Trying to open a serial port with pyserial on WinXP -> “Access denied”

Question

I'm trying to send data to an hplc pump via the serial port using python and pyserial. I tested the cable and the pump under linux (a gentoo derivative), where it worked perfectly, albeit as root. Now i have to use the code on a WinXP machine, where i always get an "Access denied" error when trying to open the port (i adjusted the parameters to COMxx instead of ttySxx, the port is found). I tried the serial port of the computer, as well as a USB2Serial adapter. I heard that WinXP was quite restrictive when it comes to trying to address some port with self written code. Is there a simpler workaround for this problem than installing linux?

# -*- coding: utf-8 -*-

import sys
import time
import serial
from threading import Thread

"""
usage: cmdCapture workDirectory pictureTime pressureTime
"""

print 'successful import is successful'

workDir=sys.argv[1]
pressureThresh=float(sys.argv[3])

class doCapture(Thread):
def __init__ (self, workDir, pressureThresh):
    Thread.__init__(self)

    self.workDir=workDir
    self.pressureThresh=pressureThresh
    self.pressureTimer=0

-> here i set the serial port

    self.ser=serial.Serial(port='\\.\COM1', baudrate=9600, bytesize=serial.EIGHTBITS, parity=serial.PARITY_NONE, stopbits=serial.STOPBITS_ONE, timeout=1)

-> here the error happens

    self.ser.open()

def getPressure(self):
    self.ser.write('PR')
    return self.ser.read(size=8), timer.timer()

def run(self):
    self.pressureTimer=time.timer()
    while 1:
        if self.pressureTimer<=(time.timer()-self.pressureTime):
            self.p=getPressure()
            print self.p

myCapture=doCapture(workDir, pressureThresh)
myCapture.start()

Answer 1

Try opening the port as \\.\COMxx

Also make sure that the port isn't already open from another application. I recommend that you use Hyperterminal to see if the port is open.

Answer 2

.close() before I called the .open() worked for me as well

Answer 3

No need to call .open if you already pass the serial port name/number in the constructor!

Answer 4

When you execute

self.ser=serial.Serial(port='\\.\COM1', baudrate=9600, bytesize=serial.EIGHTBITS, parity=serial.PARITY_NONE, stopbits=serial.STOPBITS_ONE, timeout=1) 

the serial port is already open. On Windows, you get an error if you run .open() if the port is already open, but you don't on Linux. That's where the discrepancy is.

Answer 5

Calling .close() before opening the port solved a problem that was driving me nuts!

I had it working on another machine running vista 64-bit using com0com virtual ports, not a hitch.

I was trying to work on the exact same scripts on my Windows 7 box - nada - XP Mode same thing. Access Denied or couldn't find the port (when changing up how the port was addressed). HyperTerminal, of course, recognized and worked with everything flawlessly.

4 hours later I find this little nugget and now everything is humming along just fine.

It's a good thing I enjoy programming...GAH!

Answer 6

I had a similar problem when i was trying to get accelerometer values from TI Chronos. In the device manager i just disabled the COM port and enabled it again. Worked

Answer 7

Be sure the port is not open by any other program. (That's worked for me)