GET AJAX - Json size issue on fast updates

I am using AJAX to get soft realtime values ​​from a JSON file.

In fact, on my server, the C program updates the Json file with some information (200ms period). In my webpage, I am getting this Json file to display this information with a 250ms timer. I am using Ajax to get Json file.

The problem is the resizing of the Json file, for example:

My Json file:

{ "data1": { "val1": "12", "val2": "125"}, "data2": { "val1": "253", "val2": "0"} }

      

        
        
        
      

    

And become:

{ "data1": { "val1": "12", "val2": "14"}, "data2": { "val1": "253", "val2": "0"} }

      

        
        
        
      

    

Error occured, response from Ajax request:

{ "data1": { "val1": "12", "val2": "14"}, "data2": { "val1": "253", "val2": "0"} }\u0

      

        
        
        
      

    

"\ u0" appears to replace the missing character.

The same appears when the file gets longer, the answer is:

{ "data1": { "val1": "12", "val2": "14"}, "data2": { "val1": "253", "val2": "0"}

      

        
        
        
      

    

The last curly brace is missing.

My Javascript code for Ajax:

function getXMLHttpRequest() {
    var xhr = null;

    if (window.XMLHttpRequest || window.ActiveXObject) {
        if (window.ActiveXObject) {
            try {
                xhr = new ActiveXObject("Msxml2.XMLHTTP");
            } catch(e) {
                xhr = new ActiveXObject("Microsoft.XMLHTTP");
            }
        } else {
            xhr = new XMLHttpRequest(); 
        }
    } else {
        alert("Your browser doesn't support the XMLHTTPRequest object ...");
        return null;
    }
    return xhr;
}

function getDataAjax(callback, phpFile) {

    xhr = getXMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4 && (xhr.status == 200 || xhr.status == 0)) {
            callback(xhr.responseText);
        }
    };

    xhr.open("GET", phpFile, true);
    xhr.send(null);
}

      

        
        
        
      

    

The Json file is correct, but there is no Ajax response.

Thank you for your help.