Filter
Top Products
100 Chapter 3
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);