Serial port shows activity when not in use (PXI 8106)

[keptat]

I have an application that uses the serial port to communicate with my external device, but when my application is NOT RUNNING my external device detects activity in the serial port (Tx). As soon as my application (or any other application) uses the serial port, it behaves properly.
 
This happens in any of my PXI 8106 and their integrated serial ports, this behavior does not occur on other computers or with USB-to-serial adapters (in the same PXIs)
 
My external devices wake up continiously by interruptions, and cause them to overwork

How can I configure the serial port when NOT IN USE and prevent Tx from sending pulses?

[Henrik_Volkers]

just a guess:

Maybe the port is tri stated (switch to high impedance) when not in use. So you dev. input catch noise over the cable? 

If that is possible, maybe some 10k from GND to the inputs (and a propperly shielded cable) help.