My Arduino freezes if power removed from the HMI I have an Arduino (Pro Mini) connected to an HMI display over serial. When the user pushes a button on the HMI screen, it activates a relay attached to one of the digital pins on the Arduino.

For safety reasons, if the Arduino loses communications with the HMI, I want it to turn off the relay. So, when the button has been pushed, it triggers a polling sequence to verify that the button is still being pushed (I'm not relying on the button release event, again for safety).

The following code works perfectly if I disconnect either of the signal lines (tx or rx) or ground from the HMI. If I disconnect the +5 volts, however, the Arduino freezes completely, and therefore never shuts down the relay.

What could be causing this, and what can I do to overcome the problem?

The code is as follows, however, it works as expected if the data lines or ground are severed.

int getNumber(const char *reqString)
{
  sendString(reqString);

  long startTime = currentMillis;
  long waitTimer = currentMillis - startTime;

  bool responded = false;
  int endcount = 0;
  byte inbyte;
  byte bytebuffer[80];
  int bytecount = 0;

  while (waitTimer &lt; 100 &amp;&amp; !responded)
  {

      if (HMI.available())
      {
        inbyte = HMI.read();

        if (inbyte == 0xFF)
        {
          endcount++;
          if (endcount == 3)
          {
            responded = true;
            endcount = 0;
          }
        }
        else
        {
          endcount = 0;
        }

        bytebuffer[bytecount] = inbyte;
        bytecount++;
      }

      // Update the wait Timer.
      waitTimer = millis() - startTime;
  }

  // If we received a response, return the first data byte. If not, return 0
  if (responded)
   {
     Serial.println("Received response");
     return bytebuffer[1];
   }
   else
   {
 Serial.println("No response");
     return 0;
   }
}
It seems that Software Serial is waiting for "high" on RX 

Only a hardware solution comes to my mind 







simulate this circuit &ndash; Schematic created using CircuitLab

This way Arduino RX will be released on HMI power loss

Later edit

You can also use the watchdog timer to unfreeze your code when unresponsive. It will reset the Arduino if wdt_reset() is not called in a given amount of time and you can make the cleaning you need before using Software Serial again. Here is a useful tutorial.