Ajax向php提交按钮点击，然后提交到串行设备

我正在尝试通过ajax和php让按钮在串行设备arduino上启动一个功能，但似乎无法弄清楚。

这是我的网页：

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script type="text/javascript" src="http://code.jquery.com/jquery-1.11.0.min.js"></script>
    <script type="text/javascript">
	$(function() {	
		$('#contact_form').on('submit', function (e) {	
			e.preventDefault();
			$.ajax({	
    			type: "POST",
				url: '/test/SubmitFormWORefresh.php',
				data: $('#contact_form').serialize(),
    			success: function() {
					alert('form was submitted');
      				
				}
  			});
  			return false;
		});
	});
	</script>
<meta charset="utf-8">
<title>Enroll</title>
</head>
<div id="contact_form">
<form name="contact" action="">
  <fieldset>
    <input type="submit" name="rcmd" class="button" id="submit_btn" value="Enroll" /><br />
  </fieldset>
</form>
</div>

这是我的 php：

<?php
$verz="1.0";
$comPort = "/dev/ttyACM0"; /*change to correct com port */
$PHP_SELF="index.php"; //This php file locate it from root
if (isset($_POST["rcmd"])) {
	$rcmd = $_POST["rcmd"];
	switch ($rcmd) {
     	case Stop:
		$fp =fopen($comPort, "w");
		sleep(2);
  		fwrite($fp, 1); /* this is the number that it will write */
  		fclose($fp);
  		break;
		
  		case Enroll:
		$fp =fopen($comPort, "w");
		sleep(2);
  		fwrite($fp, 3); /* this is the number that it will write */
  		fclose($fp);
  		break;
		default:
 		 die('Crap, something went wrong. The page just puked.');
	}/*end switch case*/
}/*end if statement*/
?>

当我运行它时，我得到一个对话框，显示表单已提交，但串行设备没有响应它。任何帮助将不胜感激。