"RS232 voltage level is different from TTL level.The RS-232
standard defines the voltage levels that correspond to logical one
and logical zero levels. Valid signals are plus or minus 3 to 25
volts. minus 3 to plus 3 voltage range is undefined.The
range near zero volts is not a valid RS-232 level; logic one is
defined as a negative voltage, the signal condition is called
marking, and has the functional significance of OFF. Logic zero is
positive, the signal condition is spacing, and has the function
ON.
The most commonly used RS-232 level converter is MAX232. This IC includes charge pump which can generate RS232 voltage levels (-10V and +10V) from 5V power supply which is required to convert a TTL signal in to RS232 level. The TTL signal is very small in comparison to RS232 level thus we can not directly connect a TTL device with RS232 device as it may damage the TTL device with high level voltage of RS232."
The most commonly used RS-232 level converter is MAX232. This IC includes charge pump which can generate RS232 voltage levels (-10V and +10V) from 5V power supply which is required to convert a TTL signal in to RS232 level. The TTL signal is very small in comparison to RS232 level thus we can not directly connect a TTL device with RS232 device as it may damage the TTL device with high level voltage of RS232."
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