Using MIDI with Arduino, will only send when USB plugged in

Hi All,

My first post here, and I am looking for a little help with a project I am doing. I am using my Arduino UNO to send MIDI out through the serial TX pin, as a result of some inputs. I have serial data being sent out, but I also have an LED lighting up whenever I send out serial data. This is what happens (sorry I can't post my code, its sort of a complicated function and I would rather not):

With USB plugged in: The arduino exhibits the correct response, which is to say that the "TX" LED lights up at the same time that the LED on pin 13 does (thats the pin I attached it to).

With USB uplugged, external power on: The "TX" pin no longer lights up, but the LED on pin 13 does, when it is supposed to.

So: what's the deal? Is the arduino still sending out serial data, just not telling me any more? Unfortunately, I just noticed the MIDI to USB converter I was using has a huge black mark on the cable... I am assuming I connected it incorrectly at some stage and burned it somehow, so I have no way of testing whether it is sending out serial data at the moment, but I will probably buy a new one tomorrow and test it out.

Thanks in advance for any replies!

If you'd posted your code and/or a wiring diagram we wouldn't need to guess what was happening. It should not need much code to demonstrate the behaviour you describe.

My guess is that you are using the hardware serial port used for the USB serial output for the MIDI output. You can't share serial ports like that - if you are using two serial connections you need two serial ports. If you only have one hardware serial port then you could use a software serial port for the second one.