Glossary · Maintenance and diagnostics
Diagnostic buffer
Also known as: Diagnostics buffer
German: Diagnosepuffer
In Siemens SIMATIC controllers, the diagnostic buffer is a ring buffer in the CPU that stores diagnostic and operating events such as mode changes, module faults and program errors in chronological order with a timestamp. When it is full, the oldest entries are overwritten.
- Maintenance
- Vendor product
In one sentence
The diagnostic buffer is a CPU ring buffer in Siemens SIMATIC controllers that logs faults, mode changes and program errors with timestamps.
Example
After an unexpected stop, the service technician reads the diagnostic buffer in the engineering tool and finds that a missing I/O module sent the CPU to STOP.
How it applies
- Commissioning: The diagnostic buffer is usually the first place to look when a controller changes to STOP or a module reports an error, because it shows the order of events.
- Maintenance: Service technicians read the buffer online with the engineering software, on the CPU display or through the web server, depending on the controller. Because it is a ring buffer, entries should be saved before they are overwritten.
- Operation: Some plants copy buffer entries into a central Event log or alarm archive so that events survive a CPU exchange.
- Documentation: Troubleshooting instructions should tell the reader how to open the buffer, how to export it and which typical entries point to which cause. A DITA troubleshooting topic with condition, cause and remedy fits this well.
Diagnostic buffer vs. alarm log
The diagnostic buffer records controller-level events such as hardware faults and program errors. An alarm log on the HMI records process alarms defined by the machine builder. Both are needed for a complete picture of a failure.
Keep in mind: Product names, buffer sizes and access methods differ between controller families and firmware versions and change over time. Check them against the vendor's current documentation.