The Differences Between Forcing And Controlling Variables; Principle Of Forcing With S7-300 Cpus (All Cpus Except 318-2 Dp) - Siemens SIMATIC S7-300 Hardware And Installation Manual

With S7-300-CPUs, forcing is the same as "cyclical modify"
PIQ
transfer
Execute force
job for outputs
OS: operating system execution
Figure 11-1 Principle of Forcing with S7-300 CPUs (all CPUs except 318-2 DP)

The differences between forcing and controlling variables

Table 11-1
Characteristics/Function
Memory bit (M)
Timers and counters (T, C)
Data Blocks (DB)
Inputs and outputs (I, Q)
Peripheral Inputs (PI)
Peripheral Outputs (PO)
User program can overwrite
control/force values
Maximum number of force
values
Cross-reference
Details on test functions of the software are found in the STEP 7 Online Help and
in the STEP 7 Programming Manual.
S7-300 Programmable Controller Hardware and Installation
A5E00105492-02
Testing functions, Diagnostics and Fault Elimination
Execute force
job for inputs
PII
User program
OS
transfer
Forced value
The differences between forcing and controlling variables
Forcing with
CPU 318-2 DP
and S7-400
Yes
-
-
Yes
Yes
Yes
-
256
PIQ
transfer
Forced value
overwritten by T
PQW!
T PQW Execute force
job for outputs
Forcing with
S7-300
without 318-
2 DP
-
-
-
Yes
-
-
Yes
10
Execute force
job for inputs
PII
OS
transfer
Forced value
Controlling
Variables
Yes
Yes
Yes
Yes
-
Yes
Yes
-
11-3