Calculating Component Values for MIDI synth
If you create a free account at midi.org, you can get the current version of the MIDI spec here.  The spec has a good description of the electrical interface.  It's worth looking at.

The recommended MIDI OUT circuit from the spec is shown below.  It has two 220 ohm resistors.  When you connect this to the MIDI IN circuit in your schematic, there are THREE 220 ohm resistors in the loop, two at the source end and one on the destination end.  This gives you around 5ma of LED drive current.

It makes sense to design the MIDI OUT this way so that external to the device, you always have a resistor between the +5V and driver and the outside world.  This helps to assure that nothing fries if either of them is accidentally shorted to ground.