Turning off motors if Serial Communication fails

Using Arduino Programming Questions
8

Which means it might be even better to put it in processInput().

So then you could write loop() very cleanly as

void loop() 
{
  int now=millis();

  if (Serial.available())         // Serial Received "Event"
  { 
    processInput();
    last=now;  // reset serial timer
  }

  if ( now - last > interval)       // Serial Timer Expired "Event"
  {
    digitalWrite( M1B, LOW);  // turn off motors
  }
  
}

Note that "else" is not actually necessary.

Grabbing now=millis() at the start of the loop is a often handy.

Better variable names might include:
last ==> lastSerialProcessed or serialTimerStarted (or comm instead of serial)
interval ==> serialTimeout or serialTimeOutInterval

Cheers,
John