Agilent Technologies Model A.08.xx manual Opening a Session, Device Sessions

Programming Examples

C Programming Examples using VTL

The visatype.h header file defines most of the VISA types. The VISA types are used throughout VTL to specify data types used in the functions. For example, the viOpenDefaultRM function requires a pointer to a parameter of type ViSession. If you find ViSession in the visatype.h header file, you will find that ViSession is eventually typed as an unsigned long.

Opening a Session

A session is a channel of communication. Sessions must first be opened on the default resource manager, and then for each device you will be using. The following is a summary of sessions that can be opened:

•A resource manager session is used to initialize the VISA system. It is a parent session that knows about all the opened sessions. A resource manager session must be opened before any other session can be opened.

•A device session is used to communicate with a device on an interface. A device session must be opened for each device you will be using. When you use a device session you can communicate without worrying about the type of interface to which it is connected. This insulation makes applications more robust and portable across interfaces. Typically a device is an instrument, but could be a computer, a plotter, or a printer.

NOTE	All devices that you will be using need to be connected and in working condition
	prior to the first VTL function call (viOpenDefaultRM). The system is
	configured only on the first viOpenDefaultRM per process. Therefore, if
	viOpenDefaultRM is called without devices connected and then called again
	when devices are connected, the devices will not be recognized. You must close
	ALL resource manager sessions and re-open with all devices connected and in
	working condition.

Device Sessions

There are two parts to opening a communications session with a specific device. First you must open a session to the default resource manager with the viOpenDefaultRM function. The first call to this function initializes the default resource manager and returns a session to that resource manager session. You only need to open the default manager session once. However, subsequent calls to viOpenDefaultRM returns a session to a unique session to the same default resource manager resource.

Next, you open a session with a specific device with the viOpen function. This function uses the session returned from viOpenDefaultRM and returns its own session to identify the device session. The following shows the function syntax:

viOpenDefaultRM (sesn);

viOpen (sesn, rsrcName, accessMode, timeout, vi);

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