Microcontrollers serial port (often called a SCI, COM, or ASYNC port) often do not generate the proper voltages required for connecting to computer RS-232 serial port. In this case special converter needed. Most microcontrollers run on a single supply voltage, that is usually +5 volts. In rough terms, logical 1 on these devices indicates that +5 is the voltage on the output pin. Logical 0 specifies that 0 volts is on the line.
The RS-232-C standard specifies that the voltage on the wire for sending a logical 0 are from +5v to +15v. The voltage for sending a 1 are from -5v to -15v. Most microcontrollers not capable of generating these voltages. So, to connect a microcontroller serial port to a true RS-232 device, you need to convert the TTL voltages of 0 and +5 into voltages between about -10 volts and +10 volts.
MAX232 is the chip that makes this work for you. It runs on a single chip supply (+5 volts), contains double charge pump voltage doubler and a +10v to -10v voltage inverter and requires a few external capacitors. There are some versions of this chip. The voltages output are used to generate the RS-232 compliant signals of two serial ports on the same physical package.
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