The NI PCI-6509 is a completely software timed DIO card. This
means that the setup time and acquisition time for a digital input
operation are dependant on how fast your computer can execute software
commands. There is no internal sample clock on these boards to be
used to clock a digital input or output operation. The time
required to perform consecutive digital reads using the DAQmxReadDigitalLines() in a loop will depend
on how fast your system can process and send commands to the NI
PCI-6509. For clocked digital input and output operations, you
will need a High Speed Digital board (653x family) or M-series board.
For more information on the difference between software and hardware
timing, check out the section in the NI-DAQmx Help about timing. (NI-DAQmx Key Concepts >> Timing and Triggering >> Timing).