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I am using a 3rd party Arduino board made by Stensat (from their experimenters kit). The documentation that comes with it says that in the Arduino environment, the board that should be selected is the ATmega328 5V.

I cannot, however, open the serial monitor in my program. It gives me the error message "Board at COM3 is unavailable." I have not had a problem using the serial monitor before in other sketches but now it's not working. I can't even select what port to use; the option is grayed out. I checked and made sure that the board I have selected is correct as well. Please help!

  • I am assuming you are using windows. If you open the Windows device manager (mmc devmgmt.msc) what if anything do you see under Ports (COM & LPT) section? My first guess would be that the device can be found at a different COM port. – Kolban Jun 10 '15 at 20:47
  • It MAY be a driver issue. There are two main clone ICs that are used in place of the "genuine" FTDI USB to serial bridge IC's. If the 'Unolike' product you are using uses the other IC compared to others you have used ou may need to add the correct drivers in Windows. The documentation should tell you. – Russell McMahon Jun 10 '15 at 23:17
  • The device is listed as being on COM3, and I went through the instructions in the documentation again to reinstall the drivers but it did not change anything. I can upload programs to my board, but I cannot open the serial monitor or select a port – Caleb Jun 17 '15 at 17:51
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    I'm voting to close this question as off-topic because it has been abandoned for years without information to make it specifically answerable ever being provided. "Answers" will be merely speculation or anecdotes of situations that may be superficially similar but likely as not have different causes. – Chris Stratton Mar 4 '18 at 18:53
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    I'm voting AGAIN to close this question as off-topic because it has now been abandoned EVEN LONGER without any information to make it specifically answerable ever being provided. – Chris Stratton Jun 3 '18 at 12:11
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I confront with same error and I try to solve it. after Some search I cant find anything useful for answer this question on WEB so I try to understand what Happened And finally find the answer... this problem occur because we connect ESP8266 to Arduino and then connect Arduino to PC(or laptop) via USB and PC USB port can not provide enough power for both Arduino and ESP8266 so the port(com3) is lost and we can not communicate with Arduino and ESP8266.... for solve this problem we should separate power of Arduino and ESP8266. for this we can use Adapter or USBtoTTL convertor. I use USBtoTTL convertor and take power for ESP8266 from it. I connect GND of ESP8266 to USBtoTTL one and connect CH_PD and VCC of ESP8266 to USBtoTTL 3.3v port. then connect RX of ESP8266 to TX of Arduino and TX of ESP8266 to RX of Arduino... now everything is OK and PC give 2 separate port(COM3 and COM4) to Arduino and ESP8266 and I can communicate with both Arduino and ESP8266.

I hope this is be useful for others...

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