Why would I get less than 5 volts from pin 49 on my USB-6509 DIO board when I connect it to my project board?

[ronbtom]

I can do the separate power supply for the circuit and the USB-6509 to control the DIO pins with my LabVIEW program.  That configuration works.  does that mean there is a problem with my New USB-6509 DIO box?  I really need the DIO box to supply the power as well.  this system will be a field deployable test unit.  To include a separate power supply for the 5 volts for each deployable unit complicates things. 

[alcava.cr]

You get some power from the lines when using the external power supply so that means the fuse is not broken (although it will be a good idea to check its integrity). When you mix things the issue appears, I'm thinking on a ground loop or, as previously mentioned, the circuit is consuming more than what you think. Either case, it is always a good practice to keep everything isolated.

[ronbtom]

alcava,  I will take a look at the fuse, but figure it probably is okay since I do get 5 Volts through cable.  However, my problem is not when I mix things.  I have the problem when I am using just the USB-6509 to supply the 5 volts from pin 49 and the other DIO lines to set switches.  So, the 6509 does produce 5 volts on pin 49.  I can connect it to the NI green breakout board and still have 5 volts on pin 49.  Once I connect a lead from the breakout board to my circuit board the voltage drops to 3.34.  I checked my circuit and it is drawing < 60ma.  I am currently checking my circuit for a floating ground possibility.  If there are no ground problems, then is there a good chance that my 6509 is bad?  I purchased it about 6 months ago.  I really would like to make this work with just the 6509.  This is going to be used in the field.  Having to add a bench power supply to provide the constant 5 volts makes it difficult to send it around to different locations.  Thank  you  for your assistance in helping to resolve this issue.

[ronbtom]

The problem is solved!  215 mA from the USB port just doesn't provide enough current to keep the voltage at 5 volts.  We tried it again the the 6509 external power supply with the same results.  But, there is a proper process to using the external power supply.  It is necessary to connect the power supply to the 6509 and then connect the USB cable for control.  Now it works!  When the USB port is connected to the 6509 and the computer first, the 6509 will not internally switch from USB power to the external power.

Thanks to everyone that provided suggestions in an effort to resolve this issue.