Introduction | OPC Technology | OPC Specifications
OPC Data Access Specification | DCOM Configuration | References

OPC Data Access Specification

OPC Item | OPC Group | OPC Server | OPC Client | The Namespace

OPC Group

OPC Group objects are containers of OPC Item objects. They define some characteristics of a group of items like:

• Update rate
• Active state
• Deadband
• Communication type
• Share type

The update rate of an OPC Group informs how much time is between two reads of a group from the server.

The active state indicates if an is active or not.

The deadband helps to define how much time is between a call from server to client.

Four communication types are allowed: synchronous, asynchronous, refresh and subscription. On the synchronous read, the client calls the server for a value and wait for it. This method is recommended to data process obtained fastly. Slowly obtained data process can do the system be slow. On the asynchronous read, the client calls the server for a value and do not wait for it because the response can delay. When the refresh is selected, the client reads all active items in active groups. With the subscription communication type activated, the server read all items and send only the modified one.

The data can be obtained from two different ways: from cache or from device. On the first one, the data is read from a cache on the server. On he second one, the data is obtained directly from the device.

A group can be shared in two different ways: public or local. Public groups can be accessed from multiple clients, while local groups can be accessed only from local clients.