ERROR Input/output error

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OpenC0C(): CreateFile("\.\COM3") ERROR Input/output error (5)

Reconnecting in 3s…

when trying to start server. what could be ?

  Download latest Blynk library here:

  Blynk is a platform with iOS and Android apps to control
  Arduino, Raspberry Pi and the likes over the Internet.
  You can easily build graphic interfaces for all your
  projects by simply dragging and dropping widgets.

    Downloads, docs, tutorials:
    Sketch generator: 
    Blynk community:  
    Follow us:        

  Blynk library is licensed under MIT license
  This example code is in public domain.

  =>          USB HOWTO:

  Feel free to apply it to any other example. It's simple!

/* Comment this out to disable prints and save space */
#define BLYNK_PRINT DebugSerial

// You could use a spare Hardware Serial on boards that have it (like Mega)
#include <SoftwareSerial.h>
SoftwareSerial DebugSerial(2, 3); // RX, TX

#include <BlynkSimpleStream.h>

// You should get Auth Token in the Blynk App.
// Go to the Project Settings (nut icon).
char auth[] = "my token";

void setup()
  // Debug console

  // Blynk will work through Serial
  // Do not read or write this serial manually in your sketch
  Blynk.begin(Serial, auth);

void loop()

So, you are not going to give any further details?? Like what you are trying to do that gets this error… Thankfully I still use this USB Serial link for one of my projects, so I recognise it :wink:

So, no, you are not trying to start the server :stuck_out_tongue: You are trying to start the USB to IP Serial interface… AKA blynk-ser script

And the most likely issue is that you have something else already connected to COM3 (IDE Serial monitor??), or it is being blocked by your Antivirus or something.

Shut off the IDE Serial monitor and run it again. If that doesn’t clear it up then temporarily disable your AV and try again.

1 Like

Following this link tutorial I realized that I was needing to close arduino IDE that was conflicting with script when trying to reach COM3. As sugestion maybe script could be improve displaying this kind error/warning when this is happening so then we will fix it fast.

Thanks Gunner

Yep :slight_smile: Glad you got it working.

Well, as is it a common understanding since DOS days and current Serial setup with the IDE (not just with Blynk), that Serial can only communicate with ONE thing at a time. But even accounting for non-computer savvy, having warnings and notices for every little niggly bit can get overwhelming on their own.

Besides, someone would still miss it anyhow. We already have a Welcome Topic notice to search this forum before posting… and this particular answer would have been found, but reading that rarely happens :expressionless: