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118 Chapter 2
Programming Fundamentals
C Programming Using VTL
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);
The session returned from viOpenDefaultRM must be used in the sesn
parameter of the viOpen function. The viOpen function then uses that
session and the device address specified in the rsrcName parameter to
open a device session. The vi parameter in viOpen returns a session
identifier that can be used with other VTL functions.
Your program may have several sessions open at the same time by
creating multiple session identifiers by calling the viOpen function
multiple times.
The following summarizes the parameters in the previous function
calls:
sesn This is a session returned from the viOpenDefaultRM
function that identifies the resource manager session.
rsrcName This is a unique symbolic name of the device (device
address).
accessMode This parameter is not used for VTL. Use VI_NULL.
timeout This parameter is not used for VTL. Use VI_NULL.