To trigger acquisition on the 4472 from a DIO line on a 6731, you would need to follow recommendations similar to Sal's. You could route the DIO line to a RTSI line on the PXI backplane, and then set up the 4472 to trigger acquisition off that RTSI line. You could also use an external connection and wire the DIO line to the "External Trigger" input on the 4472, and configure it to trigger off that signal. Probably the most important thing for you to do is make sure that everything that you want to trigger off that DIO line is setup and waiting for it before the trigger occurs.
You can also use the example finder to find examples that will work with each individual device. In the bottom left-hand corner of the LabVIEW example finder, there is a button that you
can click on to select or add the different pieces of hardware you want examples for. After you have configured that correctly, if you limit results to that hardware (check box), the only examples that you will see will be work with that piece of hardware. There are numerous examples of triggering on a digital line for the 4472. Hopefully this get's you moving. Good luck!
Logan S.