Yesterday I was configuring a usb to serial converter to communicate with a CNC controller (over RS232). However I was unable to transmit any data and so I brought the oscilloscope to see what was happening. I disconnected my converter and connected the oscilloscope and all the data was there and my oscilloscope had no problems decoding it with the same parameters i had used with my usb converter. Confused, I disconnected my scope and connected my converter back up again, but sill nothing. After a few times back an fourth I ended up connecting my scope up while the converter was still connected and crucially I also had a cat /dev/ttyUSB0 running and as expected the data was right there at the scope, but to my big surprise it was also there in my terminal! I then tried to disconnect my scope resulting in no more data being sent to my laptop.

The only explanation I could think of was a faulty ground connection. The scope was grounded through the wall socket same as the CNC and thus had the same ground potential. My laptop on the other hand was running on battery and only shared ground with the CNC through RS232 pin 5 (signal ground). However it seems this pin was not correctly connected on the CNC side and thus I was unable to transmit any data. Experimenting a bit I tried to connect ground to the connector shield instead and everything worked perfectly.