Technical Reference Manual - InduSoft

Communication with Other Devices
Configuring an OPC Client connection to an OPC Server
The OPC Client task/worksheet is used to communicate with any system that implements the OPC Server
protocol.
This task implements the OPC standard as described in the OLE for Process Control Data Access Standard
Version 1.0A document, which is available at the OPC Foundation web site.
Note: Before using the OPC Client task/worksheet in your project, you must make sure the OPC
server software is properly installed and configured on the system to which you want to connect.
To configure a new connection, insert a new OPC Client worksheet on the Comm tab of the Project Explorer.
Sample OPC Client worksheet
Use the following parameters in the configuration table for OPC:
• Description text box: Type a description of the OPC task for documentation purposes only. (The OPC Client
task ignores this information.)
• Server Identifier: Type the name of the server you want to connect. If the server is already installed on the
computer, you can select the server name from the list.
• Disable: Type a tag or a constant with a value other than 0, to disable communication with the OPC server.
Specify 0, or leave the field blank to enable communication.
• Read Update Rate: Specify how often the server should update this group (in milliseconds). Specify 0 to
indicate the server should use the fastest practical rate.
• Percent Deadband (valid for analog items only): Specify how much percent change in an item value should
cause a notification by the server.
• Status: Type the name of a tag to receive the status of the connection. Good status is 1.
• Remote Server Name: Node name or IP address of server on node network.
• Read before writing checkbox: Check this option to force your project to read the original values of items on
the OPC server just before writing new values to the server. The project does this by first buffering the
new values to be written and then reading the original values from the server. Only after the project is
synchronized with the server are the new values written from the buffer to the server.
• Read after writing checkbox: Check this option to force your project to read back the new values of items on
the OPC server just after the project has written those values.
Caution: The Read before writing and Read after writing options are offered because the OPC Client/
Server specification says that the value of an item on the client — in this case, your project —
should not change unless the server sends the change. That way, the client always stays in sync
with the server.
Your project, however, may be designed to change those values according to runtime processes or
user input. Therefore, the best way to change the values while staying in sync with the server is
to make it seem like the changes originate on the server. With both options enabled, the following
sequence of events happens on every scan of the OPC worksheet:
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