Items

An item, also known as a tag or signal, represents real-time values either originating from an external system (PLC, DCS, etc.), or values calculated or derived from such values within the Apis Hive environment (calculated control value, logical signal, etc.).

In most cases, a module will bring items into the Apis Hive environment, but there are also cases when a module doesn't provide items by itself, e.g. the ApisLoggerBee module.

Item attributes

Items are typically real-time process data or calculated data, and they have several attributes associated with them. All items have the standard OPC attributes: value; quality; timestamp; rights.

Additionally, items will typically have more attributes, dependent on the items purpose, and will vary for different types of items. Such attributes can be divided into two categories: custom attributes and predefined Apis/OPC attributes (for example, descriptions and engineering units).

Custom attributes

Attributes specific to the purpose of the item. E.g. the amplitude of a sine signal.

Predefined Apis / OPC attributes

Attribute of a more generic nature. See OPC DA Item attributes and "Predefined Apis attributes".

Item types

There are three main item types; scalar values, vector values and matrices.

• A scalar value is typically a flow signal, temperature, etc. from an external system.
• A vector value typically is a spectrum from an NIR instrument, or control vector in a Model-based Predictive Control (MPC) system.
• A matrix value typically is a system matrix in an MPC system.

See also the Datatypes Overview.