3 Methods for Safety Programming
Safety Programming Guideline
Entry ID: 109750255, V1.0, 10/2017
Siemens AG 2017 All rights reserved
3.1.2 Call levels of F-FBs/F-FCs
For standard user programs, the number of call levels is limited depending on the
CPU. For safety programs, you can use a maximum of eight call levels. A warning
appears when this limit is exceeded and an error message is displayed for pure FC
and multi-instance call chains.
System instructions ("ESTOP1", "SF_DOOR", etc.) are not included in the number
of call levels.
On the system side, functions are mapped as FBs with a multi-instance call in
the protection program; this is the reason why an error message is also
displayed for FC call chains with more than eight call levels.
The program structure in Figure 3-1 shows one way of keeping the call levels
relatively flat so that the safety program remains within the limit specified here.
3.1.3 Call sequence of the blocks in the Main Safety
Recommendation
Within the Main Safety, call blocks in the following sequence:
1. Receive blocks from other CPUs (F-CPU-F-CPU communication)
2. Error acknowledgment/reintegration of F-modules/F-channels
3. Evaluation block of the sensors
4. Operating mode evaluation
5. Logic operations, calculations, evaluations, etc.
6. Control blocks for safe actuators
7. Send blocks to other CPUs (F-CPU-F-CPU communication)
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