Serial monitor when port is being used by another application

Question

I am controlling some LEDs via the USB serial port using a Tkinter/Python3x script to debug what I suspect is a hardware problem.

from tkinter import *
import serial

usbport = 'COM3'
ser = serial.Serial(usbport, 9600, timeout=1)

def init():
    print("Started")

class App:

    def __init__(self, master):

        frame = Frame(master)
        frame.pack()
        self.scale1 = Scale(master, from_=0, to=255, command=lambda ev: self.getBright(1), bd=5, bigincrement=2, length=360, width=30, label='RED')
        self.scale1.set(128)
        self.scale1.pack(side=LEFT)
        self.scale2 = Scale(master, from_=0, to=255, command=lambda ev: self.getBright(2), bd=5, bigincrement=2, length=360, width=30, label='GREEN')
        self.scale2.set(128)
        self.scale2.pack(side=LEFT)
        self.scale3 = Scale(master, from_=0, to=255, command=lambda ev: self.getBright(3), bd=5, bigincrement=2, length=360, width=30, label='BLUE')
        self.scale3.set(128)
        self.scale3.pack(side=LEFT)
        self.centre = Button(frame, text="Centre All", command=self.centre)
        self.centre.pack(side=TOP)


    def getBright(self, slider):

        if slider==1:
            brt = self.scale1.get()

        if slider==2:
            brt = self.scale2.get()  

        if slider==3:
            brt = self.scale3.get()

        ser.write(255)
        ser.write(slider)
        ser.write(brt)

    def centre(self):

        for LED in range(0, 4):

            ser.write(255)
            ser.write(LED)
            ser.write(128)

        self.scale1.set(128)
        self.scale2.set(128)
        self.scale3.set(128)


init()
root = Tk()
app = App(root)
root.mainloop()

On the Arduino Mega 2560 side I read in from serial a start byte, LED label & a brightness value...

int user_input[3];    
int start_byte;       
int LED;           // which LED to pulse? r=1 g=2 b=3
int brightness;
int i;               

// LED pins
#define REDPIN 7
#define GREENPIN 8
#define BLUEPIN 9

void setup() 
{ 

  // LED outputs
  pinMode(REDPIN, OUTPUT);
  pinMode(GREENPIN, OUTPUT);
  pinMode(BLUEPIN, OUTPUT);

  Serial.begin(9600);
} 

void loop() 
{ 
  if (Serial.available() > 2) {

    start_byte = Serial.read();
    if (start_byte == 255) {

      for (i = 0; i < 2 ; i++) {
        user_input[i] = Serial.read();
      }

      LED = userInput[0];
      brightness = user_input[1];

      switch (LED) {

        case 1:
          analogWrite(REDPIN, brightness);
          break;

        case 2:
          analogWrite(GREENPIN, brightness);
          break;

        case 3:
          analogWrite(BLUEPIN, brightness);
          break;

      }
    }
  }
}

Now I would like to see what is actually being sent via the serial port to the Arduino using the serial monitor (yes I know I can just print the values in the Python terminal that's not what I'm asking).

The problem is that the Arduino Serial Monitor is locked out. I'm guessing this is a Windows thing (Win10 x64)? Has anybody here come across this?

Answer 1

Serial data can only go to one place at a time. Either your Python program can receive the data, or the Serial Monitor can receive the data. Similarly the serial port can only receive data from one place at a time.

So Windows prevents you doing something invalid by locking you out of the serial port when something else is using it.

This is normal, and entirely desired functionality.

Your simplest option is to just print what you receive in your Python program.