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Arduino's IDE is telling me that I have something connected to serial port COM3. I can't find anything that says COM3 in Device Manager. Is there any way to figure out what it is that's connected to COM3?

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Older systems often assign the same resources to com1/com3 and com2/com4. This was a much bigger problem back in the DOS dial-up BBS days. Basically, com1/com3 used IRQ4 at 3F8h/3E8h while com2/com4 used IRQ3 at 2F8h/2E8h. The distinction was that com3/com4 were supposed to be an I/O card, while com1/com2 were supposed to be on the mainboard. Of course, it never worked out like that.

In any case, it might be showing up because of these legacy issues with com ports. Windows sees that IRQ4 is active so it thinks there may be something on com3 in addition to the Arduino on com1.

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This would be valid, but I don't have an old system or an Arduino on COM1. Nothing is connected at all to serial. – Matt Feb 1 '11 at 22:52

Indeed, you don't have an old system, and the old DOS stuff does not apply. Actually, what you have is a FT232 chip (on the arduino board itself), that "translates" the USB into a serial port. So indeed the Arduino provides a "COM3" to your PC, even if this serial port can not communicate with the outside world, only with the AVR chip on the board.

So what is connected on COM 3 is ... the arduino itself.

I actually had the same problem. If you don't have com or LPT ports (basically if you're using a laptop), apparently the device manager (in Vista) doesn't show anything. So you have to click "add hardware", allow it (admin rights) select "Install teh HW that I manually select from a list (advanced)", and there you get ... "ports", select communications ports, select the "drivers" directory from your arduino installation, and after it says "updating", you get the "USB Serial converter". Note that it could select another "COM number" : I got COM7 on my machine, likely because I did use some other USB COM adapters in the past...

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Actually it can happen that sometimes windows detects com ports which aren't there. In some cases, I got as much as 5 extra serial ports by listing them using the .Net library for serial communication. None of them were listed in device manager, while USB to serial and bluetooth to serial ports do appear there. – AndrejaKo Oct 19 '11 at 21:06

I have some display controller software running on a modern PC that creates a virtual com port in order to communicate over USB to a video matrix (sounds odd but that's what it does). Perhaps you have some similar software that behaves the same

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