I've seen a lot of suggestions that arduino (I'm using Uno rev3) can be used as a USB -> serial bridge by using the TX and RX pins with either a blank sketch, a sketch with TX and RX unused, or by grounding the reset pin and connecting RX and TX to the serial device's RX and TX pins. I've tried this and I get no response, but when using a different dongle with an FT232RL I get comms no problem. Any ideas what might be going wrong? As far as I can tell it should be a pretty straightfoward setup.
My connections are just RX, TX, and ground, but I've tried attaching the 3.3v pin as well (The other dongle has 3.3v output though I don't think it is used for anything anyways), but no success.
There isn't a lot going on schematic wise, the schematic would be: arduino TX(D1) -> device TX, arduino RX (D0)-> device RX, arduino GND -> device GND, and USB going between arduino uno and PC serial port. I had to give up my time on the machine for a bit so I can't give images but those are the connections. I did try swapping RX and TX pins just in case as well. In the instances where I was holding it in reset I just connected the arduino GND to the arduino reset.
Drawing on my vast experience with stupid blunders and as a test to take the "other device" out of the equation, I would try disconnecting that device, jumpering between the Arduino TX and RX to loop back the serial port, and see if it works with whatever terminal software you're using on the PC side.
If this doesn't work, you've got a wiring or Arduino side problem.
If it does work you're looking for a baud rate, TX vs RX connection, or something faulty on the "other device" problem.