TTL to RS232 without Maxim's circuit

I’ve asked my local vendors about the TTL to RS232 circuit with no success. I decided to look for a different way to convert Arduino’s TTL to RS232 signal. I’ve found some information on Severino’s schematic and this website: and I decided to create my own schematic. There are plenty of information about Maxim chip to do this, but in some places it’s very difficult to get some components, so I think this is the best and cheapest way to do the conversion.

I’ve never tested the attached schematic, I plan to do so, but It would be very nice if someone with appropiate knowladge verifies the circuit before the tests. It would be very nice to get some help to avoid crahsing my hardware.

Thank you so much.

  1. It is really bloated with resistors and diodes
  2. The transmit pin of the DB9 is connected via two resistors back to the receive pin. Not very useful.


I removed the non-original part of the ciruit published in How about now? I left the leds TX and RX.

Why are the transmit and receive signals connected at all?


why not try with two optocouplers ...

I've found better info:

With two optocuplers? You mean only with two OC and nothing else? I can easily get EL817.

DrJuano, update your profile some geographic info. If you have an education related e-mail address, Maxim, TI, and others will send you samples for free.

I’v made the attached circuit and did not worked for me on my Arduino Severino.

The below might have some ideas.

RS232 signals are +/- 12V not +/- 5V. While you might get away with this sometimes you will not always.

I think it's even broader than that Mike, like +/-5 up to +/-25.

From MAXIM's MAX232 datasheet: "The typical driver output voltage swing is ±8V when loaded with a nominal 5k? RS-232 receiver and VCC =+5V. Output swing is guaranteed to meet the EIA/TIA-232E and V.28 specification, which calls for ±5V minimum driver output levels under worst-case conditions."

For those not in the know: Parts like MAX232 use the capacitors and an internal voltage doubler & inversion circuits to take 3.3V power on VCC pins to make +/- 6V output swing, and 5V power to make +/-10V swing: "The MAX220–MAX249 have two internal charge-pumps that convert +5V to ±10V (unloaded) for RS-232 driver operation. The first converter uses capacitor C1 to double the +5V input to +10V on C3 at the V+ output. The second converter uses capacitor C2 to invert +10V to -10V on C4 at the V- output."

"The MAX3222/MAX3232/MAX3237/MAX3241’s internal power supply consists of a regulated dual charge pump that provides output voltages of +5.5V (doubling charge pump) and -5.5V (inverting charge pump), regardless of the input voltage (VCC) over the 3.0V to 5.5V range."

Some versions don't even use external caps, everything is internal, altho these are limited in the speed they can support.

Read all about the different versions here.