Siemens Simatic S7-300 - Principle of Forcing with S7-300 Cpus (All Cpus Except 318-2 DP)

Testing functions, Diagnostics and Fault Elimination
S7-300 Programmable Controller Hardware and Installation
A5E00105492-01
10-3
PIQ
transfer
Execute force
job for inputs
Execute force
job for inputs
T PQW
OS
PII
transfer
User program
Forced value
Forced value
overwritten by T
PQW!
With S7-300-CPUs, forcing is the same as “cyclical modify“
OS: operating system execution
PIQ
transfer
OS
PII
transfer
Forced value
Execute force
job for outputs
Execute force
job for outputs
Figure 10-1 Principle of Forcing with S7-300 CPUs (all CPUs except 318-2 DP)
Differences between Forcing and Controlling of Variables
Table 10-1 Differences between Forcing and Controlling of Variables
Characteristics/Function Forcing with
CPU 318-2 DP
and S7-400
Forcing with
S7-300
without 318-2 DP
Controlling
Variables
Memory bit (M) Yes - Yes
Timers and counters (T, C) - - Yes
Data Blocks (DB) - - Yes
Inputs and outputs (I, Q) Yes Yes Yes
Peripheral Inputs (PI) Yes - -
Peripheral Outputs (PO) Yes - Yes
User program can overwrite
control/force values
-YesYes
Maximum number of force
values
256 10 -
Cross-reference
Details on test functions of the software are found in the STEP 7 Online Help and
in the STEP 7 Programming Manual.