OPC Client Driver

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By adding OPC Client Driver tags to your application, you can communicate with OPC servers on your network, whether those are other VTScada applications, control equipment or other programs.

Driver Errors: To learn more about the cause of an error condition, refer to the Driver Summary Report and the Driver Error Report, both of which are available in the Reports page. The Show Stats button will also provide current and last error messages: Show Statistics Button Widget

Reference Notes:

The OPC interface allows for two-way communication, thus you can add both input and output tag types that exchange information with the OPC server through the OPC client drivers you configure.

Server List

Select (or create) a named server list. (Driver Server Lists) Servers for the list must be defined using the Application Configuration dialog, as described in Servers for Specific Services. Smaller sites that do not have multiple servers, or that use only the default server list, need not configure this field.

OPC Client Driver properties OPC tab

The OPC tab for the OPC Client driver tag properties folder consists of attributes used to identify and establish a connection to the control device to be associated with this driver. The OPC tab for an OPC Client driver is also used to identify the remote server node that the driver should use to transmit data.

OPC driver tag, OPC tab

OPC Server

The Server Prog ID field enables you to specify the program identifier (ProgID) for the OPC server to which you wish to connect. This will be a string of the form Manufacturer.OPC.ServerType.

Examples of a Server Prog ID are: Trihedral.VTSOPC.DA or Matrikon.OPC.Modbus.

The OPC Server vendor generally supplies the Server Prog ID. Refer to your OPC Server manufacturer's documentation for the correct details.

Remote Server Node

Use this field to specify the name of the workstation that is acting as the OPC server. The Remote Server Node field can be left blank in the event that the local workstation is the OPC server.

Analog Deadband (% of full scale)

Set a percentage by which analog values must change before a change in value is reported. If the percentage is 0, the updates will be event driven, occurring on each value change.


In the event of an interruption in communication, this option will cause the last good value to be held. You should use this option with caution because, unless there is another means to alert you to interruptions in communication, it will appear that a valid value is still being received.

Data Update Rate Controls

By default, data updates from OPC servers are sent to OPC clients only on change of value or quality, with the maximum rate at which the servers send the values to the clients set to a default of 1 second. This is the default time defined in the OPC standard. The following two settings allow the VTScada OPC Client driver to change this default behavior to meet application specific requirements.


Force Asynchronous Refresh

This option allows for an override to the default "send only on change" behavior and when selected will cause the VTScada OPC client to poll the OPC server for values at a fixed rate as defined in the individual I/O tag’s "Scan Interval" setting.

The default "send on change" behavior will still function when this option is selected. For example, if the I/O tag "Scan Interval" is set to 10 seconds, the server can still send values every second, but the setting will only act to force a refresh from the server every 10 seconds in addition to the update on change.

In some cases, use of this option this may cause excessive load on the OPC Server host as it services all of the data update requests, and should be used only when there is an application-specific need for time based updates versus the OPC standard of update on change.

Use Tag Scan Interval

If this option is selected, the VTScada OPC client driver will request that the server send the values to the client at a maximum update rate defined by the individual I/O tags’ "Scan Interval" instead of the default 1 second rate.

This feature affects only the rate at which the OPC Server updates tags in the VTScada OPC Client. The rate at which the OPC Server queries field devices depends on the server implementation and cannot be controlled from within VTScada.

If using this option, the client (VTScada) is updated with the current value at the end of each scan interval. If the value at the field device changes several times within that period, then even if those changes are read by the OPC Server, they are not sent to the client.

Accept Data Marked as

Values received by the OPC client tag are set to INVALID unless the attributes of the value are specifically set to GOOD (>= 0xC0). Use these options to accept values with other attributes as marked.