Applies To:
  • CitectSCADA

The Citect OPC server publishes all tags so external OPC clients can access them. What is the update rate (or poll rate) of the tags and can this be adjusted?

The update rate will be set by the OPC client, and you should be able to adjust it there. You set the subscription rate in the OPC client, and the OPC server will give updates according to this schedule. There's no theoretical lower limit. How often the Citect server polls the PLC however, will determine how fresh the data is.

If the server's poll time is slower than the OPC requests, then stale data will be provided to the client. When caching is enabled, all data that is read from a I/O device is stored temporarily in the memory. A minimum cache time of 300 milliseconds is recommended. If you are caching for 300ms, and requesting data via OPC every 100ms, then you will get 3 sets of the same data.

So feel free to adjust the update rate, but beware of the Citect server poll rate also.

 citect opc server