This is likely due to the fact that the 6509 is port-configurable, rather than line-configurable. For your channel string, try "Dev1/Port11/Line0:7." If this does not help, let us know what you mean by "that did not work." Did DAQmx return an error? Was the terminal not in a high impedance state?
Hope this helps,
Ryan Verret Product Marketing Engineer Signal Generators National Instruments