Hello,
Check your external connections detailed in the user manual of the NI 6515 (see the link below):
* Digital I/O Help for 6509/651x/6528 Devices
http://digital.ni.com/manuals.nsf/websearch/A448429253021B5A86256E85006A123D?OpenDocument&node=132100_US
Make sure that P<4..7>.COM (VCC) is connected to the positive pole of the power supply and that P<4..7>.GND is connected to the negative pole of the power supply. Failure to do so could permanently damage the 6515 device and the power supply if it is not protected.
Sincerely.
Matthieu Gourssies
National Instruments