Siemens SIMATIC S7-1200 Safety Programming Manualline page 44

4 Optimizing Safety Programs
Checklist
The following checklist allows you to identify and correct user-generated STOP
causes.
Table 4-1: Checklist
Overflow
Underflow or overflow can occur in arithmetic functions. This must be caught by the user in
the program. A library with arithmetic functions for the safety program is available in SIOS:
https://support.industry.siemens.com/cs/ww/en/view/109482083
Division by 0
If a division by 0 occurs in the safety program, the F-CPU goes to STOP. Prior to the
division, the divisor must be checked for 0 and the division must be conditionally skipped.
Access via HMI
An HMI is used to write (modify) data (bit memories, DBs) that is read in the safety
program. By default, communication has a higher priority than safety. This can result in
data corruption. For possible solutions, see Chapter 3.9.
Standard access to F-data
The standard user program modifies data of fail-safe tags or parts of their protection. Write
access to F-data is only allowed in the safety program.
Pointer access to F-data
Identical to standard access; can occur at runtime when there are unfavorable defaults for
generating a pointer to F-areas (inputs, outputs, data blocks, etc.).
Changing start values in F-instance data blocks
Start values may only be changed in the interfaces of F-FBs.
Read / write standard data
Read / write access to the same standard data from the safety program is not allowed.
Additional information
For more information and causes of data corruption, visit Siemens Industry Online
Support:
https://support.industry.siemens.com/cs/ww/en/view/19183712
Safety Programming Guideline
Entry ID: 109750255, V1.0,
Possible causes
10/2017
Checked
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