Technical data

Appendix
27.2 Memory Areas of S7 CPUs
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 583
27.2.3.6 Diagnostic Buffer
The diagnostic buffer displays the diagnostic messages in the order in which they occur. The first
entry contains the newest event. The number of entries in the diagnostic buffer is dependent on the
module and its current operating mode.
Diagnostic events include the following:
• Faults on a module
• Errors in the process wiring
• System errors in the CPU
• Mode transitions on the CPU
• Errors in the user program
• User-defined diagnostic events (via the system function SFC52).
27.2.3.7 Evaluating the Diagnostic Buffer
One part of the system status list is the diagnostic buffer that contains more information about
system diagnostic events and User-defined diagnostic events in the order in which they occurred.
The information entered in the diagnostic buffer when a system diagnostic event occurs is identical
to the start information transferred to the corresponding organization block.
You cannot clear the entries in the diagnostic buffer and its contents are retained even after a
memory reset.
The diagnostic buffer provides you with the following possibilities:
• If the CPU changes to STOP mode, you can evaluate the last events leading up to the STOP
and locate the cause.
• The causes of errors can be detected far more quickly increasing the availability of the system.
• You can evaluate and optimize the dynamic system response.
Organizing the Diagnostic Buffer
The diagnostic buffer is designed to act as a ring buffer for a maximum number of entries which is
dependent on the individual module. This means that when the maximum number of entries is
reached, the next diagnostic buffer event causes the oldest entry to be deleted. All entries then
move back one place. This means that the newest entry is always the first entry in the diagnostic
buffer. For the S7-300 CPU 314 the number of possible entries is 100:
The number of entries displayed in the diagnostic buffer is dependent on the module and its current
operating mode. With some CPUs, it is possible to set the length of the diagnostic buffer.