National Instruments NI USB-621x - Manual (Page 132)

Chapter 13 Bus Interface
NI USB-621x User Manual 13-2 ni.com
Programmed I/O
Programmed I/O is a data transfer mechanism where the user’s program is
responsible for transferring data. Each read or write call in the program
initiates the transfer of data. Programmed I/O is typically used in
software-timed (on-demand) operations. Refer to the
Software-Timed
Generations section of Chapter 7, Analog Output , for more information.
Changing Data Transfer Methods
USB M Series devices have four dedicated USB Signal Stream channels.
To change your data transfer mechanism between USB Signal Streams and
programmed I/O, use the Data Transfer Mechanism property node
function in NI-DAQmx.