OPC server operation

PI Interface for OPC DA can log OPC server interactions in the opcresponse.log, opcscan.log, and opcrefresh.log files. To interpret the information in these files, you must understand the basic interface architecture. PI Interface for OPC DA has two threads of operation:

• PI thread

  Interacts with Data Archive.

• COM thread

  Interacts with the OPC server.

Polled PI points

For polled PI points, the interface notifies the PI thread when it’s time to scan. The PI thread starts the data collection process and logs the time, group number, and current flag value in opcscan.log, then sets the flag. (If the flag in opcscan.log is non-zero, the last call made to the server did not return before the interface initiated another poll, and data might have been missed as a result.)

When the COM thread detects that the flag is set, it logs the time, group number and transaction ID in the opcrefresh.log file and makes a refresh call to the OPC server. When it receives the synchronous response from the OPC server, it clears the flag.

Now the OPC server can send data at any time, in an asynchronous manner. When the OPC server sends data to the interface COM thread, the time, group number and transaction ID are logged in opcresponse.log.

Advise PI points

For advise PI points, the COM thread receives callbacks only when the data from OPC server changes value. Therefore, advise points do not generate entries in the opcscan.log or opcrefresh.log files, and only the data callbacks are logged in the opcresponse.log file. Advise points can be identified in the opcresponse.log file by group numbers that range from 200 to 800.