Protective Mechanisms And Fault Detection; Error Response; Timing - Siemens Simatic Manual

Fail-safe function blocks for storage and retrieval machines
3.4.3.2

Protective mechanisms and fault detection

3. The block has know-how protection, so that the internal logic cannot be manipulated. Due to
the fact that the block is implemented as a function, then there is no instance DB. This means
that there is absolutely no possibility of externally influencing the internal processing.
4. Sign-of-life monitoring identifies if the block is either not called or is not correctly processed.
5. These mechanisms guarantee that the scale value is calculated without being able to be ma-
nipulated.
6. For transfer to the safety program at outputs OUT1 and OUT2, the masked and scaled value
is provided, logically combined with another check value.
7. The user must interconnect these two values with the corresponding inputs of the
F_SCALE_DINT block in the safety program using bit memory words.
8. By transferring the masked and scaled values to the safety program in this way, data transfer
errors of the value as well as incorrect user interconnection between SCALE_DINT blocks in
the standard and F_SCALE_DINT blocks in the safety program can be diagnosed.
3.4.3.3

Error response

9. The block itself does not signal an error; the error evaluation is carried out using the
F_SCALE_DINT block in the safety program.
Safety notes and instructions
It must be carefully ensured that the machine does not automatically restart after
a 1->0->1 change at SCALE_OK by suitably interconnecting output SCALE_OK
in the user program.
3.4.3.4

Timing

10. As a minimum, the block must be called in the same time grid as the F_SCALE_DINT block.
11. If this is not the case, then F_SCALE_DINT signals an error, i.e. its SCALE_OK output chang-
es to 0.
Note
We recommend that the SCALE_DINT block is called in the same cyclic interrupt
OB as the safety program, e.g. immediately before the F_CALL block.
S7-Distributed Safety/SIMATIC Safety - fail-safe function blocks for storage and retrieval machines
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