We are going to explore why your PLC data might be jumping erratically and how to determine if the issue is a calculation error in your logic or an external "ghost write" coming from Ignition.

When a PLC variable behaves erratically, the standard diagnostic step is to perform a Cross Reference within the programming software.

However, as demonstrated in the video, a cross-reference only shows instructions within the PLC code itself. It cannot detect writes coming from external sources like an HMI, SCADA system (such as Ignition), or other networked controllers via OPC-UA.

To isolate these "ghost writes," you can follow a systematic approach:

1. Isolate the Logic: Move the suspicious data into a temporary tag that is not linked to any external software. If the temporary tag is smooth while the original tag is jumpy, the issue is external to the PLC logic.

2. Trend the Data: Use high-speed trending to compare the "Raw" input data with the "Processed" result. Spikes in the processed result that do not exist in the raw data indicate an overwrite occurring after the calculation.

3. Check External Access: In Studio 5000, tags have an External Access property. Setting this to "Read Only" is a powerful diagnostic tool. If the jumpiness disappears once the tag is set to Read Only, you have confirmed that an external device is attempting to write to that address.

4. Audit the SCADA: Search the SCADA project for the tag name. Look for "Bidirectional" settings or "Write" scripts that might be unintentionally triggered by a configuration error.