The Principle Of Forcing With S7-300 Cpus (Cpu 312Ifm To 316-2Dp) - Siemens SIMATIC S7-300 Series Reference Manual

Forcing with the CPU 312 IFM to 316-2 DP:
Caution
!
Forced values in the input process image can be overwritten by write instructions
(e.g. T EB x, = E x.y, copying with SFC etc.) and peripheral read instructions (e.g.
L PEW x) in the user program, as well as by write instructions of PG/OP opera-
tions!
Outputs initialized with forced values only return the forced value if the user pro-
gram does not execute any write accesses to the outputs using peripheral write
commands (e.g. TPQB x ) and if no PG/OP functions write to these outputs!
Always note that forced values in the I/O process image cannot be overwritten by
the user program or PG/OP functions!
With S7-300 CPUs, forcing is the same as "cyclical modify"
PIO
OS
transfer
Forced
value
Execute force
job for outputs
OS .... Operating system execution
Figure 1-4 The Principle of Forcing with S7-300 CPUs (CPU 312IFM to 316-2DP)
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
Execute force
job for inputs
PII
User program
transfer
Forced value
overwritten by T
PQW!
T PQW
CPUs
Execute force
job for inputs
PIO
PII
OS
transfer
transfer
Forced
value
Execute force
job for outputs
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