Access items using the OPC communications protocol

In the case of OPC communications, the protocol addresses an element of data in a conversation with six characteristics: node name, program name, group name, device group, link name, and item name.

The node name and device group are optional. A fully qualified OPC item name (ItemID) is composed of the link name and item name. All other characteristics are specified through separate Communication Driver means.

To access an OPC item, the OPC client connects to the Communication Driver, either in-process or out-of-process, and creates an OPC group defining the data-acquisition properties for the collection of items to be added. OPC groups can be either public or private. Public OPC groups are shared across multiple clients. Private OPC groups are local to a single client.

Note: The Communication Drivers only support private OPC groups.

Optionally a device group, which indicates the access path to the items for Read/Write, can be specified from the Communication Driver. The following briefly describes each characteristic of the OPC protocol.

node name

Computer (host) name identifying a specific node on the network. This node name is required for remote access and is optional for local access.

program name

The registered OPC server name uniquely identifying a specific server (ProgID).

For this Communication Driver, the base instance name is OI.GESRTP.3, and the cloned instance name would be OI.GESRTP_<cloned name>.3. The version independent ProgID is OI.GESRTP.

group name

The OPC group created from the client for logically organizing a collection of items with the same data-acquisition properties between the client and the server, such as update rate.

device group

Meaningful names configured in the Communication Driver under a specific controller for the common custom attributes between the Communication Driver and the device, such as update interval.

If not specified from the client, the default device group using the global-configuration attribute values from the Communication Driver is assumed.

Functionally, a device group is equivalent to an access path (optional).

link name

The set of hierarchy node names separated by delimiters. Link names represent the specific devices on a communications path link from the hierarchy root to a specific controller as configured for this Communication Driver under the OI Server Manager.

item name

A specific data element, the leaf of the hierarchy tree of this Communication Driver, within the specified group. For example, when using this Communication Driver, an item can be a relay, timer, counter, register, and so on, in the controller.

Connecting to the Communication Driver from an OPC Client

The GESRTP Communication Driver always runs out-of-process when instantiated, whether directly in the OI Server Manager or through OPC.