Parts
MAX232: pinout, the four capacitors, RS-232 levels and what changes on a 3.3 V board
By Bulan Sarkar
The MAX232 turns a UART's 0 V and 5 V into the roughly ±7 V an RS-232 port expects, and back. Logic goes in on pin 11 (T1IN) and out on pin 14 (T1OUT); the line comes back in on pin 13 (R1IN) and leaves as logic on pin 12 (R1OUT). Pins 7 to 10 are a second pair. VCC is pin 16, ground pin 15, and pins 1 to 6 take four 1 µF capacitors that let the chip make its own +8.5 V and −8.5 V from 5 V.
This page covers the capacitors, the levels each side guarantees, how fast it can go, and the problem people hit with a 3.3 V board: the receiver output swings up to 5 V. It also covers the drop-in MAX202 and MAX3232. Every number comes from TI's MAX232 sheet (SLLS047N, February 2024) and its MAX202 and MAX3232 sheets.
Datasheet: Texas Instruments MAX232 Dual EIA-232 Drivers and Receivers (SLLS047N) (PDF)Ask Datasheet Guru about the MAX232CPE
Pinout by package
Top view, notch up: pin 1 is top left and the count runs down the left side, then up the right.
The SOIC versions (MAX232D, MAX232DW) and the MAX232I use the same pin order.
Source: Texas Instruments MAX232 Dual EIA-232 Drivers and Receivers (SLLS047N), p. 3 (Figure 4-1, D, DW, N or NS package, top view, and Table 4-1 pin functions); Texas Instruments MAX232 Dual EIA-232 Drivers and Receivers (SLLS047N), p. 1 (package information: PDIP (16) 19.3 mm × 9 mm, SOIC (16) and SOP (16))
Which one do you have?
The last letter is the package. A MAX232N is the 16-pin through-hole DIP, a MAX232D or MAX232DW the SOIC, and a MAX232CPE the same DIP under the original maker's naming. An I after the number (MAX232I) marks the −40 to 85 °C grade; plain MAX232 is 0 to 70 °C. All of them share one pinout.
Plenty of chips look like a MAX232 and are not: the MAX202 wants smaller capacitors, the MAX3232 runs from 3.3 V. Read the full part number before you choose the capacitors, and on a ready-made DB9 module read the chip itself, not the listing: some carry a MAX3232.
Absolute maximum ratings
The chip runs from 4.5 to 5.5 V and must never see more than 6 V. The receiver inputs take ±30 V, so a port driven to ±12 V by an old PC is no threat. TI also says an unpowered MAX232 can be plugged into a live RS-232 port without harm.
| Parameter | MAX232NDIP-16 |
|---|---|
| Supply voltage, absolute maximum VCC | −0.3 to 6 V |
| Supply voltage, recommended VCC | 4.5 to 5.5 V |
| Charge-pump outputs VS+, VS− | up to 15 V; down to −15 V |
| Receiver input voltage R1IN, R2IN | ±30 V |
| Driver input voltage T1IN, T2IN | −0.3 V to VCC + 0.3 V |
| Driver output short-circuit duration | unlimited |
| Thermal resistance, junction to air, DIP RθJA | 60.6 °C/W |
| Operating temperature TA | 0 to 70 °C (MAX232I: −40 to 85 °C) |
| ESD, human-body model V(ESD) | 2000 V |
The driver outputs may be shorted to ground indefinitely; TI lists the short-circuit current at about 10 mA. The charge-pump pins VS+ and VS− are rated to ±15 V, and those two pins are only for their storage capacitors. Do not run other circuits from them.
