I have a board that's powered from my bench supply. When I connect the GND, RXD, and TXD lines of my FTDI USB-to-serial cable directly to my circuit, it behaves incorrectly. I assume this is because there's a voltage drop between USB ground and my power supply's ground.

I can't power the entire circuit off USB because it uses too much current. I can't leave the cable's ground line unconnected because no output shows up in my PC's terminal program. However, if I connect the USB ground line to the board's ground through a 1K resistor, everything seems to work fine. Not sure if this is the right thing to do; what's the proper way to connect two separate grounds like this?

 

Post a schematic of how things are connected..... you may be tapping off of the wrong "ground" on one of the devices, a common ground is necessary, but I have not heard of anyone having issues with it.... are you using the -D off of the USB chip to connect to your circuits ground?? Is this a DC ground on your circuit or AC (earth ground).... only a schematic can clear up a lot of questions....

B. Morse