Hello DMcAndrew,
Typically for common handshaking type applications (burst, 8255), the acq and req lines are controlled automatically. However it seems like you would like to control these lines yourself.
It is possible for you to control acq with a software timed output. I have attached an example that does this with DAQmx. Since the control of the acq/pfi line is software timed, it would be difficult to ensure that the acquisition task does not start before the generation task (pfi2) goes low without missing a trigger. Perhaps you could wire the output from pfi2 and your req signal back to a digital lines where a pattern match for a start trigger would wait for acq and req signals to go low.
If you are just starting a new application and you are using the PXI or PCI 6533 I would recommend that you upgrade to DAQmx. To see the different names in DAQmx there is a good online document
here. There is also a new help file for DAQmx
here.
I hope this helps.
Jesse O.
Applications Engineering
National Instruments
Jesse O. | National Instruments R&D