Forcing Variables In The Cpu - Siemens SIMATIC S7-1200 Manual

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8.8 Forcing variables in the CPU

8.8 Forcing variables in the CPU
A watch table provides a "force" function that overwrites the value for an input or output point
to a specified value for the peripheral input or peripheral output address. The CPU applies
this forced value to the input process image prior to the execution of the user program and to
the output process image before the outputs are written to the modules.
● Prior to the execution of the scan, the CPU overwrites the value of the peripheral input
● At the end of the scan, the CPU overwrites the output values generated by the user
When an input or output is forced in a watch table, the force actions become part of the user
program. If you close STEP 7 Basic, the forced elements remain active for the user program
being executed by the CPU program until they are cleared. To clear these forced elements,
you must use STEP 7 Basic to connect with the online CPU and use the watch table to turn
off or stop the force function for those elements.
If the CPU is executing the user program from a write-protected memory card, you cannot
initiate or change the forcing of I/O from a watch table because you cannot override the
values in the write-protected user program. Any attempt to force the write-protected values
generates an error. If you use a memory card to transfer a user program, any forced
elements on that memory card will be transferred to the CPU.
Startup
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B
C
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with the forced value. The user program uses the forced value in processing.
program with any forced value specified for the peripheral outputs. The forced value
appears at the physical output and is used by the process.
The clearing of the I memory area is not
affected by the Force function.
The initialization of the outputs values is
not affected by the Force function.
During the execution of the startup OBs,
the CPU applies the force value when
the user program accesses the physical
input.
After copying the state of the physical
inputs to I memory, the CPU applies the
force values.
The storing of interrupt events into the
queue is not affected.
The enabling of the writing to the
outputs is not affected.
RUN
① While writing Q memory to the physical
outputs, the CPU applies the force value as
the outputs are updated.
② After copying the state of the physical inputs
to I memory, the CPU applies the force
values.
③ During the execution of the user program
(program cycle OBs), the CPU applies the
force value when the user program accesses
the physical input.
④ Self-test diagnostics are not affected by the
Force function.
⑤ Handling of communication requests and the
processing of interrupts during any part of the
scan cycle is not affected.
Manual, 11/2009, A5E02486774-01
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