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Improve OPC interface performance with scan class configuration

When configuring OSIsoft PI System OPC interface tags to collect data from many Rockwell Automation PLCs and controllers, pay attention to the tag scan class. If the OPC values are collected by PI on a polled basis, scan class configuration can significantly impact PLC communication performance.

Here's how you might improve on RSLinx™'s attempts to automatically group data requests. When assigning scan classes to tags:

  • Keep items from the same PLC and data file together in a scan class.
  • Consider using array addressing for contiguous address ranges.
  • Do not mix Advise and Polled tags in the same scan class.
  • Do not use Scan Class 1 for Polled tags.
  • Consider using unsolicited messaging for data transfer. Note that this might require PLC program changes.

These optimization recommendations apply to RSLinx Classic, PLC-5, SLC 500 System and ControlLogix. You can expect to find similar issues with other OPC servers. See related case histories.

Interface not sending data

Have an interface that connects to a PI Server but doesn't send data? Check the pipc.log file. If you see ‘-10401 No Write Access’, then there is no Trust entry for the interface. Create it either via ‘piconfig’ or PI System Management Tools. A very old PI Server may need a Proxy entry which can only be created via ‘piconfig’. See related case histories.

Eliminate range errors in PI system

Getting under and over range errors? You can get rid of these by converting float16 tags to float32. With older PI systems, especially PI2, the float16 data type conserved archive space. Now disk space is relatively cheap. Converting float16 points to float32 eliminates the under and over range digital states when the tag goes outside of the configured range. See related case histories.

Having trouble finding a PI System manual?

When searching the OSIsoft tech support site for interface manuals, the less popular ones return no results. Instead, for all PI interface manuals, look here: http://interfaces.osisoft.com/osiif/docs/ . See related case histories.

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