Siemens SIMATIC S7 System Manual page 275
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Also See for SIMATIC S7:
- System manual (866 pages) ,
- Functional safety manual (212 pages) ,
- User manual (134 pages)
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Using a trigger when monitoring or modifying PLC tags
Triggering determines at what point in the scan cycle the selected address will be monitored
or modified.
Trigger Type
Permanent
At scan cycle start
At scan cycle end
At transition to
STOP
For modifying a PLC tag at a given trigger, select either the start or the end of cycle.
● Modifying an output: The best trigger event for modifying an output is at the end of the
● Modifying an input: The best trigger event for modifying an input is at the start of the
To diagnose why the CPU might have gone to STOP, use the "Transition to STOP" trigger to
capture the last process values.
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
Description
Continuously collects the data
Permanent: Continuously collects the data at the start of the scan cycle, after
the CPU reads the inputs
Once: Collects the data at the start of the scan cycle, after the CPU reads the
inputs
Permanent: Continuously collects the data at the end of the scan cycle, before
the CPU writes the outputs
Once: Collects the data once at the end of the scan cycle, before the CPU
writes the outputs
Permanent: Continuously collects data when the CPU transitions to STOP
Once: Collects the data once after the CPU transitions to STOP
scan cycle, immediately before the CPU writes the outputs.
Monitor the value of the outputs at the beginning of the scan cycle to determine what
value is written to the physical outputs. Also, monitor the outputs before the CPU writes
the values to the physical outputs in order to check program logic and to compare to the
actual I/O behavior.
cycle, immediately after the CPU reads the inputs and before the user program uses the
input values.
If you are modifying inputs the start of the scan cycle, you should also monitor the value
of the inputs at the end of the scan cycle to ensure that the value of the input at the end
the scan cycle has not changed from the start of the scan cycle. If there is a difference in
the values, your user program may be writing to an input in error.
Online and diagnostic tools
9.7 Watch tables for monitoring the user program
275
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- Introducing the S7-1200 Plc
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