The physical communication standard defines the signal voltage of -10V for logic '1', and +10V for logic '0'. However in practise, the voltage can be ranging from +/-3V to +/-25V. Not to worry if the measured voltage is not +/-10V. Typical receiver is able detect the incoming signal with voltage as low as +/-3V.
A microcontroller like PIC16F877a uses USART (5V system). The PC (personal computer) that we have in the office/home uses the standard RS232. To enable a microcontroller to communicate with the computer, a RS232 to TTL converter is required.
IC chip maker has come up with the integrated circuit for interfacing RS232 with TTL logic (5V for logic 1, 0V for logic 0), making the interfacing work very simple. MAX232 is one of the many IC in the market which helps to convert between RS232 -/+10V and TTL +/- 5V. It is a simple voltage level converter in short. The charge pump design allows the circuit to generate +/-10V from a 5V supply, with the help from the four capacitor. With charge pump to double up the supply voltage for RS232 transmitter, there is no need to design a power supply for +/-10V.
The diagram on the left shows the schematic of the MAX232 IC circuit. It consist of only 4x 1uF 16V electrolytic capacitor, and the MAX232 IC itself. It is that simple. I have include a layout which I always use for PC to PIC16F877a microcontroller, RS232 interface.
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