Technical data
Appendix
27.2 Memory Areas of S7 CPUs
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 579
I/O Access Error (PZF) during Update of the Process Image
The default reaction of the CPU families (S7-300 and S7-400) to an error during the update of the
process image varies:
• S7-300: No entry is made in the diagnostic buffer, no OB is called, the corresponding input
bytes are reset to "0" and will remain at "0" until the fault is gone again.
• S7-400: An entry is made in the diagnostic buffer and OB85 is started for each I/O access for
each update of the corresponding process image. The faulty input bytes are reset to "0" each
time the process image is accessed.
For new CPUs (as of 4/99), you can reassign parameters for the reaction to I/O access errors so
that the CPU functions in one of the following manners:
• Generates an entry in the diagnostic buffer and starts OB85 only for incoming and outgoing
PZF (before OB 85 is called, the faulty input bytes are reset to "0" and are no longer
overwritten by the operating system until the outgoing PZF)
• Produces the default reaction of an S7-300 (does not call OB85; the corresponding input bytes
are reset to "0" and are no longer overwritten by the operating system until the fault is cleared.)
• Produces the default reaction of an S7-400 (calls OB85 for each individual access; the faulty
input bytes are reset to "0" each time the process image is accessed.)
How Often Does OB85 Start?
In addition to the reaction to PZF that is assigned as a parameter (incoming/outgoing, or for each
I/O access), the address space of a module also influences how often OB85 starts:
For a module with an address space of up to a double word, OB85 starts once, for example, for a
digital module with a maximum of 32 inputs or outputs, or for an analog module with two channels.
For modules with a larger address space, OB85 starts as often as access has to be made to it with
double word commands, for example, twice for an analog module with four channels.