There have been several discussions on this here, try to search the archives and you should find some good info. Also search NI's website, they have some good info there as well.
To get you started try,
http://zone.ni.com/devzone/conceptd.nsf/2d17d611efb58b22862567a9006ffe76/0989d3d9dafae64e8625680400679736?OpenDocument
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/f4146c66990c46388625695f00726581?OpenDocument
http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/f3c4ee6ee2c23ef38625646b007882c8?OpenDocument
http://www.advmeas.com/goodies/parallelport.html
These are a few links that should get you started. I think what you'll find is that you can use VISA for writes to port only. To get bi-directional commu
nication you will have to use the in/out port functions.
I hope this helps point you in the right direction.
Brian