Issue
What is the polling rate (refresh rate) of Scada tags by an OPC server?

Product Line
Scada 7.x
Power Scada Expert 8.x

Environment
OPC server

Cause
The Citect OPC server publishes all tags so external OPC clients can access them. User needs to know how fast the tags can be polled.

Resolution
The update rate will be set and adjusted by the OPC client. The subscription rate in the OPC client, and the OPC server will give updates according to the schedule. There's no theoretical lower limit.
How often the Citect server polls the PLC will determine how up to date 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 an I/O device is stored temporarily in the memory.
A minimum cache time of 300 milliseconds is recommended. If the server is caching for 300ms and data is requested via OPC client every 100ms, then 3 sets of the same data will be read.

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