Writing to the NUS TX characteristic

[espruino-discuss2]

Posted at 2023-05-08 by user155409

I'm attempting to connect to the NUS service to send javascript commands to the Bangle from an external javascript file. I can see the service ['6e400001-b5a3-f393-e0a9-e50e24dcca9e'] and the TX characteristic ['6e400002-b5a3-f393-e0a9-e50e24dcca9e'] from javascript. I can even write to it, but the Bangle doesn't actually execute the commands.

This is what my JS looks like:

// I have a button in the html that attempts to send a command to the BangleJS
commandButton.addEventListener('click', async () => {
    
    // I'm attempting to send javascript commands of the form that would work in the Web IDE directly to the characteristic -- str2ab converts this string into an arrayBuffer for sending 
    await sendCommand(device, str2ab("var on = false;setInterval(function() {on = !on;LED1.write(on);}, 500);"), DoneSending());
    console.log('Wrote command to characteristic');
  });

  // this function does the work of actually getting the service and writing to the characteristic
  async function sendCommand(device, msg, callback) {
    return getPrimaryService().then(function(s) {
      return s.getCharacteristic(NUS_TX_CHARACTERISTIC_UUID);
    }).then(function(c) {      
      function sender(resolve, reject) {
        if (msg.length) {
          c.writeValue(msg).then(()=>{
            sender(resolve, reject);
          }).catch(reject);
        } else  {
          resolve();
        }
      }
      return new Promise(sender);
    }).then(function() {
      if (callback) callback();
    });
  }

My callback gets called and things appear succesful, but the Bangle doesn't actually execute the JS that I sent it. Do I have to format it some other way? Do something else to actually invoke it? I'm so stumped! Any help would be amazing--thank you!

[espruino-discuss2]

Posted at 2023-05-08 by @fanoush

well, TX is output, you should write to RX

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Posted at 2023-05-08 by user155409

oops, good point! RX doesn't work either though, so I don't think that's the problem (I kept switching them back and forth to see if that's what was wrong :)).

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Posted at 2023-05-08 by @fanoush

your string may be too long, sending 72bytes will
work only if negotiated MTU is >=72, also you are not sending end of line

and BTW there is ble uart library you may use
https://www.espruino.com/BLE+UART

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Posted at 2023-05-08 by user155409

oh excellent--thank you so much, I will try it!

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Posted at 2023-05-09 by @gfwilliams

You probably also need to write a newline character at the end \n. It's currently as if you're writing the data on the input line but not hitting return :)