Arduino Uno, weird characters sent to COM (SoftwareSerial)

Question

I'm trying to send ASCII characters from Arduino UNO to a computer serial port. I'm using a cable with a male COM connector, attached to the computer's serial port, and three wires (TX, RX and Ground) on the Arduino side. I used pins 12, 13 and Gnd as shown in the picture:

And I am running this piece of code to send a string every second on the serial port:

#include <SoftwareSerial.h>

SoftwareSerial mySerial(13, 12); // RX, TX

void setup()  
{
  mySerial.begin(9600);
}

void loop()
{
  mySerial.println("Hello world");
  delay(1000);
}

But, when I read that port (at the right 9600 speed), instead of "Hello world" I get strange characters instead:

I thought I had set the pin erroneusly, so I swapped RX with TX, but I get a different but still wrong output:

What am I doing wrong?

Answer 1

The Arduino UART produces TTL level signals, that is 5V for high and 0V for low. A PC's RS232 port expects full RS232 voltages which can be -9V to +9V and are inverted.

Either use a TTL level serial adapter (such as those from FTDI) to interface to the PC. Or use a level converter like the MAX232.

Answer 2

Like Toby says you need an EIA-232 (the name RS-232 is obsolete) transceiver. The Arduino's UART will output +5 V when idle and for a logical "1", and 0 V for a logical "0". EIA-232 works with inverted levels, so the +5 V becomes typically -12 V, and the 0 V becomes +12 V.

If you connect the UART directly to the PC's EIA-232 port it may see the +5 V as a low, but the 0 V will be undefined, so it may interpret your data any way.

But the received data is a much bigger problem, and you're very lucky that the AVR's I/O pins have protection diodes:

The RxD line from the PC will be -12 V when idle, and +12 V when it's sending a logic "0". Both levels are way beyond the maximum allowed values for the AVR, but the diodes will clamp them, and the limited drive current from the PC's EIA-232 will be lower than the maximum allowed 40 mA. Without the protection diodes it's very likely that that I/O pin (and maybe more) would be damaged.