Elements and Attributes

DABACON databases contain element instances. An element instance will be of a specified type. The type of the element determines what attributes it may have. For example, a Box could have attributes XLEN, YLEN, ZLEN and position. In the AVEVA base product, a Box has many more attributes than this! The database schema defines:

• What element types can exist.

• What attributes are allowed for each element type.

• The permitted element relationship within the hierarchy. For example, an Equip item is allowed to own a Box, but a Box is not allowed to own an Equip.

Attributes are of different types. The type of an attribute is relevant when accessing attribute values within a PML expression. For example, the expression (XLEN EQ TRUE) is invalid since XLEN is a real rather than a logical. The types of attribute allowed are:

Reference type attributes are pointers to other elements; for example, from a Nozzle to a Branch. They are also used to reference between databases; for example, from a Design item to its Catalogue representation.

Within the AVEVA base product, the valid attributes for an element can be output by giving the command Q ATT. As well as the attributes defined in the schema, there are User-Defined Attributes (UDAs) and pseudo-attributes. Pseudo-attributes are not output by Q ATT. Pseudo-attributes are calculated internally by the AVEVA base product. For example, at a Flange the user may query the p-point information or the arrive/leave tube length. A full list of pseudo-attributes is provided in Data Model.

Each element instance has a unique reference number. This number never changes, and can even be preserved when the database is reconfigured, and so it can be used to reference an AVEVA base product element from outside the AVEVA base product.

There are also User Defined Element Types (UDETs) which allows users to change the schema; for example an administrator can define a :PUMP element as a special case of an EQUIpment, with modified lists of attributes and members.