Armand,
Please reference the User Manual for the PCI-6534. Specifically, pages 2-10 and 2-22 in the Continuous Output sections explain how to leverage the onboard memory using either LabVIEW or the NI-DAQ C interface. For LabVIEW you will set the Pattern Generation Loop Enable attribute to ON in the DIO Parameter VI.
653X User Manual
http://digital.ni.com/manuals.nsf/websearch/EDE443F5C700373A862569B90072D26F?OpenDocument&node=4575_US
Regards,
Justin Britten
Applications Engineer
National Instruments