Reading analog voltage from slow inputs + graph

In this example we will plot on graph voltage measured on one of Red Pitaya slow analog inputs. We take Reading analog voltage from slow inputs example as a basis. Web UI

You also need new .js file:

jquery.flot.js - for drawing graphs

<script src="js/jquery-2.1.3.min.js"></script>
<script src="js/jquery.flot.js"></script>
<script src="js/pako.js"></script>
<script src="js/app.js"></script>

Add graph placeholder using this string in index.html:

< div id='placeholder'></div>

In app.js we should draw signal value on graph. Change APP.ws.onmessage() callback. Now we should decompress message and push it to stack. Data arrives quite faster than we can process it. That’s why we should firsly save it, and then process.

var data = new Uint8Array(ev.data);
var inflate = pako.inflate(data);
var text = String.fromCharCode.apply(null, new Uint8Array(inflate));
var receive = JSON.parse(text);

if (receive.signals) {

Processing of signals is also located in APP.processSignals() function, which is called every 15ms by APP.signalHandler(). In this function we draw points according to values and update graph:

var pointArr = [];
var voltage;

for (sig_name in new_signals) {

    if (new_signals[sig_name].size == 0) continue;

    var points = [];
    for (var i = 0; i < new_signals[sig_name].size; i++) {
        points.push([i, new_signals[sig_name].value[i]]);


    voltage = new_signals[sig_name].value[new_signals[sig_name].size - 1];

$('#value').text(parseFloat(voltage).toFixed(2) + "V");

APP.plot.draw(); Controller

As in a previous tutorial we will read values from pins using controller. In main.cpp we should make changes.

As you remember we added signal in global variables:


Now SIGNAL_SIZE_DEFAULT should be 1024. We will send 1024 points to Web UI.

In rp_app_init() we should set signal update interval:


SIGNAL_UPDATE_INTERVAL is also our constant. It is 10ms. It means how often program will call function void UpdateSignals(void). In this function we will read value from AIpin0 and write it to signal:

rp_AIpinGetValue(0, &val);

val - is buffer variable, which will get value from AIpin0. We should write this value to data vector in last position. First measurement should be deleted from this vector.

g_data.push_back(val * GAIN.Value());

After all steps write data to signal and it will be sent to server.

for(int i = 0; i < SIGNAL_SIZE_DEFAULT; i++)
    VOLTAGE[i] = g_data[i];