This example describes the steps required to access the COM object from Visual Basic. Building a simple project will be described to illustrate this process. The Vektrex Scope driver will be used as an example.

Step 1: Making the COM Object Available to the Project

Once a new project is created, select References from the Project Menu. A dialog similar to that shown below will appear. This box contains a list of the registered COM objects on the computer. Select the IVI Vektrex Scope object and press OK.

Step 2: Creating an Instance of the Object

At the top of the Visual Basic Code window type the line shown in the diagram below. The diagram shows Visual Basic’s Intellisense in action, once the space after New is pressed a list of possible options appears. Select VektrexScope. As well as creating the object, this line of code sets MyScope to be a reference to the default Interface inside the object – in this case the top-level interface of the instrument specific functions.

Step 3: The Form_Load Procedure

When a Visual Basic program is run, the form_load procedure is executed first, so this is an ideal place to place the call to the driver’s Initialize function. The diagram below shows this code, and also shows the user starting to enter the code that will run the Initialize function (at the bottom of the Intellisense window).

The next diagram shows the help “tooltip” that is displayed when the spacebar is pressed after Initialize.

The finished Form_Load procedure is shown below:

If more help is needed to complete the function, click on the word Initialize and press the F1 key, this will open the driver’s help file to the appropriate place.

Step 4: Coding a Function

The diagram below shows a completed Visual Basic procedure that uses a property in the Instrument Specific Acquisition Interface. Once again, Intellisense will help with each step, including listing options for any enumerated types. This code assumes that on the Visual Basic form is a button named Get Number of Averages for the user to press and a text box named Averages where the returned value will be displayed. This code snippet also shows how the procedures perform error handling  – if an error occurs, an error handler routine will be called.

Step 5: Tidying Up at the end of the Program

The code snippet below is run when the user presses a button on the form named Close.

Here the COM object’s Close function is run, the references are released and the program is stopped.

Step 6: Error Handler

If an error occurs inside the COM object, information about what happened is passed back to Visual Basic inside an error object. This simple error handler causes a dialog box to appear with the error number and description. When the user clears the dialog box, program execution continues. 

Discussion

This example has shown how to create a client application in Visual Basic using the instrument specific interfaces. Note that this application is not using any of the interchangeability features (it is not using the compliant interfaces and it references a particular driver, VektrexScope, directly).