Hello WaltF,
I am not sure why you receive an error when using "built-in" as your CJC option. It simply might be that the driver doesn't support that kind of configuration for the particular pieces of hardware you are using.
I did find a KnowledgeBase article linked below that gives a step-by-step process for setting up the CJC when using the BNC 2120. It sounds like it might be a scaling issue, but go ahead and take a look at it, hopefully it will help.
Take care,
E.Lee
http://digital.ni.com/public.nsf/websearch/E45ADD30F609AEC4862569CF00588DC9?OpenDocument
Eric
DE For Life!