Levels, current and speed
| Parameter | MAX232NDIP-16 |
|---|---|
| Supply current ICCVCC = 5.5 V, outputs open, 25 °C | 8 typ, 10 mA max |
| Driver output high VOH3 kΩ to ground | 5 V min, 7 V typ |
| Driver output low VOL3 kΩ to ground | −7 V typ, −5 V max |
| Driver input levels VIH, VIL | high ≥ 2 V, low ≤ 0.8 V |
| Driver short-circuit current IOSVCC = 5.5 V, output at 0 V | ±10 mA typ |
| Receiver thresholds VIT+, VIT−5 V, 25 °C | rising 1.7 typ, 2.4 V max; falling 0.8 min, 1.2 V typ |
| Receiver hysteresis Vhys | 0.2 min, 0.5 typ, 1 V max |
| Receiver input resistance rI | 3 to 7 kΩ, 5 kΩ typ |
| Receiver output VOH, VOL | high ≥ 3.5 V at 1 mA; low ≤ 0.4 V at 3.2 mA |
| Data rateone driver switching | 120 kbit/s typ |
| Driver transition slew rate SR(t)−3 V to +3 V | 3 V/µs typ (30 V/µs max) |
| Receiver delay tPLH, tPHL | 500 ns typ |
Into a 3 kΩ load each driver gives at least 5 V high and at most −5 V low, typically ±7 V. The receivers switch near 1.7 V going up and 1.2 V going down, with about 0.5 V of hysteresis. An open receiver input reads as high at its output, which is the idle state of a serial line, so a disconnected cable does not spray garbage into the UART. A driver input that floats also idles high through an internal pull-up.
The supply current is 8 mA typical and 10 mA at most with the outputs open, so the chip draws 40 mW on 5 V and at most 55 mW at 5.5 V. In the DIP package at 60.6 °C/W that warms the die by only 3.3 °C. Loaded outputs draw more, through the pump.
What it is good for
- Connecting an Arduino, 8051 or PIC UART to a PC's 9-pin COM port.
- Talking to bench instruments, CNC controllers, UPS units and PLCs that still have an RS-232 port.
- Building a console port for a router, a modem or an embedded Linux board.
- Learning: it shows a charge pump, level shifting and line drivers in one cheap chip.
It is the wrong part for a USB link (use a USB-to-serial chip such as the CH340G), for RS-485 or for a 3.3 V board without extra parts. Above 120 kbit/s, or on long cables, choose something faster.
Worked example: a microcontroller UART to a COM port at 115200 baud
A microcontroller's TX goes into T1IN, R1OUT goes back to its RX, and T1OUT and R1IN go to a 9-pin connector wired as a PC would expect (pin 2 to the PC's receive, pin 3 from its transmit, pin 5 ground). The MAX232 runs from 5 V. Check each direction, the speed and the capacitors.
1. Logic into the driver. T1IN needs 2 V for a high. A 5 V board clears it easily; a 3.3 V board does too, by 1.3 V. Driving the MAX232 from 3.3 V logic is fine.
2. Line out. The worst driver high is 5 V into 3 kΩ. TI times its edges between −3 V and +3 V (Figure 6-3), so the worst high still clears +3 V by 2 V. A typical 5 kΩ receiver at the far end takes 1.4 mA at 7 V.
3. Speed. At the typical 3 V/µs transition slew rate, one edge takes 2 µs to cross from −3 V to +3 V. One bit at 115200 baud lasts 8.68 µs, so the edge eats 23 % of it. 115200 is 96 % of the 120 kbit/s TI quotes: it works on a short cable, but 9600 or 19200 leaves far more room on a long one.
4. Logic out of the receiver. R1OUT is at least 3.5 V high at 1 mA and can rise close to the 5 V supply. That is fine for a 5 V board and too much for a 3.3 V pin rated to about 3.6 V. A 10 kΩ / 20 kΩ divider brings 5 V down to 3.33 V, but the worst high becomes 2.33 V. If the input wants 0.7 × 3.3 V = 2.31 V, the margin is only 23 mV.
5. Or change the chip. A MAX3232 runs from the same 3.3 V. Its receiver gives at least VCC − 0.6 V = 2.7 V and can never exceed the rail. That leaves 390 mV of margin and needs no extra parts.
6. Capacitors. Four 1 µF parts (ceramic is fine; observe polarity on tantalum or electrolytic). C3 and C4 hold about +8.5 V and −8.5 V, and the pins are rated to ±15 V, so pick 16 V parts. If C3 goes to VCC instead of ground, as TI allows, it sees only 3.5 V. Add 1 µF from VCC to ground at the chip.
Worked example in numbers
- Line high, worst
- 5 V
- Edge at 115200
- 23 %
- Margin, 3.3 V divider
- 23 mV
- Margin, MAX3232
- 390 mV
Tips and checks
- Use 1 µF on a MAX232. 0.1 µF belongs to the MAX202 and MAX3232; on a MAX232 it lets the pump rails sag under load.
