How One Test Lab Used RTA’s A-B PLC Historian
to Uncover a Recurring PLC Failure

Overview

A well-known, international product development facility was conducting multi-day duration tests to validate equipment performance under extended run conditions. For unknown reasons, every Friday at 11:00 PM, their MicroLogix PLCs consistently faulted, bringing critical testing to a halt. With no personnel on-site, identifying the root cause became nearly impossible.

To diagnose the issue, the team turned to Real Time Automation’s 1848ETC-HST A-B PLC Historian.

Challenge

The facility’s test setup includes three MicroLogix PLC control systems running simultaneously. Every Friday evening, all three units faulted within minutes of each other, disrupting ongoing long-term tests and wasting valuable development time.

Without real-time visibility into system behavior, engineers faced two problems:

1. No contextual data at the time of the faults.
2. No way to isolate each test system’s data when all were logged together.

Traditional troubleshooting methods, manual data review, incremental code edits and retesting were inefficient and risked missing transient conditions.

Solution

The engineering team installed an 1848ETC-HST to log operational data from all three PLCs and push it to a Microsoft SQL Server for detailed analysis. The device quickly began capturing high-resolution time-series data from each control loop.

However, because all three PLCs’ data was stored in a single SQL table, separating events by PLC seemed cumbersome. What the engineering team didn’t understand is that they had several options to identify this data:

1. Create a new string tag in each PLC (i.e. “PLC 22”) to identify the PLC and read that tag whenever data was collected from the PLC.
2. Create a string value in the data model for each PLC in the 1848ETC-HST units so that the PLC identification string would be included whenever the model was published to the SQL table.
3. Use three different 1848ETC-HST units and publish data to three different SQL tables.

Working with RTA application support, the engineering team ultimately chose to use three 1848ETC-HST units in this application as that best suited their control system architecture.

Results

After several weeks of data collection and analysis, the customer pinpointed the root cause:

A single PLC routine exceeded the MicroLogix’s 1.5-millisecond watchdog timer. When the routine occasionally took longer than expected, just a fraction of a millisecond over the limit, the PLC faulted and shut down all tests simultaneously.

By increasing the watchdog threshold to 2.5 milliseconds, the customer eliminated the recurring Friday night fault condition and successfully completed full-duration test runs without interruption.

Ongoing Optimization

With the release of 1848ETC-HST Revision 2.01, the customer leveraged a new scheduled logging feature, allowing precise data capture only during the critical fault window from Friday 12:00 PM to Saturday 12:00 PM.

This enhancement:

• Reduced historian storage usage by eliminating irrelevant data,
• Minimized analysis time, and
• Focused engineering attention on the exact time window of interest.

Key Takeaways

• Problem: Repeated PLC watchdog faults halted testing during off-hours.
• Solution: Deployed RTA’s 1848ETC-HST to log, segregate and analyze system data.
• Outcome: Identified timing routine exceeding watchdog limit and corrected issue.
• Impact: Prevented test interruptions, reduced diagnostic time and improved data handling efficiency.

Conclusion

By integrating the 1848ETC-HST Historian, the customer transformed a recurring, undiagnosed failure into a resolved, data-backed insight. The historian’s detailed timestamping, flexible scheduling and SQL integration gave engineers the visibility they needed to restore reliability and confidence in their testing operations, demonstrating the real-world diagnostic power of RTA’s edge historian technology.