MAX232DR

The MAX232DR from Texas Instruments is a dual RS-232 driver/receiver housed in a 16-pin SOIC package, operating from a single 5V supply with integrated capacitive charge pump circuitry that generates the TIA/EIA-232-F voltage levels (typically plus/minus 9V output) without requiring external dual supplies.

The device contains two independent drivers and two independent receivers, providing full-duplex communication capability at data rates up to 120 kbps. Each driver converts TTL/CMOS logic levels (0 to 5V) into RS-232 compliant voltage levels, while each receiver converts RS-232 input levels into TTL/CMOS compatible outputs. The receivers feature a typical threshold of 1.3V with 0.5V hysteresis for noise immunity, and can accept input voltages up to plus/minus 30V.

The internal charge pump uses a voltage doubler and voltage inverter, requiring only four external 1uF capacitors (C1+, C1-, C2+, C2-) to generate the plus and minus supplies needed for RS-232 output levels. The V+ pin (pin 2) outputs approximately 2xVCC (about 10V), and the V- pin (pin 6) outputs approximately -2xVCC (about -10V). These generated voltages are used internally by the drivers and should not be used to power external circuits beyond a small bypass capacitor.

The MAX232DR is one of the most widely used RS-232 interface ICs in the electronics industry, found in virtually every device that requires serial communication via RS-232. The DR suffix denotes SOIC-16 package with tape and reel packaging (2,500 units per reel). The device is RoHS3 compliant, MSL Level 1 (unlimited), and classified as EAR99. Note that TI lists MAX232D as discontinued, with MAX232DR as the recommended replacement with plus/minus 2kV ESD protection.

The MAX232DR operates as a dual-channel RS-232 level translator, converting between TTL/CMOS logic levels (0-5V) and TIA/EIA-232-F voltage levels (plus/minus 5V to plus/minus 15V) using an integrated charge pump to generate the required high-voltage rails from a single 5V supply.

Charge Pump Operation: The internal charge pump consists of two stages. The first stage is a voltage doubler that uses an external capacitor (C1) to double the VCC supply, producing approximately 2xVCC (about 10V) at the V+ output (pin 2). The second stage is a voltage inverter that uses another external capacitor (C2) to invert the V+ voltage, producing approximately -2xVCC (about -10V) at the V- output (pin 6). Two additional external capacitors (C3, C4) serve as energy storage for the V+ and V- rails respectively. All four capacitors are typically 1uF (non-polarized ceramic recommended). During each charge pump cycle, the internal oscillator alternately charges and transfers charge between the capacitors, maintaining the V+ and V- supplies at the required levels for RS-232 output compliance.

Driver Operation: Each driver takes a TTL/CMOS logic input (T1IN or T2IN) and converts it to an RS-232 compliant output (T1OUT or T2OUT). When the input is logic HIGH (greater than 2.4V), the output swings toward V- (typically -8.5V). When the input is logic LOW (less than 0.8V), the output swings toward V+ (typically +8.5V). Note that the driver inverts the signal: a HIGH TTL input produces a negative RS-232 voltage (mark), and a LOW TTL input produces a positive RS-232 voltage (space), which is the correct polarity for RS-232 communication. The driver output impedance is typically 300 ohms, and the output is current-limited for short-circuit protection.

Receiver Operation: Each receiver takes an RS-232 input (R1IN or R2IN) and converts it to a TTL/CMOS logic output (R1OUT or R2OUT). The receiver has a switching threshold of approximately 1.3V with 0.5V hysteresis. Input voltages above the threshold are interpreted as logic LOW on the output (inverted), while input voltages below the threshold are interpreted as logic HIGH. The hysteresis provides noise immunity against signal reflections and ground potential differences common in RS-232 cables. The receiver inputs can accept plus/minus 30V without damage, providing robust protection against voltage spikes common in long cable runs.

Signal Flow in Typical Application: The UART TXD output (TTL) connects to T1IN; T1OUT drives the RS-232 cable (DB9 pin 3). The RS-232 cable (DB9 pin 2) connects to R1IN; R1OUT drives the UART RXD input (TTL). This provides bidirectional level translation between the microcontroller and the RS-232 serial port.