- Fit the capacitors right at pins 1 to 6. TI says the C1 and C2 traces matter most, because those nodes switch fastest.
- TX goes to TX-in and RX to RX-out on the chip side: MCU TX to T1IN, R1OUT to MCU RX. On the connector side the crossover is already done.
- Join the grounds. A link with only TX and RX wired may work on the bench, then fail once the two boards sit on different supplies.
- On a 3.3 V board use a MAX3232. If you must use a MAX232, divide R1OUT and check your input's VIH in its datasheet.
Substitutes
The MAX232I drops straight in. The MAX202 and MAX3232 have the same pins and can replace it if you also change the capacitors: 0.1 µF for the MAX202 and for a MAX3232 on 3.3 V. For new boards the MAX3232 is the better choice: it works on 3.3 V and 5 V, has ±15 kV ESD on the line pins and draws 0.3 mA typical against the MAX232's 8 mA.
Bench check: is it working?
- Power it from 5 V with the four capacitors fitted. Measure pin 2: about +8.5 V. Pin 6: about −8.5 V. If either is near 0 V, a capacitor is missing, reversed or the wrong pins.
- Tie T1IN (pin 11) to ground. T1OUT (pin 14) should read about +7 V. Tie it to 5 V: about −7 V.
- Loop T1OUT to R1IN (pin 14 to pin 13). R1OUT (pin 12) should now follow T1IN: high when it is high, low when it is low.
- With that loop in place, send characters from the UART at 9600 baud and read them back. Every byte should echo.
- Measure the supply current with nothing connected: about 8 mA, 10 mA at most. Much more points to a shorted capacitor or a damaged part.
What we'd do
On a 5 V board, use the MAX232 with four 1 µF ceramic capacitors rated 16 V, keep them tight to the pins, and stay at 115200 baud or below. On a 3.3 V board, or for any new design, fit a MAX3232 with 0.1 µF capacitors instead. It has the same pins, and its receiver output stays within the 3.3 V rail.
Questions people ask
What is the MAX232 pinout?
Pins 1 to 6 are the pump capacitors (C1+, VS+, C1−, C2+, C2−, VS−). Pin 7 T2OUT, 8 R2IN, 9 R2OUT, 10 T2IN, 11 T1IN, 12 R1OUT, 13 R1IN, 14 T1OUT, 15 GND, 16 VCC.
What capacitor values does the MAX232 need?
Four 1 µF capacitors, plus 1 µF from VCC to ground. Ceramic is fine. The MAX202 and MAX3232 use 0.1 µF.
Can a MAX232 work with an ESP32 or other 3.3 V board?
The transmit side works, because 3.3 V is above the 2 V the input needs. The receive side can send up to 5 V back into the 3.3 V pin. Use a MAX3232 on 3.3 V, or divide R1OUT and accept a thin margin.
What voltage does a MAX232 output?
About ±7 V typical, at least ±5 V into a 3 kΩ load, from charge-pump rails near ±8.5 V. A logic high comes out negative.
What is the maximum baud rate of a MAX232?
TI quotes 120 kbit/s typical with one driver switching. 115200 works on short cables; for faster links the MAX3232 is rated to 250 kbit/s.
Buying MAX232 in India
The MAX232N or MAX232CPE in DIP-16 and ready-made DB9 modules are on most Indian component sites. Check the chip on a module: many carry a MAX3232, which needs different capacitors and can run on 3.3 V. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes. The logic levels lesson explains why a 5 V output and a 3.3 V input do not always agree.
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Datasheets this page was checked against
- Texas Instruments, MAX232 Dual EIA-232 Drivers and Receivers (SLLS047N) (SLLS047N, February 1989, revised February 2024)
- Texas Instruments, MAX202 5-V Dual RS-232 Line Driver and Receiver With ±15-kV ESD Protection (SLLS576G) (SLLS576G, July 2003, revised February 2024)
- Texas Instruments, MAX3232 3-V to 5.5-V Multichannel RS-232 Line Driver and Receiver With ±15-kV ESD Protection (SLLS410O) (SLLS410O, January 2000, revised June 2021)
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