Handling Events Example Discussion

Programming with events presents some new functions to use. The first two functions you notice are viEnableEvent() and viDisableEvent(). These functions tell the VISA driver which events to listen for—in this case VI_EVENT_VXI_SIGP, which covers both VXI interrupts and VXI signals. In addition, these functions tell the driver how to handle events when they occur. In this example, the driver is instructed to queue (VI_QUEUE) the events until asked for them. Notice that instr is also supplied to the functions, since VISA performs event handling on a per-communication-channel basis.

Once the driver is ready to handle events, you are free to write code that will result in an event being generated. In the example above, this is shown as a comment block because the exact code depends on the device. After you have set the device up to interrupt, the program must wait for the interrupt. This is accomplished by the viWaitOnEvent() function. Here you specify what events you are waiting for and how long you want to wait. The program then blocks, and that thread performs no other functions, until the event occurs. Therefore, after the viWaitOnEvent() call returns, either it has timed out (5 s in the above example) or it has caught the interrupt. After some error checking to determine whether it was successful, you can obtain information from the event through viGetAttribute(). When you are finished with the event data structure (eventData), destroy it by calling viClose() on it. You can now continue with the program and retrieve the data. The rest of the program is the same as the previous examples.

Notice the difference in the way you can shut down the program if a timeout has occurred. You do not need to close the communication channel with the device, but only with the VISA driver. You can do this because when a driver channel (defaultRM) is closed, the VISA driver closes all I/O channels opened with it. So when you need to shut down a program quickly, as in the case of an error, you can simply close the channel to the driver and VISA handles the rest for you. However, VISA does not clean up anything not associated with VISA, such as memory you have allocated. You are still responsible for those items.