RS-232 Interface to PIC
The standard rs-232 (com port) interface specifies +/- 12 volt signals. It is possible to work quite well with +/- 5 volts.
This circuit works by 'stealing' the negative voltage from the COM port and using it to communicate back.
The RECV line is connected to the rs-232 output line from the PC com port. It normally sits at -12 volts. This voltage is fed thru D8 and charges C2 with a negative voltage. The negative voltage goes thru R4 to Q2's collector. With Q2 off, the output (thru R3) is about -5 volts. When Q2 is turned on, the output goes to +5 volts. This is sufficient to feed the input of the PC com port.
Output from the PC feeding the RECV line also goes thru R1 to switch Q1 which feeds an input to the PIC (pin 7). The pic does 'software serial' to read this line, although PICs with hardware UARTs could be used here.
The extra circuit D1/R7/C7 going to the MCLR (reset) line is activated by a 'break' condition from the PC. This allows the PC to force a hardware reset of the PIC remotely.