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posts/2015-06-05.02/article.m4

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+define(`TITLE', `WiFi Powermeter')
+define(`DATE', `2015-06-05')
+define(`CONTENT', `
+The WiFi Powermeter is a electric power meter which sends its measurement results every minute using the MQTT protocol via WiFi to an MQTT broker in the local network.
+
+Here is the open test setup:
+
+<a href="http://a385e5.files.wordpress.com/2015/06/img_3151.jpg"><img class="alignnone size-medium wp-image-593" src="http://a385e-5.de/wp-content/uploads/IMG_3151-300x225.jpg" alt="IMG_3151" width="300" height="225" /></a>
+
+<!--more-->
+
+
+
+This is how it looks in its box:
+
+<a href="http://a385e-5.de/wp-content/uploads/IMG_3217.jpg"><img class="alignnone size-medium wp-image-594" src="http://a385e-5.de/wp-content/uploads/IMG_3217-225x300.jpg" alt="IMG_3217" width="225" height="300" /></a>
+
+I used it to measure the power consumption of the fridge in the basement or of the PC of my son and so on.
+
+On the MQTT broker there is one subscriber who transfers all the measurements into a MongoDB, where it now waits for further analysis.
+
+The hardware is rather simple: an Arduino Mega (since it has more than one serial interface), an Arduino WiFi shield and on top a RS485 adaptor for the Modbus communication.
+
+For the software I'm using the Modbus library from <a href="https://code.google.com/p/simple-modbus/">https://code.google.com/p/simple-modbus/</a>. Unfortunately, it disappeared right there, don't know why. Seems, that I have to be really careful with the files. The MQTT library I'm using is from <a href="http://knolleary.net/arduino-client-for-mqtt/">http://knolleary.net/arduino-client-for-mqtt/</a>.
+
+With the couple of the WiFi library and the MQTT library I ran into two problems.
+
+First, it was not possible to establish an MQTT connection at all. Strange, since via Ethernet it wasn't a problem at all. I found, supported by Google that that the WiFi library and the shield handles a <tt>stop()</tt> on a fresh client socket somewhat strange: The socket can not be open or will be closed immediately after opening. Since at least in my case I always give a fresh <tt>WiFiClient</tt> into the <tt>PubSubClient</tt> I removed the <tt>connected()</tt> call from the top of the <tt>connect</tt> method in <tt>PubSubClient.cpp</tt>.
+
+Second, I ran into the problem that messages I published via PubSubClient using an Arduino Mega and an Arduino Wifi Shield which are large than about 80 octets did not appear at the broker. I found by googling that it is not possible to send more than 90 octets using the Wifi Shield and the related library at once: http://mssystems.emscom.net/helpdesk/knowledgebase.php?article=51
+And that exactly what I experienced too: I increased the buffer size one by one and at 90 octets it stopped and the WiFi library lost the connection.
+
+Using this patch, which I applied against release 1.9.1 everything works fine now:
+
+[code language="cpp"]
+42,43c42
+&lt; //   if (!connected()) {
+&lt;    if (true) {
+---
+&gt;    if (!connected()) {
+217d215
+&lt;       // Serial.print(&quot;pub len: &quot;); Serial.println(length-5);
+219,220d216
+&lt;    } else {
+&lt;      // Serial.println(&quot;connection lost&quot;);
+290,308c286
+&lt; 
+&lt; //   Serial.print(&quot;write len: &quot;); Serial.println(length+1+llen);
+&lt; //   size_t olen = length + 1 + llen;
+&lt; //   rc = _client-&gt;write(buf+(4-llen),length+1+llen);
+&lt; 
+&lt;    const size_t SEND_AT_ONCE = 64;
+&lt;    size_t remains = length + 1 + llen;
+&lt;    // Serial.print(&quot;write len: &quot;); Serial.println(remains);
+&lt;    const uint8_t *writebuf = buf + (4 - llen);
+&lt;    bool result = true;
+&lt;    while ((remains &gt; 0) &amp;&amp; result) {
+&lt;      size_t actuallySendChars = (remains &gt; SEND_AT_ONCE) ? SEND_AT_ONCE : remains;
+&lt;      // Serial.print(&quot;tbs: &quot;); Serial.println(actuallySendChars);
+&lt;      size_t sentChars = _client-&gt;write(writebuf, actuallySendChars);
+&lt;      result = sentChars == actuallySendChars;
+&lt;      remains -= sentChars;
+&lt;      writebuf += sentChars;
+&lt;    }
+&lt; 
+---
+&gt;    rc = _client-&gt;write(buf+(4-llen),length+1+llen);
+311,312c289
+&lt; //   return (rc == 1+llen+length);
+&lt;    return result;
+---
+&gt;    return (rc == 1+llen+length);
+357,362c334
+&lt;    //Serial.print(&quot;rc: &quot;); Serial.println(rc);
+&lt;    if (!rc) {
+&lt;      //Serial.println(&quot;would stop&quot;);
+&lt;      _client-&gt;stop();
+&lt;      // while (true);
+&lt;    }
+---
+&gt;    if (!rc) _client-&gt;stop();
+[/code]
+
+
+Sources for the firmware are here: <a href="http://a385e-5.de/wp-content/uploads/WiModbusGateway.zip">WiModbusGateway</a>
+')