Siemens S7-300 - Page 160

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S7-300 Automation System, Hardware and Installation: CPU 312IFM - 318-2 DP
11-2 A5E00203919-01
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The STEP 7 testing function with program status extends the CPU cycle time! In
STEP 7 you can customize the maximum permitted increase in cycle time (not for
CPU 318-2 DP). In this case, set process mode for the CPU parameters in
STEP 7.
• Single-step Mode
When testing in single-step mode, you can process your program instructions in
sequence (= single-step) and set break points. This is only possible in testing
mode and not in process mode.
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The Force function can be used to declare permanent values in specific variables
of a user program or CPU (also: inputs and outputs) which cannot be overwritten
subsequently by the user program.
For example, you can use it to jumper sensors or switch outputs permanently,
irrespective of the user program.
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This could result in severe injury or even death, and damage to property.
Incorrect use of the Force function could result in death or severe injury, and
damage to machinery or even the entire plant.
Always follow the safety instructions in the 67(3PDQXDOV.
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The forced values in the LQSXWVprocess image can be overwritten by write
commands (such as T EB x, = E x.y, copy with SFC, etc.) and by read I/O
commands (such as L PEW x) in the user program, or by write PG/OP function. 
2XWSXWV initialized with forced values only return the forced value if not accessed
by the user program via peripheral write instructions (e.g. TPQB x) or by PG/OP
write functions!
Always ensure that forced values in the I/O process image cannot be overwritten
by the user program or PG/OP functions!