Hi Tony,
From what I understand, you are trying to make your modules bidirectional so that they can accept both input and output, thereby preventing you having to do writes every time you want to do a read. The 9401 is a bidirectional module and be configured for bidirectional line measurements using four-bit nibble. You can have all input, all ouput or one nibble as input and the others as ouput--basically combine them whichever way you want. Please refer to
this knowledgebase article on how to configure the nibble bits. Hope this helps.
Ipshita C.
National Instruments
Applications Engineer