title: Arduino + GSM SIM900 returns ÿ char
body: <p>So, when I send at example "AT" via Serial, GSM always returns me ÿ char. "AT" returns ÿÿ - any one char returns one ÿ. Where is the problem? Maybe connection is wrong? Should I use any resistors and connect GSM TX also to the GND?</p>
  
  <p>UPDATE: GSM responds only on CR and NL giving ÿ of each. Any ideas?</p>
  
  <p>Code:</p>
  
  <pre><code>#include &lt;SoftwareSerial.h&gt;
  
  #define rx 10
  #define tx 11
  SoftwareSerial gsm(rx, tx);
  
  void setup()  
  {
     Serial.begin(9600);
     gsm.begin(9600);
  }
  
  void loop()
  { 
      if(gsm.available())
         Serial.write(gsm.read());
  
      if(Serial.available())
         gsm.write(Serial.read());
  }
  </code></pre>
  
  <p>Interfacing:</p>
  
  <p><img src="https://i.stack.imgur.com/L0h1q.jpg" alt="Interfacing"></p>