Opening A Session; Device Sessions - Agilent Technologies ESA-E Series Programmer's Manual

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.
• A device session is used to communicate with a device on an
All devices that you will be using need to be connected and in working
NOTE
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:
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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.
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.
Chapter 3