Subtracting in Tag Logging

When you say it doesn't work, what exactly does not work ?? What does your current script look like ?

Have you read the relevant section in the manual ? I have included an extract below :

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Function for Conversion of Tag Values

Double Function Name (Double dLastValue,

Double dCurrentValue,

DWORD dwCount,

Boolean fArchivingCycle);

Description

This function may be used to convert a tag value prior to archiving. This function is assigned in the "Parameters" tab control in the "Process tag properties" dialog.

Parameter

dLastValue: last acquired value (from the last acquisition cycle)

dCurrentValue: currently acquired value

dwCount: current acquisition cycle since the last acquisition cycle (e.g., archiving cycle is 4 x 1 second acquisition cycle; resulting in a count of 1 to 4 until the count starts up again)

fArchivingCycle: TRUE = Cycle is archiving cycle (default); FALSE = Cycle is acquisition cycle

Application Example

For example, you may use this function to implement your own calculation of mean value. This will then carry not the actual tag value but the calculated mean value instead.

The following function will determine the geometric mean of the acquired process values:

double CalculateGeometricMean (double dLastValue,double dCurrentValue,DWORD dwCount,BOOL fArchivingCycle)

{

static double dCurrentProduct = 1.0;

double dReturnValue = 0.0;

if (!fArchivingCycle)

{

// 1. continiously calculate the product of all values with every scan cycle

dCurrentProduct *= dCurrentValue;

printf ("current value=%f current product=%f archiving cycle flag=%d\r\n",dCurrentValue,dCurrentProduct ,fArchivingCycle);

dReturnValue = dCurrentProduct ; // optinonally-the return value is actually not of interest in a scan cycle

}

else

{

// 2. if this is the archiving cycle, use the product of all scan values to calculate

// the geometric mean value

dReturnValue = pow (dCurrentProduct ,(double) 1 / dwCount);

printf ("archiving cycle: geometric mean value=%f\r\n",dReturnValue);

printf ("\r\n");

dCurrentProduct = 1.0; // reset current product

// the finally calculated value will be returned - this value will be archived

// since this is the archiving cycle

}

return dReturnValue;

}

Yes, it is... here's a sample of how to store information in .txt, in VB

Dim Preset1
Preset1 = ReadValue("Tagname")
Set fs = CreateObject("Scripting.FileSystemObject")
Set A = fs.CreateTextFile("c:\preset.txt", True)
A.WriteLine (Preset1)
A.Close

That one creates a file c:\preset.txt, and inserts a line with the tag.
To read the data from a .txt, also in VB

Dim Preset1
Open "c:\preset.txt" For Input As #1
Line Input #1, Preset1
Close #1


This one opens the file c:\preset.txt, reads a line from it and stores the value from that line in the tag "Preset1".
What I'd do is create files with names in the format yyMMdd.txt.

Does that help?

Dim Today, FileName Today = Now ' Assign current system date and time. 

 
Today = Format(Today, "yyMMdd") 'Formats the tag "Today", to yyMMdd








FileName = "c:\" & Today & ".txt" ' If today is 20/09/06, FileName = c:\060920.txt
If FileExists(FileName) Then ' This one's to check if your file exists