This is a monodirectional MIDI to RS232 (serial) interface, it play only MIDI to personal PC “RS232″. The MIDI output connector indicated on the left picture  dosen’t work because the buffer inside pic16f84 is too short. source

This small realization, based on one of the most common IC (MAX232) is designed to create a small and convenient TTL to RS232 and vice versa convertor.All you need is 4 caps, one IC and 2 connectors. If you want to add a small regulator on the board (already foreseen on the PCB) you just need to add a 78L05 regulator and a cap.If you do not need to regulate the power supply, you can just replace the 78L05 by a strap (pin1 to pin 3) and drop the 78L05, C1 and C2.source