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Arduino has become quite costly where I come from (2/3 of the price of Raspberry PI) so I have created an Arduino Severino clone.

I had used the device with the RS-232 Usb-Serial converter. But after a (long) while, the device stopped working. My friend came up with a point that RS-232 communication ranges between +6 to -6 V (with thresholds at +3 and -3 volts).

So I have tried FT232 which uses 0 to 5 volts instead. I have a little Chinese FTDI chip designed to do that: usb to serial front side

usb to serial back side

However, after connecting it, it behaves unexpectedly. (it had crashed my PC, one of the communication LEDs is on all the time...) I realised that the Arduino board might get broken for a different reason (an accidental short-circuit is very likely).

So for which type of communication is this "Arduino" designed?

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  • Please see my updated answer.
    – Ricardo
    Jun 28, 2014 at 1:04

4 Answers 4

Reset to default
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Looking at the schematic of that board it uses a very crude signal inversion for the RS-232. This allows it to work of a standard RS-232 signal direct from a computer - just.

It's far from ideal and really should use a MAX232 (or similar) chip for the interface.

Your FT232 adaptor should bypass that chunk of the circuit and connect direct to pins 0 and 1 of the Arduino clone. The DTR pin of the FT232 should connect to the RESET pin of the Arduino clone via a 100nF capacitor to allow the IDE to reset the board to enter the bootloader. Oh, and of course, connect the ground to your Arduino clone too (something Arduino users often forget).

You can follow the serial breakout board portion of the Arduino breadboard tutorial and apply it to your board.

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The arduino you linked to is designed for RS232-level serial signals, which can be guessed from the use of a DB09 connector.

The FT232 print you show is for 5V TTL (or maybe 3.3V 'TTL') signaling. Not compatible with your arduino. (BTW: that chip is not Chinese at all, it is made by www.ftdichip.com IMO they make the best usb-to-serial chip/driver products.)

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  • Thanks for the additional info. I've edited my question thanks to you.
    – Tomáš Zato
    Jun 27, 2014 at 21:45
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    You are assuming, of course, that it is a REAL FT232 chip, and not a dodgy Chinese copy... zeptobars.ru/en/read/FTDI-FT232RL-real-vs-fake-supereal
    – Majenko
    Jun 27, 2014 at 21:59
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The Arduino Severino (Single Sided Serial version 3 - S3V3rino) is designed to work with RS-232 serial interface, not with the TTL serial signal levels of your FT232 board.

So, to use your FT232 board to connect your Severino to a USB port, you'll need connections like those in the schematics below:

Serial TTL to ATmega328 schematics

So, basically you need these connections:

  FTDI |  ATmega328           | Arduino Severino 
----------------------------------------------------
  DTR  |  Pin 1 (thru cap C6) | RESET (thru cap C6) 
  RXI  |  Pin 3               | TX (D1)  
  TXO  |  Pin 2               | RX (D0)  
  VCC  |  Not connected       | Not connected
  CTS  |  Not connected       | Not connected  
  GND  |  Pin 8 & 22          | Ground

That's basically the same Majenko suggested (but with a picture added).

I'm not sure you can make these connections directly into the Severino without damaging the board or anything else. You'll probably need to cut the TX/RX traces that go from the ATmega328 to the DB9 female connector on the Severino and wire them to the FTDI header pins.

Best would be to modify the Severino EagleCAD schematics to remove all the RS-232 circuitry and replace it by TTL connector as shown in my schematics above and, with that, etch and assembly a new board. It's a lot of work, but can be done.

I hope this helps.

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A typical 5V Arduino will use logic-level serial - that is 0 and 5V signalling.

RS232 has signal levels of plus or minus 3 to 15 volts. RS-232 drivers and receivers must be able to withstand indefinite short circuit to ground or to any voltage level up to ±25 volts

From Wikipedia

Many RS232 interfaces will receive 0-5V signals.

I believe the original serial Arduino had circuitry to tolerate RS232 signal levels on it's inputs.

Since most computers now have USB rather than RS232 ports, Most users of Arduinos use a USB to logic-level serial interface. Some of the most popular chips for this are from FTDI such as the FT232RL on your Chinese clone of a "FTDI Friend". In this case all the signals are logic-level 0-5V (or 3.3V if you change the jumper link).

So the following are probably OK

 PC DE-9 RS232    ----- Severino DE-9
 PC DB-25 RS232   ----- Severino DE-9
 PC USB port ---- "FTDI Friend" ---- Arduino 0(RX), 1(TX), Reset, GND and (optionally) VCC.
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  • Well, no I've got two responses claiming exactly the opposite. What should I do? :D
    – Tomáš Zato
    Jun 27, 2014 at 21:43
  • @TomášZato: I suspect you don't have a true RS232 signal level anywhere. I'd use a voltmeter if in doubt.
    – RedGrittyBrick
    Jun 27, 2014 at 21:54

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