When variables are configured and used with in an ST program they are, by default, cached prior to the program execution.
You can use the NOCACHE keyword in your Logic programs to prevent the program from using a cached value. This is required when you need a value to accumulate each time a program executes, for example, when a total count value needs to increase when an input value changes.
If the NOCACHE keyword is not used, the ST program uses the value that is in place when the program is first executed. This means that when a series of values are reported for one of the ST program's inputs, the value of the variable that is affected by the inputs will be the same.
NOCACHE and SQL queries
The use of NOCACHE is particularly useful when used with SQL queries that are executed from within an ST program to ensure that the query is executed when program is executed under a read lock rather than a write lock when the program is triggered.
Example:
In the example below, the NOCACHE keyword is used to prevent the program from calculating the Total value as 0.0 + the current value of the Value variable. So, if the program receives 10 values for the Value variable, the program will use the default value of 0.0 for the Total value on the first execution, but will not use 0.0 on subsequent executions. If the NOCACHE keyword was not used, the program would use the 0.0 value for the Total value for each execution.
PROGRAM TotalValues
VAR
FirstTime:BOOL := TRUE;
END_VAR
VAR
Value AT %M(Input Point Value): REAL;
END_VAR
VAR NOCACHE
Total AT %M(Total Point): REAL;
END_VAR;
IF FirstTime THEN
Total := 0.0;
FirstTime := FALSE;
END_IF;
Total := Total+Value;
END_PROGRAM
In this example, the FirstTime value is a Boolean value that is used to define whether the program is being executed for the first time. By default, it is set to TRUE.
VAR
FirstTime:BOOL := TRUE;
END_VAR
The Value variable is a real value that the program reads from the Input Point Value database item.
VAR
Value AT %M(Input Point Value): REAL;
END_VAR
The Total value is a NOCACHE value (it is not cached and is retrieved at the time that the program executes). The Total value is a real value that is calculated by the program each time it executes. The result of the calculation is written to the Total Point database item.
VAR NOCACHE
Total AT %M(Total Point): REAL;
END_VAR;
To define a NOCACHE value, you have to create a separate VAR list for the values to which the NOCACHE feature applies. The VAR list has to be started as VAR NOCACHE. The values need to be entered beneath the VAR NOCACHE definition in the same way as with normal VAR lists, and the VAR list needs to be closed with END_VAR.
The If statement determines whether the Total value is 0 or is another value. If the FirstTime value is TRUE then the Total value is 0.0 (the FirstTime value is only TRUE the first time that the program executes). If the FirstTime value is FALSE, the FALSE value is maintained and the Total value is the current Total value + the value of the Value variable.
This means that the first time the program executes, the Total value is 0.0. Before the first execution of the program completes, it sets the FirstTime value to FALSE so that the next execution of the program will cause the Total value to change from 0.0. At the end of the first execution of the program, the Total value is set to the same value as the Value variable (Total=0.0 + Value = Same value as Value variable).
On subsequent executions of the program, the FirstTime value will be FALSE and so the Total will be the current Total value + the value of the Value variable. The program will not use 0.0 as the Total value each time, as the 0.0 is not cached.