Programming Example
Programming in C using the VTL
36
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.
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.
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.
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