Using NI-VISA to Handle Events from a PXI/PCI Device

Often, PXI/PCI devices use interrupts to request service from the system. VISA handles an interrupt from a device using its event-handling model. There are two mechanisms for handling events in the NI-VISA event-handling model—the queuing mechanism and the callback mechanism.

Queuing Mechanism

You can use the queuing mechanism in LabVIEW and LabWindows/CVI. With this technique, all occurrences of a specified event are placed in a queue. Your program can periodically poll the queue for event information or pause the program until the event has occurred and has been placed in the queue. The queuing mechanism is generally useful for noncritical events that do not need immediate attention, because you must explicitly poll for the occurrence of an event.

When using the queuing event-handling mechanism, you must manually poll the event queue to determine which events have occurred. The LabVIEW function to do this is VISA Wait on Event, and the corresponding LabWindows/CVI function is viWaitOnEvent(). When using these functions, a program waits for a specified amount of time for the event to be placed in the VISA event queue. The program pauses in this function until the event occurs or the function times out. When the specified event occurs, specific information about the event is passed back via this function.

Callback Mechanism

The callback mechanism is available in LabWindows/CVI but not in LabVIEW. This technique involves having a section of code called automatically by the VISA driver whenever a particular event occurs. The function invoked if a particular event occurs is called a callback function. The callback mechanism is useful when your application requires an immediate response. It is possible to use both queuing and callbacks in the same application.

When using callbacks, you must associate an interrupt-handling function with a particular event before you enable events using the viEnableEvent() function. The function you use to associate a handler with an event is viInstallHandler(). After calling this function and then enabling events, the function you have specified using viInstallHandler() is called asynchronously when the interrupt occurs.

Event Functions

There are a few important functions to be familiar with when using events in VISA. The first two functions enable or disable the event handling mechanism in NI-VISA. The LabVIEW function VISA Enable Event and the LabWindows/CVI viEnableEvent() function tell the VISA driver to begin waiting for a particular event. The call to enable a VISA event must specify the VISA resource to monitor for events, the type of event you want to acknowledge, and which event-handling mechanism to use. In the case of PXI/PCI devices, the type of event you enable is the VI_EVENT_PXI_INTR. This event tells VISA to place events from that particular PXI/PCI device in the queue or to use a specified callback function when the event occurs. The LabVIEW function VISA Disable Event and the LabWindows/CVI viDisableEvent() function tell the VISA driver to stop handling the specified type of events from the specified device. For more detailed information about setting up and handling PXI/PCI interrupts, refer to Events.