9 pin D-SUB male connector at the computer
at the device like a cellular phone
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