Troubleshooting Data Mismatch Between PI System and DCS Historian: Investigating Missing Value Spikes

When integrating OSIsoft PI System with external historians like InfoPlus.21 (often tied to DCS systems), engineers sometimes encounter discrepancies between the source (DCS) data and the PI System archives. One common and perplexing issue is missing or underreported high-value spikes—values appear in the DCS but are not reflected in the PI data, despite both systems recording at the same frequency (e.g., once per second).

The Scenario

A PI System is connected to an InfoPlus.21 Historian. The DCS records values every second, as does the PI System. However, a rapid increase (spike) spanning two seconds in the DCS appears to be missed or smoothed over in the PI archives. After verifying that date formats are aligned and no data editing occurred in InfoPlus, attention turns to the PI Interface configuration—specifically, the Location3 parameter.

Common Causes for Data Discrepancies

There are several potential causes for PI not capturing sudden data spikes:

1. Snapshot and Archive Compression Settings

• Compression is designed to reduce archive size by dropping values that don't show significant change (based on Compression Deviation).
• A rapid spike may not be recorded if it falls within the compression settings (i.e., the spike's begin and end points are within the same compression deadband).
• Solution: Try setting the target tag's Compression and/or Exception parameters to zero during troubleshooting to force PI to record all values. Observe if the spike appears.

2. PI Interface Scan Class Configuration

• Ensure the scan class is configured to sample the source data at the desired interval (e.g., 1 s).
• If the scan interval is longer than the spike's duration, the spike may be missed.
• Solution: Confirm that the scan class matches the data rate of the source historian.

3. PI Interface Location Parameters (e.g., Location3)

• Many interfaces rely on Location3 to determine the scan/read mode. For the PI Interface to InfoPlus.21, setting Location3 to zero may instruct the interface to use a specific scanning method—such as time-based or event-based scanning.
• Solution: Consult the PI Interface Manual for InfoPlus.21 to correctly set Location3. Experiment with its value and observe changes in data capture.

4. Latency and Data Pipeline Bottlenecks

• Network latency or bottlenecks between PI Interface Node and the InfoPlus.21 Server could cause data loss or delayed delivery.
• Solution: Monitor interface logs and ensure robust network connectivity.

Step-by-Step Troubleshooting Guide

1. Verify Time Settings: Confirm that both systems (PI and DCS/InfoPlus) are using synchronized clocks (NTP recommended) and that time zones and date formats do not mask true-time alignment issues.
2. Set Compression and Exception to Zero: For the tag in question, temporarily set both Compression and Exception settings to zero to rule out data filtering.
3. Check Scan Class Interval: Ensure the PI tag's scan class matches or is faster than the real data change rate in InfoPlus.21.
4. Review and Adjust Interface Location Parameters:
   • Set Location3 to 0 if specified by the PI Interface Manual.
   • Restart the interface (if safe to do so) after configuration changes.
5. Review Logs: Examine PI Interface logs for dropped data or timestamp mismatch messages.
6. Compare Raw Data: Use PI SMT (PI System Management Tools) to compare raw data points in PI with the original InfoPlus data.

Conclusion

Data loss or discrepancies when connecting PI System to external historians often stem from interface configurations or PI tag settings. Compression and exception settings, scan frequency, and interface-specific parameters (like Location3) can all impact data fidelity. By systematically checking each area—not just the PI System but also source historian data acquisition settings—engineers can ensure critical process spikes and changes are properly captured for analysis and compliance.

Further Reading:

• PI System Compression and Exception Settings Explained
• PI Interface for InfoPlus.21 Manual

Have you experienced similar challenges connecting PI to legacy historians? Share your troubleshooting tips in the comments below!