Have you tried to do a serial loopback test on the cables to see if the cable is the problem?
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000YFtECAW&l=en-US
Has this board ever worked before? Have these instruments ever worked before? This sounds like there is a hardware problem somewhere, you will just need to narrow down exactly where.
Maggie M.
Application Engineer
National Instruments.
http://www.ni.com/support