Electronic – arduino – How to read and display data from a CO2 logger connected via RS-232

arduinoserial

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.

Best Answer

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);

Related Solutions

Electronic – arduino – Applescript to Arduino LCD only works with Serial Monitor

I think you are bitten by CR-LF (carriage return [0x0d, '\r'], line feed [0x0a, '\n']). Sometimes CR comes first followed by a LF; sometimes the LF comes first, followed by the CR; sometimes only a CR is sent; sometimes only a LF is sent.

It really depends on the application and/or operating system you use. I'd advice to change your sketch in such a way that it doesn't print the actual characters, but prints the hexcodes. That will be the easiest way to troubleshoot this problem.

Don't filter the incoming data, so remove the '\n'-check:

if (recieved == '\n')

Replace:

lcd.print(inData);

by:

char ascii[ 8 ];
sprintf( ascii , "0x%02x " , inData );
lcd.print( ascii );

to view the data in hex.

Once you figured out what codes are arriving at your Arduino, you can change your code accordingly. Make your code robust as possible, as CR/LF comes in many variations. I'd even account for a NULL (0x00) character.

Electronic – arduino – Trasferring Arduino chip from prototype platform to a PCB (Serial Communication)

Transferring to PCB

See How do I create a permanent application from something prototyped with Arduino? for general guidance. Doesn't cover serial comms specifically.

Serial comms

For serial comms you can connect a suitable USB-to-serial cable such as an FTDI cable. You'd connect it to header pins on your PCB that you linked to the appropriate pins on the ATmega.

Or you can add the USB interface to your PCB design using the same FTDI FT232RL chip and support components used on the older Arduinos. See page 27 of the data sheet. I believe you can omit the level convertor (as you are connecting the ATmega IC not a +/- 12V serial device). See also SparkFun's breakout as an example.