The MAX232 is an integrated circuit first created in 1987 by Maxim Integrated Products that converts signals from a TIA-232 (RS-232) serial port to signals suitable for use in TTL compatible digital logic circuits. The MAX232 is a dual driver/receiver and typically converts the RX, TX, CTS and RTS signals.
The drivers provide TIA-232 voltage level outputs (approx. ± 7.5 volts) from a single five volt supply via on-chip charge pumps and external capacitors. This makes it useful for implementing TIA-232 in devices that otherwise do not need any other voltages.
The later MAX232A is backwards compatible with the original MAX232 but may operate at higher baud rates and can use smaller external capacitors – 0.1 μF in place of the 1.0 μF capacitors used with the original device. The newer MAX3232 is also backwards compatible, but operates at a broader voltage range, from 3 to 5.5 V. 
Pin-to-pin compatible versions from other manufacturers are ICL232, SP232, ST232, ADM232 and HIN232. Texas Instruments makes compatible chips, using MAX232 as the part number.
It is helpful to understand what occurs to the voltage levels. When a MAX232 IC receives a TTL level to convert, it changes a TTL logic 0 to between +3 and +15 V, and changes TTL logic 1 to between -3 to -15 V, and vice versa for converting from TIA-232 to TTL. This can be confusing when you realize that the TIA-232 data transmission voltages at a certain logic state are opposite from the TIA-232 control line voltages at the same logic state. To clarify the matter, see the table below. For more information, see RS-232 voltage levels.
|TIA-232 line type and logic level||TIA-232 voltage||TTL voltage to/from MAX232|
|Data transmission (Rx/Tx) logic 0||+3 V to +15 V||0 V|
|Data transmission (Rx/Tx) logic 1||-3 V to -15 V||5 V|
|Control signals (RTS/CTS/DTR/DSR) logic 0||-3 V to -15 V||5 V|
|Control signals (RTS/CTS/DTR/DSR) logic 1||+3 V to +15 V||0 V|
The MAX232(A) has two receivers that convert from RS-232 to TTL voltage levels, and two drivers that convert from TTL logic to RS-232 voltage levels. As a result, only two out of all RS-232 signals can be converted in each direction. Typically, the first driver/receiver pair of the MAX232 is used for TX and RX signals, and the second one for CTS and RTS signals.
There are not enough drivers/receivers in the MAX232 to also connect the DTR, DSR, and DCD signals. Usually, these signals can be omitted when, for example, communicating with a PC's serial interface. If the DTE really requires these signals, either a second MAX232 is needed, or some other IC from the MAX232 family can be used. Also, it is possible to connect DTR (DE-9 pin #4) directly to DSR (DE-9 pin #6) without going through any circuitry, which provides an automatic (brain-dead) DSR acknowledgment of the incoming DTR signal.
The MAX232 family was subsequently extended by Maxim to versions with four receivers and transmitters (the MAX238) and a version with eight receivers and transmitters (the MAX248), as well as several other combinations of receivers and transmitters.
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