Voltage supply ? Problem

I have a BV502
it is working 100% ok if I supply 5V+ from the Usb to serial interface to the pin next to tx on the BV502.
However if I disconnect this supply and supply 5V from an external supply to the +ve pin on top row thalthough the board led is lit but I cannot communicate with the 502 from the computer ( no yellow writting at all )!!
There is continutity between the the V+ pins and and both G pins

I think you will find that the ground has not been considered. The serial still needs a ground to work, if that is coming form the external supply then it needs to be connected to the external ground.