How to read and display data from a CO2 logger connected via RS-232?

Question

I got a Licor CO2-Logger (Li-820) for a scientific experiment. The Li-820 outputs an analog signal (voltage) and also offers a serial communication via a RS-232 port. I would like to sniff the serial communication with an Arduino Uno and display the CO2 values on a LCD display such that I can control the analog signal that is logged in a computer system. I want to display both the analog and the digital signal.

I use a RS-232 level shifter to connect the Licor CO2 analyzer to the Arduino Uno, and I can sniff the COM port successfully both with the Arduino serial monitor and a sniffer program. Over the RS-232 port the Li-820 device outputs an XML-like line that is as follows (Line breaks and indentation added for readability):

<li820>
  <data>
    <celltemp>5.1252350e1</celltemp>
    <cellpres>9.7159633e1</cellpres>
    <co2>5.2527637e2</co2>
    <co2abs>7.7893261e-2</co2abs>
    <ivolt>1.1386718e1</ivolt>
    <raw>3950702,3808028</raw>
  </data>
</li820>

I would like to parse that information for the relevant part with the Arduino Uno which is the <co2>5.2527637e2</co2> ("CO2" value) and first output it to the serial monitor. Next, I will display that value on a LCD display. This last step should be a minor problem.

So, how can I parse the information for the relevant bits and then display it to the serial monitor?

I looked into many examples on the Internet. A modified version of the working code from here was the closest I got.

Answer 1

I'm assuming that you can already read the string into a buffer so I will start at the buffer. Your code should go something like this (you will need to import the string library):

String wholeString = String(yourLoadedCharBuffer);
int startTag = wholeString.indexOf("<co2>");
int endTag = wholeString.indexOf("</co2>");

String co2FullString = wholeString.substring(startTag+5,endTag-1);

This gives us "5.2527637e2", you add 5 to the start tag because it is 4 characters long and you want to start at the "5", and you subtract 1 from the endtag, because we don't want the first character of it.

int eStart = co2FullString.indexOf("e");
String decimalString = co2FullString.substring(0,eStart-1);
String powerString = co2FullString.substring(estart+1, co2FullString.length-1);

char decimalCharArray[] = decimalString.toCharArray();
char powerCharArray[] = powerString.toCharArray();

float decimal = atof(&decimalCharArray[0]);
float power = atof(&powerCharArray[0]);

float finalCO2Value = pow(decimal,power);

Answer 2

This may be obvious to you, but the output is being sent in XML format. Getting an XML parsing library for arduino will go a long way to meeting your needs.