can generate separate Event Notifications for each branch. When the state represented by a ConditionBranch does not need further attention, a final Event Notification for this branch will have the Retain Property set to False. Clause 4.4 provides more information and use cases. Maintaining previous states and therefore the support of multiple branches is optional for Servers.

Conceptually, the lifetime of the Condition instance is independent of its state. However, Servers may provide access to Condition instances only while ConditionBranches exist.

The base Condition state model is illustrated in Figure 1. It is extended by the various Condition subtypes defined in this standard and may be further extended by vendors or other standardisation groups. The primary states of a Condition are disabled and enabled. The Disabled state is intended to allow Conditions to be turned off at the Server or below the Server (in a device or some underlying system). The Enabled state is normally extended with the addition of sub-states.

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Figure 1 – Base Condition state model

A transition into the Disabled state results in a Condition Event however no subsequent Event Notifications are generated until the Condition returns to the Enabled state.

When a Condition enters the Enabled state, that transition and all subsequent transitions result in Condition Events being generated by the Server.

If Auditing is supported by a Server, the following Auditing related action shall be performed. The Server will generate AuditEvents for Enable and Disable operations (either through a Method call or some Server / vendor – specific means), rather than generating an AuditEvent Notification for each Condition instance being enabled or disabled. For more information, see the definition of AuditConditionEnableEventType in 5.10.2. AuditEvents are also generated for any other Operator action that results in changes to the Conditions.

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