Siemens SIMATIC S7-1200 Manual page 263

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.
Note
Digital I/O points assigned to HSC, PWM, and PTO cannot be forced
The digital I/O points used by the high-speed counter (HSC), pulse-width modulation (PWM),
and pulse-train output (PTO) devices are assigned during device configuration. When digital
I/O point addresses are assigned to these devices, the values of the assigned I/O point
addresses cannot be modified by the force function of the force table.
Startup
A
B
C
D
E
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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.
The storing of interrupt events into the
queue is not affected.
The enabling of the writing to the
outputs is not affected.
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①
While writing Q memory to the physical
outputs, the CPU applies the force value as
the outputs are updated.
②
When reading the physical inputs, the CPU
applies the force values just prior to copying
the inputs into I memory.
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 or writes the physical
output.
④
Handling of communication requests and self-
test diagnostics are not affected by the Force
function.
⑤
The processing of interrupts during any part of
the scan cycle is not affected.
11.6 Using the force table
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