The OPC Data Access model is based on a hierarchy of OPC item values. This hierarchy can be represented as a tree data structure that can have an arbitrary depth. The MetaServer and MacroView data model is more restrictive in that values are always defined as an attribute of an entity. An entity is a named data structure with a predefined set of attributes, which are defined by the type of the entity. A MacroView configuration consists of a list of entities expressed solely as a list with no hierarchical or graph relationships.
The OPC MetaBridge application presents the OPC item hierarchy as a list of entities. The entities list are extracted from all branches in the OPC item hierarchy that have at least one branch node with an OPC item value. Empty OPC branches or branches that only have sub branches are not included in the derived entity list presented to MetaServer clients. The attributes of each entity are then defined by all of the OPC items in the associated item branch. In the example mapping diagram below, the "Square Waves" branch maps to a "Square Waves" entity, which has attributes named Boolean, Int1,Int2, ... and Uint4. The OPC MetaBridge entity list can be viewed as a flattened view of the OPC item hierarchy.
Note: For historical reasons, MacroView entity names are restricted to capital letters with a maximum of eight characters in the name. Attribute names are similarly required to capitalized with a maximum length of eleven characters. These MacroView naming restrictions don't actually apply to the MetaServer protocol where entity and attribute names can be of any length and mixed case. For this reason the OPC item names can be simply split into an entity name and an attribute name when mapping to the MetaServer protocol data model. If the client application that uses the OPC MetaBridge entity data is a MacroView program, there will need to be a mapping to an eight character capitalized entity name and an eleven character capitalized attribute name. The metaimg driver performs this additional mapping during its operation.