Technical data

Testing with the Variable Table
20.8 Forcing Variables
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 465
20.8.3 Differences Between Forcing and Modifying Variables
The following table summarizes the differences between forcing and modifying:
Feature / Function Forcing with
S7-400 (incl.
CPU 318-2DP)
Forcing with
S7-300 (without
CPU 318-2DP)
Modify
Bit memory (M)
yes
−
yes
Timers and counters (T, C) − −
yes
Data blocks (DB) − −
yes
Peripheral inputs (PIB, PIW, PID)
yes
− −
Peripheral outputs (PQB, PQW, PQD)
yes
−
yes
Inputs and outputs (I, Q)
yes yes yes
User program can overwrite the modify/force
values
−
yes yes
Replacing the force value effective without
interruption
yes yes
−
The variables retain their values when the
application is exited
yes yes
−
The variables retain their values after the
connection to the CPU is broken
yes yes
−
Addressing errors permitted:
e.g. IW1 modify/force value: 1
IW1 modify/force value: 0
− − The last becomes
effective
Setting triggers Always trigger
immediately
always trigger
immediately
once or every cycle
Function only affects variable in visible area
of active window
Affects all force
values
affects all force
values
yes
Note
1. With "Enable Peripheral Outputs," the force values for forced peripheral outputs become effective
on the corresponding output modules; the modify values for peripheral outputs, however, do not.
2. With forcing, the variable always has the forced value. This value is read during each read access
to the user program. All forms of write access are ineffective.
3. With permanent modifying, read access to the program is effective and remains so until the next
trigger point.