User`s guide

34 Agilent VISA User’s Guide
3 Programming with VISA
Opening a Session
A session is a channel of communication. Sessions must first
be opened on the default resource manager, and then for
each resource you will be using.
• 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 resource session is used to communicate with a
resource on an interface. A session must be opened for
each resource you will be using. When you use a 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.
Resource Manager Sessions
There are two parts to opening a communications session
with a specific resource. 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 return a unique session to the same
default resource manager resource.
Resource Sessions
Next, open a session with a specific resource using the
viOpen function. This function uses the session returned
from viOpenDefaultRM and returns its own session to
identify the resource session. The following shows the
function syntax.
viOpenDefaultRM(sesn);
viOpen(sesn, rsrcName, accessMode, timeout,
vi);