MAX232

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MAX232 chip in DIP-16 package
The die of a MAX232
MAX232 pinout

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 transmitter / dual receiver that typically is used to convert the RX, TX, CTS, RTS signals.

The drivers provide TIA-232 voltage level outputs (about ±7.5 volts) from a single 5-volt supply by 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 receivers reduce TIA-232 inputs, which may be as high as ±25 volts, to standard 5 volt TTL levels. These receivers have a typical threshold of 1.3 volts and a typical hysteresis of 0.5 volts.

The MAX232 replaced an older pair of chips MC1488 and MC1489 that performed similar RS-232 translation. The MC1488 quad transmitter chip required 12 volt and -12 volt power,[1] and MC1489 quad receiver chip required 5 volt power.[2] The main disadvantages of this older solution was the +/- 12 volt power requirement, only supported 5 volt digital logic, and two chips instead of one.

Versions[edit]

The later MAX232A is forward 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.[3] The newer MAX3232 and MAX3232E are also backwards compatible, but operates at a broader voltage range, from 3 to 5.5 V.[4][5]

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.

Voltage levels[edit]

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 and −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

Applications[edit]

TIA-232 to TTL converters that use MAX232

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.

Derivatives[edit]

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.

References[edit]

External links[edit]