Issue with serial communication Arduino and python3

Question

I've some problem writing serial bytes from a python code to arduino. The python code had to write to serial port a number that the arduino receive.

Python3 code:

import serial
import time
ser = serial.Serial ('/dev/ttyACM0',)
ser.baudrate = 115200
ser.write(str(3).encode()) #or (b'3')
ser.write(str('\n').encode())

Arduino code:

void setup(){
Serial.begin (115200); //Comunicazione seriale 115200 bit
servomotore.attach(3);
pinMode(2,OUTPUT);
pinMode(12,OUTPUT);
pinMode(13,OUTPUT);
digitalWrite(2,HIGH);
digitalWrite(12,HIGH);
servomotore.write(180);
}
/*Il loop comprende due funzioni; sensori e Mappa, attivate ogni 15 gradi di movimento del servomotore, 
sensori rileva le distanze, Mappa invia i valori al seriale, ogni ciclo del radar produce 24 valori in centimetri*/
void loop() {
  char buffer[] = {' ',' '};
if (Serial.available() > 0) {


  Serial.readBytesUntil('n', buffer, 2);
  int incremento = atoi(buffer);

If I run this code I can't see output, no error or print, I need to exit with ctrl+c. Arduino doesn't receive nothing. The Arduino code is longer, this is the only part that I can't understand in this moment, it's only a part of a most complex project

readBytesUntil('n') is never going to return if you never send a n! — jasonharper
– jasonharper, Commented Mar 30, 2018 at 21:18
Is this 'n' an EOF caracters? to indicate the end of line? How can I write this to serial in python? — Antonio Del Cogliano
– Antonio Del Cogliano, Commented Mar 30, 2018 at 21:31
You are specifying the literal letter n. \n would be the newline character, which is more commonly used as a read terminator - but you're not sending that from the Python side, either. To do so, you'd use serial.write(), exactly as you did for the 3. — jasonharper
– jasonharper, Commented Mar 30, 2018 at 21:35
I changed the code, but it doesn't work, even with the /n, I can't understand why — Antonio Del Cogliano
– Antonio Del Cogliano, Commented Mar 30, 2018 at 22:14
Did you call Serial.begin(115200) in your Arduino setup function? You should show a Minimal, Complete, Verifiable Example which means the setup part of the Arduino code as well as your loop code. — patthoyts
– patthoyts, Commented Mar 30, 2018 at 22:45

patthoyts · Accepted Answer · 2018-03-31 11:52:14Z

I cannot reproduce your problem. Here is how I tested it. On the Arduino Uno setup the following program:

void setup()
{
    Serial.begin(115200);
    Serial.println(F("Serial test"));
}

char buffer[80] = { 0 };

void loop()
{
    if (Serial.available() > 0)
    {
        Serial.readBytesUntil('\n', buffer, sizeof(buffer));
        Serial.print(F("read: ["));
        Serial.print(buffer);
        Serial.println("]");
        memset(buffer, 0, sizeof(buffer));
    }
}

The following python3 script successfully reads and writes to this Arduino firmware:

#!/usr/bin/env python3

import sys
from serial import Serial

def main():
    ser = Serial('/dev/ttyACM0',)
    ser.baudrate = 115200

    print(ser.readline())

    ser.write(str(3).encode())
    ser.write(str('\n').encode())

    print(ser.readline())
    return 0

if __name__ == '__main__':
    sys.exit(main())

I get the following output once the device is programmed and plugged in:

~ $ python3 so-check-serial.py 
b'Serial test\r\n'
b'read: [3]\r\n'

Alternative interrupt based serial input

I'd like to add that I would not normally write a serial handler like this in Arduino. The following uses the serial interrupt to buffer the input and set a flag when the line is read completely. In this example we can wait or do other things until a whole line has been receivied (ie: keep blinking at the right times):

#include <Arduino.h>

volatile String buffer;
volatile bool inputComplete = false;

void serialEvent()
{
    while (Serial.available())
    {
        char c = (char)Serial.read();
        if (c == '\n')
            inputComplete = true;
        else
            buffer += c;
    }
}
void setup()
{
    Serial.begin(115200);
    Serial.println(F("Serial test"));
    pinMode(LED_BUILTIN, OUTPUT);
    digitalWrite(LED_BUILTIN, LOW);
}

void loop()
{
    if (inputComplete) {
        inputComplete = false;
        Serial.println(buffer.c_str());
        buffer = "";
    }
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
    delay(500);
}

Thank you! I solved the problem looking at your code, the wrong code was in the arduino side, everything works properly!

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