add watchdog example

examples/watchdog.js

@@ -0,0 +1,42 @@
+// create an empty modbus client
+//var ModbusRTU = require("modbus-serial");
+var ModbusRTU = require("../index");
+var client = new ModbusRTU();
+
+// open connection to a tcp line
+client.connectTCP("192.168.1.115");
+
+// the global last_connection time for the watchdog
+var last_connection = new Date();
+
+/* check time of last_connection and restart-reading if needed
+ */
+var watchdog = function() {
+    var time_diff = new Date() - last_connection;
+    if (time_diff > 1000) {
+        console.log('Oyyy ... we did not get data for 1 sec, will try again.');
+        run();
+    }
+}
+
+/* if last request was good - send a read request evry 0.5 sec
+ * and update last_connection
+ */
+var run = function() {
+    console.log('Send a request for input registers.');
+    client.readInputRegisters(0, 10, function(err, data) {
+        if (!err) {
+          console.log(data.data);
+          last_connection = new Date();
+
+          setTimeout(run, 500);
+        }
+    });
+}
+
+/* run the watchdog
+ */
+setInterval(function() {
+    watchdog();
+}, 500);
+