Writing to COM port on C# or python

Question

im trying to write a string to a COM4 which is a usb-serial adapter using Visual Studio C# and got the following error:

Access to the port 'COM4' is denied.

My program is extremely simple:

serialport.Open();
serialport.WriteLine("test");
serialport.Close();

I also tried using pyserial with python and got:

Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    ser.write("string")
  File "C:\Python27\lib\site-packages\serial\serialwin32.py", line 255, in write
    raise SerialException("WriteFile failed (%s)" % ctypes.WinError())
SerialException: WriteFile failed ([Error 6] The handle is invalid.)

I know this question isn't really related to programming and is probably a Windows 7 thing but I was hoping someone has already had a similar problem.

Turtle · Accepted Answer · 2010-09-04 05:52:35Z

1

Try this:

Right click on your executable and choose "run as administrator". Windows 7 by default runs programs without admin privileges. I'm betting that the ability to write to a COM port requires running a program as an Administrator.

edited Sep 4, 2010 at 5:52

answered Sep 4, 2010 at 4:18

Turtle

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4 Comments

Turtle Over a year ago

Did you try restarting your computer? Seriously, unplugging the device might screw things up, and this would fix that. Take a look at social.msdn.microsoft.com/Forums/en-US/vcgeneral/thread/…

kirbuchi Over a year ago

The problem is I won't be able to to this for a python script. Haven't tried yet with the exe file.

ACP Over a year ago

ya i didnt restart it. Ok i ll let you know once i do that

Hans Passant Over a year ago

It doesn't have anything to do with permissions, the port is simply in use by another process.

Ed Swangren · Accepted Answer · 2010-09-04 06:10:49Z

1

You cannot have multiple simultaneous connections to a serial port. The port is likely in use.

answered Sep 4, 2010 at 6:10

Ed Swangren

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Comments

Community · Accepted Answer · 2017-05-23 09:57:55Z

1

It had nothing to do with permissions neither with the port being in use. I found a related question which said it was because of the version I had installed (64 bit Python and 32 bit pyserial) I feel so silly.

What I still can't figure out is why it also didn't work on C# but that's OK. I'll just use python.

edited May 23, 2017 at 9:57

CommunityBot

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answered Sep 7, 2010 at 4:52

kirbuchi

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