Trouble sending data from Matlab to Python via serial port

Question

to better understand serial port communication, I am trying to write some sample code, where Matlab has a loop running, which continously sends data to a serial port, while a Python script running on the same windows machine listens to this port, then receives and prints any received data.

In Matlab, I have written a simple loop, which sends 99 test signals to Port COM1,

% Setup a serial port and open it
tep=serial("COM1", "Baudrate", 9600);
fopen(tep);

% this loop is supposed to send a number to a serial port repeatedly
n = 1; % counter variable
while n < 100
    fprintf(tep,"50"); % Send data to serial port
    n = n + 1;      % Counter variable
    pause(0.5)      % Sleep to make this loop run for a total of 50s0
    fprintf('run'); % Test output within matlab to check, whether it runs
end

% finally close the serial port
fclose(tep);

as far as I can tell, this Matlab part works, as it prints "run" every hald second.

The Python listener:

import serial
import time
# set up the serial line same as in Matlab
ser = serial.Serial('COM1', 9600)
time.sleep(2)

# Read and record the data
data =[]                       # empty list to store the data
for i in range(50):
    b = ser.readline()         # read a byte string
    string_n = b.decode()  # decode byte string into Unicode  
    string = string_n.rstrip() # remove \n and \r
    flt = float(string)        # convert string to float
    print(flt)
    data.append(flt)           # add to the end of data list
    time.sleep(0.1)            # wait (sleep) 0.1 seconds

ser.close()

# show the data

for line in data:
    print(line)

Running the script in Python results in the following error:

serial.serialutil.SerialException: could not open port 'COM1': PermissionError(13, 'Zugriff verweigert', None, 5)

Obviously the port is already in use by Matlab, as it sends information to it, but I don't understand, why that is a problem. Shouldn't it be fine, for one program to send data to it and for another to receive data from it?

Kind regards.

Marcos G. Marcos G. · Accepted Answer · 2019-10-10T04:36:58

I'm afraid you cannot connect to the same serial port from two different processes.

That means if you open the serial port from Matlab for writing you cannot open it from Python to read.

The serial port is intended to send data from a computer to another computer or device not to share data between different applications (to do that there are better ways like writing to a file or to a chunk of shared memory).

Having said that, if what you are trying to do is for debugging purposes or just learning you might want to investigate com0com which allows you to create a pair of virtual (software) serial ports. This would be the same as having a couple of hardware (real) serial ports wired to each other. Since now you have two ports you can send data from Matlab on one and read from Python on the other.

This is one of the most recurring questions about serial ports, so you should be able to find a lot of good resources. You might want to start here.

Trouble sending data from Matlab to Python via serial port

1 Answers