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I've recently jumped on the Arduino bandwagon and am attempting to upload my very first sketch to my Arduino Uno installed on COM port 3. However, I get errors that another device is using COM port 3 and I need to disable that device.

Specs: Win XP, all current SPs. Arduino drivers 1.0.3, dated 12/10/2012, downloaded 12/25/2012. Drivers installed successfully without error messages. Windows Device Manager shows Arduino Uno as the device on COM3. The Uno is connected via USB. The status bar at the bottom of the Arduino Software window indicates Uno @ COM port 3. The Arduino software preferences are set to Arduino Uno (as opposed to one of the other form factors...Duemilanove, etc.).

I can write and compile sketches just fine. When I try to upload, compilation happens fine, but it'll eventually crap out telling that COM3 is in use by a device, and I need to stop that device and try again.

I feel like I'm travelling back to the early 90s and need to manually fix contentious COM ports...a phase of reality I never wanted to go back to...and have been spoiled by plug-n-play to the point where I've forgotten most of what I know about configuring ports from the command line...what is what I suspect I may need to do.

I've not found solutions in Arduino's help, Arduino's forums, or LadyAda.net's troubleshooting guides.

Anyone ever encounter this situation before or have any advice on how to counter it? This MAY be a more appropriate conversation for SuperUser...so if an admin wants to move it...

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  • \$\begingroup\$ What does WinXP Device Manager say that makes you sure you have the right COM port? \$\endgroup\$
    – jippie
    Dec 26, 2012 at 18:05
  • \$\begingroup\$ @jippie - COM3 is the only COM port in use (LPT1 and LPT2 are assigned to printers) and the properties of what's connected is Arduino somethin'-somethin'. Of course, I'm sittin' in a coffeeshop right now and don't have my Arduino with me to give you the exact indicators...but I can grab a screen shot later this afternoon. \$\endgroup\$
    – dwwilson66
    Dec 26, 2012 at 18:08
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    \$\begingroup\$ Easy test => does the device disappear when you unplug it from USB? \$\endgroup\$
    – jippie
    Dec 26, 2012 at 18:11
  • \$\begingroup\$ Yes...it's MIA right now, and I've unplugged it. \$\endgroup\$
    – dwwilson66
    Dec 26, 2012 at 18:25

2 Answers 2

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There's an easy way to force windows to drop anything on that port and that's to rename/reassign it. I've done it myself when the serial gets abruptly disconnected and windows doesn't handle it. To do this you need to:

1) Go to Control Panel-> Device Manager

2) Look for the COM port and right click, go to properties.

3) In the Advanced tab there is a button that is also called advanced. In it you can reassign the COM3 to another COM. Disregard any warning that "This port is being used" because since you don't have anything connected to those ports they're not being used.

4) Once you reassign it (and press OK to whatever it complained about to force it), then connect and disconnect the Arduino. It should show up in the new port and work.

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  • \$\begingroup\$ This did it. I'm on COM4 right now, no problems at all, and my sketch is doing what it's supposed to so. Not what I envisioned...but that's another post. :) \$\endgroup\$
    – dwwilson66
    Dec 27, 2012 at 12:22
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    \$\begingroup\$ I'm happy too hear. Unfortunately this issue is extremely common on windows. But I guess COM ports are not on Microsoft's priority. \$\endgroup\$
    – Gustavo Litovsky
    Dec 27, 2012 at 18:09
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I remember vaguely that something similar happened with me too. If i remember correctly after some abrupt stop of Arduino IDE. The COM port wasn't really used by anything else, but somehow windows (7) didn't close the connection and as much as I hate to do it, had to restart the PC. That fix it for me. YMMV, but try that as it is simplest and quick way to be sure that nothing use that virtual port.

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  • \$\begingroup\$ Arduino UNO too. \$\endgroup\$
    – zzz
    Dec 26, 2012 at 18:38
  • \$\begingroup\$ I should have added that I'd already tried turning it off and on again. I always forget to include that since it's so second nature to me. :) \$\endgroup\$
    – dwwilson66
    Dec 27, 2012 at 12:20
  • \$\begingroup\$ That's because you think Arduino IDE stopped, when in fact just the GUI went away, while processes using the COM port were left behind. \$\endgroup\$
    – Dmitry Grigoryev
    May 10, 2017 at 10:20
  • \$\begingroup\$ Sound pretty logical, but I am pretty sure I was unable to find a process related to Arduino, that hold the port open. So most probably it is via some system process. Btw I think GUI is very integral part of IDE. Don't know how Windows work exactly in fringe cases, but in principal there is no need to have working process for something to be considered used in the moment, and denied access to. \$\endgroup\$
    – zzz
    May 11, 2017 at 8:20

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