Diagnostic Buffer
The Diagnostic Buffer contains a timestamped log of events recorded by the PLC itself — hardware faults, I/O errors, block errors, mode changes, and operator actions. It is the first place to look when diagnosing unexpected stops or alarms.
Reading the buffer
Section titled “Reading the buffer”The table shows the 50 most recent events, newest first. Each row contains:
| Column | Description |
|---|---|
| Timestamp | Date and time the event was recorded by the PLC (PLC clock, not PC clock) |
| Status | Whether the event is incoming (fault appeared) or outgoing (fault cleared). Both states in the same entry mean it appeared and cleared within a single cycle. |
| Description | Human-readable text. S7 PLC Buddy requests all available text attributes (short_text, long_text, help_text) and shows the most descriptive one available. |
| Event Reference | Internal Siemens event code. Useful when searching SIOS or contacting support. |
Exporting to CSV
Section titled “Exporting to CSV”Click the Export CSV button (top-right of the diagnostic buffer table) to download the current buffer as a .csv file.
The CSV has four columns:
Timestamp,Status,Description,Event Reference2026-07-15T08:23:41.000Z,Incoming,Power supply voltage fault,16#0F0EThe timestamp is in ISO 8601 UTC format, making it easy to sort and filter in Excel or any data tool.
What kinds of events appear?
Section titled “What kinds of events appear?”- Hardware faults — faulty I/O modules, missing devices, power supply issues
- Software faults — programming errors (e.g. OB not loaded, access errors)
- Mode changes — STOP → RUN transitions, warm/cold restarts
- Operator actions — mode changes made via HMI or TIA Portal online tools
- Communication events — PROFINET line structure changes, DP master/slave errors
- Time synchronisation — NTP sync events
Using the AI Agent with the diagnostic buffer
Section titled “Using the AI Agent with the diagnostic buffer”The AI Agent can read the diagnostic buffer directly via its get_diagnostic_buffer tool. You can ask questions like:
“Why did the PLC stop at 08:23 this morning?” “Are there any recurring hardware faults in the buffer?” “Summarise the last 20 diagnostic events in plain language.”
See AI Agent Overview to set up the agent.