Serial communication error between Arduino and Python

Question

I want to send values from a Python program on a Raspberry Pi to an Arduino. My RPi program looks like that:

import time, serial
time.sleep(3)
ser = serial.Serial('/dev/ttyACM0', 9600)
ser.write(b'5')

My Arduino program:

void setup() {
    Serial.begin(9600);                    // set the baud rate
    Serial.println("Ready");               // initial "ready" signal
}

void loop() {
    char inByte = ' ';
    if(Serial.available()) {         
        inByte = Serial.read();
        Serial.println(inByte);
    } 
    delay(100)
}

My problem is that when I run the program in Python and afterwards open the serial monitor in the Arduino IDE, it only shows "ready", but not the sent value. If I want to have the monitor open at the same time as the program, there is the error:

Device or resource busy: '/dev/ttyACM0'

I am using Python 3.6 by the way.

I am sorry if this is an easy error, but I am very new to serial and Arduino.

This makes sense, as the serial monitor you are opening in the Arduino IDE is the same serial interface that is being used by your python program, so you can't use them at the same time. Have you tried doing a read on the python side to see if the byte is being sent back from your println? — Easton Bornemeier
Possible duplicate of serial.serialutil.SerialException: [Errno 16] could not open port: [Errno 16] device or resource busy: '/dev/ttyACM0' — gre_gor
Possible duplicate of avrdude: ser_open(): can't open device "/dev/ttyACM0": Device or resource busy — Parth Sarthi Sharma

MihanEntalpo MihanEntalpo · Accepted Answer · 2018-08-09T04:44:39

You can connect to your Arduino serial port from only one application at a time. When you are connected to it via Serial port Monitor, Python couldn't connect to it.

You have two variants of solution:

Use serial sniffer instead of Arduino IDE's Serial Monitor. There are another answered question on this topic: https://unix.stackexchange.com/questions/12359/how-can-i-monitor-serial-port-traffic

Don't use any serial monitor, use Python! You can just continiualy read from serial, and print what have been received, something like this:

import time, serial
time.sleep(3)
ser = serial.Serial('/dev/ttyACM0', 9600)
# Write your data:
ser.write(b'5')

# Infinite loop to read data back:
while True:
    try:
        # get the amount of bytes available at the input queue
        bytesToRead = ser.inWaiting() 
    if bytesToRead:
        # read the bytes and print it out:
        line = ser.read(bytesToRead) 
        print("Output: " + line.strip())
except IOError:        
    raise IOError()

Serial communication error between Arduino and Python

1 Answers