PreserveVariables (Preserve Variables)
The PreserveVariables instruction is used to retain in memory the values for all variables declared by the Dim or Public statements.
Syntax
PreserveVariables
Example
In the following example program, the PreserveVariables instruction is placed at the beginning of the program. All variables will reflect the last known value if the datalogger experiences a power loss or the program is stopped using the Stop and Retain Data option.
The program can be tested by manually typing in values for Test() or setting Flag(4) high (which, in turn, sets high a control port). If power is removed from the datalogger and reapplied, the variables are reinitialized to the last known value.
PreserveVariables
Public RefTemp, TCTemp(2)
Public Flag(4) As Boolean, PortHigh AS Boolean
DataTable (TCTemp,True,-1)
DataInterval (0,1,Min,10)
Average (2,TCTemp(),FP2,False)
Maximum (2,TCTemp(),FP2,False,True)
Minimum (2,TCTemp(),FP2,False,True)
EndTable
BeginProg
Scan (1,Sec,3,0)
TCDiff (TCTemp,1,mv200C,U1,TypeT,RefTemp,True,0,15000,1.0,0)
CallTable (TCTemp)
If Flag(4) Then
PortHigh=True
Else
PortHigh=False
EndIf
PortSet (C1,PortHigh )
NextScan
EndProg
Remarks
The PreserveVariables instruction is placed in the declarations section of the program prior to the BeginProg instruction. When PreserveVariables is present in a program, the values of variables will not be reinitialized after a recompile (due to power loss, Constant Table change, File Control restart, etc.), provided there are no changes to the declaration of variables (Public, Dim, local variables in functions), or to data table fields. Hence, if a program is run with the same variables as the previous running program, variables’ values are preserved if the program is run with the option to retain data (Do not erase data files). If the Erase data files option is chosen, the variables are reinitialized.
NOTE: PreserveVariables preserves values that have been written to variables on the main scan interval. It does not preserve intermediate values that are held in memory, such as the temporary values used to calculate an average over an output interval.