I am reasonably new to this, and I am trying to find the calculations used to choose the component values in this circuit.

Concerning the Optocoupler( an article about optocoupler: http://www.apogeeweb.net/article/69.html), from what I can work out:

6N139 Forward Voltage = 1.4V Supply Voltage = 5V

so: 5-1.4/220= 16.36mA through the optocoupler.

But the MIDI standards state that it should be 5mA current loop?

Also, how was the other values calculated? Thank you