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Automated Tasks, the use of scheduled execution of Scripting within your environment
 Introduction
Sometimes you simply do not need an application to stay open and continually
run, or stay open and sleep for the majority of time before doing anything. Sometimes you just need a small application that will run on a schedule.
The OPC Web Client, because it is so versatile, does give you this possibility! With a little bit of Windows Shell
scripting and the us of a scheduler, you can automate tasks quickly and easily, and this document will show you how...
Intended Audience
Scheduling
There are many scheduling applications out there. In a world of Windows computers, there is a scheduler already available that is often overlooked... "AT".
AT is a command-line utility that allows you to specify an application to run, and a time in which to run it. AT can
be used to schedule a script to run also as it is not distinguished any differently to an application.
AT can be used to schedule "something" to run once! or to run on a regular/cyclical basis.
It is beyond the scope of this paper to detail every feature of the AT command. Our advice is to read the Help on
this command or seek the documentation from Microsoft. For help, simply open a Command Prompt and type:
... followed by ENTER.
Once you have defined your application and schedules, you are set!
Using Scripts for Automation
Windows Shell Scripting (filenames are suffixed with .VBS extension) allows you to create scripts using VB.
There is no need for a development environment, no compilation and complication! Simply create a new text file and edit it using NotePad or any other ASCII text editor. Simply save the file with a ".VBS" extension and you
have just created a Windows Shell Script that you can run like a Batch-file/executable!
A Typical Case Scenarios for Automated Scripts
There are many use-cases where automated scripts can be of use. However, one use-case stands out as the one that is most commonly needed
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I need to be able to read a tag (or some tags) at midnight and to dump those values into a specific (or unique) text file.
After I have captured and logged this data, another application will read the values from that/those file(s).
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This would've been previously been accomplished via a dedicated application running all the time. The
application would be asleep the majority of the time only to wake-up at night and log the data to a file before going back to sleep. This is not needed anymore!
A Step-By-Step Example Of a Scripted Data Logging Application
Defining "the problem"
In this example we are going to log a single value of a Tag to a text file named "mydata.txt". This
application will need to run at midnight, every night.
Defining the Script
There are many scripts available in our source-code section.
Let's see how a script might look in our example. We are going to use the "Greenhouse" OPC Server that is installed with the OPC Web Client
. We are going to log a single line of data as follows:
TAG:<tagname>;VALUE:<value>;QUALITY:<quality>;TIMESTAMP:<timestamp>
Let's begin by first, reading our Tag's value:
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Option Explicit
Dim EasyDA
Set EasyDA = CreateObject("OPCLabs.EasyDA3.0")
Dim strOPCServer
strOPCServer = "OPCLabs.KitServer.2"
Dim strTag
strTag = "Simulation.Random"
Dim VTQ
Set VTQ = EasyDA.ReadItem("", strOPCServer, strTag)
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Const FOR_WRITING = 2
Dim strFileName
strFileName = "C:\OPC\MyData.txt"
Set objFSO = CreateObject("Scripting.FileSystemObject")
Set objFile = objFSO.OpenTextFile(strFileName, FOR_WRITING, True)
Dim strWriteLine
strWriteLine = "TAG:" & strTag & ";VALUE:" & VTQ.Value & ";TIMESTAMP:" & VTQ.TimeStamp
objFile.WriteLine(strWriteLine)
objFile.Close
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You can simply join the two tables together to form a single script file.
This script will make a connection to the OPC Server, read a Tag value, write it to a file and then close the file and exit the script.
What if the file is locked?
What if the OPC Server is not available?
What about error trapping?
These are questions that I'm not going to go into here. There are some very obvious things that you can
do, but you are at the mercy of VBScript's infamous:
On Error Resume Next
You do have to assume errors will arise, and you do have to plan for them. A pseudo paradigm for
handling errors in this scripted-example may be:
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On Error Resume Next
Make the connection to the OPC Server
If no error occurred then:
Try to read the Tag
If no error occurred then:
Try to open/create the text file
If no error occurred then:
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Creating the scheduled event
As mentioned previously, we are going to use the "At" command which is available in all fo the
NT-Based OS's. Simply open an MS-DOS/Command-Prompt window and type the following:
... followed by ENTER.
The following information should be shown:
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The AT command schedules commands and programs to run on a computer at
a specified time and date. The Schedule service must be running to use
the AT command.
AT [\\computername] [ [id] [/DELETE] | /DELETE [/YES]]
AT [\\computername] time [/INTERACTIVE]
[ /EVERY:date[,...] | /NEXT:date[,...]] "command"
\\computername Specifies a remote computer. Commands are scheduled on
local computer if this parameter is omitted.
id Is an identification number assigned to a scheduled
command.
/delete Cancels a scheduled command. If id is omitted, all the
scheduled commands on the computer are canceled.
/yes Used with cancel all jobs command when no further
confirmation is desired.
time Specifies the time when command is to run.
/interactive Allows the job to interact with the desktop of the user
who is logged on at the time the job runs.
/every:date[,...] Runs the command on each specified day(s) of the week o
month. If date is omitted, the current day of the month
is assumed.
/next:date[,...] Runs the specified command on the next occurrence of th
day (for example, next Thursday). If date is omitted,
current day of the month is assumed.
"command" Is the Windows NT command, or batch program to be run.
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We are going to specify that our script is going to run at midnight, every night!
Enter the following AT command:
AT 12:00:00am /every:Monday,Tuesday,Wednesday,Thursday,Friday MidnightRun.vbs
... then press ENTER.
Hopefully, "nothing will happen" meaning you will not see an error or the Help screen.
To see what scheduled events are defined, simply type in the following:
At
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Status ID Day Time Command Line
-------------------------------------------------------------------------------
1 Each M T W Th F 12:00 AM MidnightRun.vbs
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Summary
The purpose of this paper was to show you how to automate tasks using a little Windows Shell Scripting along
with the scheduling abilities of the Windows AT command.
Hopefully I have explained and shown how easy it is to use the OPC Web Client in a simple and versatile manner such as this.
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