Trouble with creating serial object in Python

Question

I'm writing a program that is supposed to communicate with an Arduino-unit using the serial-object. Within the init-method of a class this piece of code can be found:

    try:
        self.rotor  = serial.Serial(port = "COM22", baudrate=115200, timeout = 0.1, writeTimeout = 1)
    except serial.SerialException, e:
        print "Error when connecting to collimator: ", e

When I run it I get this error message:

SerialException: could not open port 'COM1': WindowsError(2, 'The system cannot find the file specified.')

I asked the computer to open COM22, and it responds that it cannot open COM1. What's that about? The Arduino-unit is plugged into COM22.

I have another program that I haven't written myself, but which utilizes the same class library. This program works, but I don't understand how. Is there some sort of initialization of the serial-object that I have missed to do?

The error might be happening somewhere else. Your try catch would still catch that error if the error was happening here in the code. — justengel
– justengel, Commented Oct 7, 2014 at 11:58

Brett Lempereur · Accepted Answer · 2014-10-07 12:23:18Z

From the source code for the Win32Serial object in the PySerial SVN trunk (http://svn.code.sf.net/p/pyserial/code/trunk/pyserial/serial/serialwin32.py):

def open(self):
    """\
    Open port with current settings. This may throw a SerialException
    if the port cannot be opened.
    """
    if self._port is None:
        raise SerialException("Port must be configured before it can be used.")
    if self._isOpen:
        raise SerialException("Port is already open.")
    # the "\\.\COMx" format is required for devices other than COM1-COM8
    # not all versions of windows seem to support this properly
    # so that the first few ports are used with the DOS device name
    port = self.portstr

So changing your code to:

    try:
        self.rotor  = serial.Serial(port = r"\\.\COM22", baudrate=115200, timeout = 0.1, writeTimeout = 1)
    except serial.SerialException, e:
        print "Error when connecting to collimator: ", e

Should work correctly.

user2536262 · Accepted Answer · 2014-10-10 13:45:07Z

0

I later found the error to stem from a wrongly defined path to the module with the class defintion. The path was directed to an older version of the same file.

answered Oct 10, 2014 at 13:45

user2536262

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