serial.serialutil.SerialException: [Errno 16] could not open port: [Errno 16] device or resource busy: '/dev/ttyACM0'

Question

I'm trying to print serial output from Arduino Uno onto my screen using Pyserial python library.

Here is my Arduino code. It just produces and prints random numbers to serial monitor:

void setup() {
Serial.begin(9600);

}

void loop() {
    long rand = random(10);
    Serial.print(rand);
}

My python code is just supposed to print the values from serial onto the command line, here is the code:

#!/usr/bin/python

import serial

ser = serial.Serial("/dev/ttyACM0",9600)
while True:
    thing = ser.readline()
    print(thing)

While Arduino is printing random numbers to the serial monitor, I run my python script and get the error:

Traceback (most recent call last):
  File "/home/archimedes/anaconda3/lib/python3.6/site-packages/serial/serialposix.py", line 265, in open
    self.fd = os.open(self.portstr, os.O_RDWR | os.O_NOCTTY | os.O_NONBLOCK)
OSError: [Errno 16] Device or resource busy: '/dev/ttyACM0'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "pythonSerialTest.py", line 6, in <module>
    ser = serial.Serial("/dev/ttyACM0",9600)
  File "/home/archimedes/anaconda3/lib/python3.6/site-packages/serial/serialutil.py", line 240, in __init__
    self.open()
  File "/home/archimedes/anaconda3/lib/python3.6/site-packages/serial/serialposix.py", line 268, in open
    raise SerialException(msg.errno, "could not open port {}: {}".format(self._port, msg))
serial.serialutil.SerialException: [Errno 16] could not open port /dev/ttyACM0: [Errno 16] Device or resource busy: '/dev/ttyACM0'

Solution 1

You can't have two programs use the same serial port.

Close the serial monitor in the Arduino IDE.

---

PS: You are also trying to read lines in Python, but you aren't sending them from the Arduino.

Print the numbers as lines with Serial.println(rand);.

Solution 2

I was getting the resource busy error because Python was trying to access the serial monitor, but since I uploaded my code to Arduino with the command: sudo make upload monitor clean, my PC was getting access to the serial monitor from Arduino, which prevented Python from being able to access the serial monitor from Arduino. Now I just upload the code to Arduino with sudo make upload clean and exclude the command monitor if I plan to use Python to access the serial monitor.

Comments

[skrrgwasme]

Just to make sure I'm understanding this correctly: Your Arduino is connected to a computer via a serial-to-USB cable, and your Python script is running on the computer trying to read what the Arduino is writing to the serial port? Is that right?

[skrrgwasme]

Also, take a look at this similar question: stackoverflow.com/q/40951728/2615940. It's not quite a duplicate, but may have some useful info.

[Ramces Gonzalez]

Hi, yes, that is correct. Thanks, I will take a look at that link.

[Ramces Gonzalez]

Thanks for the suggestion, I tried closing the serial monitor for the Arduino and then I ran the python script. However, this time, nothing happens, the cursor just sits below the command line. I think that it is waiting for the Arduino to stop printing values to serial. I'll try adding a Serial.end(); to my Arduino code.

[Ramces Gonzalez]

I tried using Serial.end() but the python script gave the same error: 'resource busy'

[gre_gor]

Serial.end() just disables the serial on the Arduino. It doesn't enable access of multiple programs to a single serial port.

[Ramces Gonzalez]

I changed Serial.print(rand); to Serial.println(rand); still get the same error 'resource busy', I'm out of ideas for now.

[Ramces Gonzalez]

Ok, I made a little progress, I excluded the line Serial.begin(9600); in my Arduino code and the following program ran without issues: #!/usr/bin/python import serial ser = serial.Serial("/dev/ttyACM0",9600) print(ser.name) ser.close() the output was /dev/ttyACM0

[Ramces Gonzalez]

However, I still can't get the python program to print out each value coming from the Arduino

[gre_gor]

Without Serial.begin(9600); Arduino won't send anything. If you get "resource busy" then something else on your computer is connected to that serial port. Read: Kill process that raises Device or resource busy: '/dev/ttyUSB0'?