Technical data

36 Programmer’s Guide
4 Programming Example
• 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.
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 (resource name)
parameter to open a device session. The vi parameter in viOpen
returns a session identifier that can be used with other VTL functions.
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.