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

A simple RS232 example project that takes all the power it needs from the serial port. Use it to power your microcontroller and communicate between serial port and microcontroller. Takes advantage of the fact that pc serial ports will accept 0-5V rather than the RS232 standard of around negative 10V to positive 10V. Very convenient – no external power required! Note that you may use either a 5.1V zener or a 5V source

This is a programmable infrared (remote control) transmitter, which can be controlled from a PC serial port. It is capable of sending many remote control formats, including the Philips RC-5 standard.The controller will accept commands on the serial port. Settings are: 19200 bps, 8 bits, no parity, 1 stopbit, no flow control (XON/XOFF or RTS/CTS). Commands consist of hex coded bytes and must be written on the port as ASCII characters separated by space, terminated by ENTER (ASCII char 0d) 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

Here is an inexpensive way to get slow-moving analog data directly into a spreadsheet on your computer! A simple circuit connects to a serial port and a short Qbasic program gathers and saves 12 bit data to a file. A spreadsheet automatically retrieves the file and displays the data in real time.This simple interface can be built right into your project, turning it into a computer peripheral device.source