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

PBUS is an RS485-like multi-drop bus for interconnecting PIC and other microcontroller-driven devices. Protocol and software is written by Jap. Just include a separate pbus library file into your project and add your command handlers for the specific device in a few lines.PBUS assigns a unique device ID in the range 1-f for every controller on the bus. The maximum number of slave devices on one bus is maximized in 15. Current protocol is one master, multiple slaves.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

The output of the Tx mixer is buffered by a 2N4401 emitter follower to drive the low impedance input side of a 10.7 MHz IF transformer. The transformer is returned to 75 meters by an additional, external 330 p cap. A second IF transformer is used to make a double tuned band pass filter, although for 75 meters.source

The circuit starts with a standard Collpits crystal oscillator. In order to provide a 600 Hz frequency shift, a trimmer cap and 2N7000 FET is added in series with the crystal. During transmit, the 2N7000 is turned on, effectively shorting out the trimmer cap and lowering the oscillator frequency. During receive, the ’7000 is turned off, allowing the trimmer cap to raise the frequency. The trimmer cap is adjusted to give a shift of about 600 Hz. source