I will post a picture of my circuits when I get the chance so that you can tell me if I am connecting anything wrong.
We need a picture of your circuit a schematic of what it is and the software you are using and what you are connecting to it.
tested with both resistor values for 3.3V and 5V
I can get a signal by hooking up Ch1 of an oscilloscope to pins 2 and 3 of the the 6N138 in the midi schematic posted above, but when I put CH2 on any mix of 5, 6, or 8 of the 6N138, all I get is a constant HIGH voltage of whatever voltage I have going into pin 8
You can't do that because there is no common connection between the MIDI input and the output of the opto. You should only measure one side at a time and move the scope's ground over.
Circuit 3:-The minimum operating voltage of the TL071 is 6V, you are only feeding it with 3 volts. There is no common reference between the input signals and the TL071 so even if the supply voltage was correct then it still wouldn't work.
Circuit 1:-I am concerned that you sayThe resistor values should be the same. However on a 3V3 system the 220R RX pull up resistor should go to the 3V3 supply not 5V. Lets hope you have not blown it. I would also disconnect the resistor on pin 7 and let it float.Finally what way round is that connector. Their is often some confusion as to if you are looking at the holes the plug connects to or the back of the socket, you should try swapping these two input connectors over.
So what you are saying is that regardless the voltage, the circuit layout should stay the same?
However, in that regard, I did try flipping the direction relative to the 6n138 and still no dice
The TL071 works fine and is doing its job, even at 3.3V.
Yes I can cause each channel on the oscilloscope that I am using can has its own ground.