Serial loopback with Arduino doesn't work

[ThomasS]

Hi,
I'm trying to establish a serial communication with my Arduino Uno using pyserial. To test it, I programmed a simple loop back example, where the data I send with my python program over USB is send back unchanged. However, I get weird results. I already checked that the data the Arduino receives is OK (it is). When I just send data from the Arduino to the Computer (e.g. Serial.print('a')) the data is received correctly by my python program. I already tried to use ArduinoData.read(1) instead of ArduinoData.readline() in python and Serial.print(), Serial.println() and Serial.write() in Arduino. I get similar results each time.

I already looked in several forums and the pyserial documentation but couldn't find anything that helps.

I hope someone here can help me. Thanks!

Here is my python code:

import serial

delay = 0

ArduinoData = serial.Serial('COM8', 9600)
print("port is open!")


while(delay<1000):
    delay+=1
    ArduinoData.write(b'a')
    
        
data = ArduinoData.readline()
#data1 = ArduinoData.read(1)

print(data)
#print(data1)

ArduinoData.close()

Arduino Code:

byte data;

void setup(){
  pinMode(13, OUTPUT);
  Serial.begin(9600);
  }

void loop() {
  //wait for data
  
  while(!Serial.available());
  data = Serial.read();
  
  Serial.println(data);
  delay(500);
  
}

console output:

Output:
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'133\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'133\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'88\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'133\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'133\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'133\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'97\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'97\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'133\r\n'
>>> 
=== RESTART: C:\Users\thoma\Documents\studium\learn python\mySerial\test1.py ===
port is open!
b'88\r\n'

[deanhystad]

What is this supposed to do?

while(delay<1000):
    delay+=1
    ArduinoData.write(b'a')

[ThomasS]

For some reason the Arduino wouldn't receive any data when send right after the port is opened. I found this solution to wait after it's open in some forum.

[deanhystad]

The code does not wait, it writes b'a' 1000 times. Even if you did implement a wait (looping 1000 times is hardly a wait) it doesn't make any sense to do so because the start of the Arduino code and the start of the Python program are not simultaneous.