5. Initial Programming
253
In case the CPU is initialized without the stored application, it automatically goes to Stop Mode, as it
happens when a software exception occurs.
Monitoring, Writing and Forcing Variables
After Logging into a CPU, the user can monitor the project variables in 3 different ways:
Right in the POU (in the declaration area of the variables or in the program area)
Through the monitoring window (View menu -> Watch)
Via Trace functionality, which writes and shows the values of the variables internally in the
CPU.
ATTENTION:
In online mode, there is a limit of 25.000 inputs for monitorable variables in POUs edited with ST
Editor. The user will get a compile error message notifying the limit is exceeded.
In addition, the user can also write or force values in project variables.
The writing command (
CTRL + F7
) writes a value into a variable that can be overwritten by
instructions executed in the application in its next cycle.
On the other hand, the force writing command (
F7
) writes a value into a variable without allowing it
to be changed until the forced variables are released.
ATTENTION:
The variables forcing can be done in the CPU only in the Online mode.
Diagnostic variables cannot be forced, only written, because the CPU provides diagnostics that
should be able to be overwritten.
When a forced writing is done into a redundant variable of the Active CPU, the MainTask execution
time will be impacted, in both Active and Stand-by CPUs. This occurs because both CPUs will
exchange information about the forced variables in each cycle. Therefore, when forcing variables in a
redundant system, the user should consider the time added to the task execution time. Table 5-7
exemplifies the medium execution time added to the MainTask in this case:
Active CPU
Stand-by CPU
Execution Time
50 ms
100 ms
200 ms
50 ms
100 ms 200 ms
Increase with 10 forcing
2.4 %
2.2 %
1.7 %
4.0 %
3.4 %
2.0 %
Increase with 50 forcing
12.0 %
9.2 %
6.0 %
18.0 %
12.0 %
8.0 %
Increase with 128 forcing
26.0 % 21.0 %
16.0 %
56.0 %
34.0 %
22.5 %
Table 5-7. The Influence of the Variables Forcing in a Redundant PLC
For further information about variable forcing in a redundant system, see Redundant Data
Synchronization.
ATTENTION:
When a CPU presents forced variables and it is de-energized, the variables will lose the forcing in
the next initialization.
The limit of forcing for the Hadron Xtorm CPUs is 128 variables, regardless of the CPU model and
its configuration.