I want to connect two MIDI keyboards by connecting the MIDI OUT of the first keyboard to the MIDI IN of the second keyboard, but I don't have the connector and I'm nowhere near a music store to buy one. And for that, I want to make my own DIN to DIN MIDI connector. I have two male 5 pin DIN connectors, and the question is, do I have to put some kind of isolation (usually an optocoupler) between the connectors? Or is it safe to just solder them directly without some form of isolation? As far as I know, optocoupler isolation usually found in MIDI to USB interface/adapter. I don't know if a MIDI to MIDI connector is supposed to be electrically isolated or not.
The isolation is handled on the device, not the cable. A normal MIDI (MIDI to MIDI) cable does not contain optocouplers or other isolation. You see the isolation on the USB adapters because the adapter is serving as one of the 'devices'.