Pixl onWrite can receive one character only from nRF Connect or BLEScanner

I think you might need to specify a length:

NRF.setServices({
  0xEDCB : {
    0xBA98 : {
      maxLen : 20, // <- this line here
      description: "MyCharact",
      writable  : true,
      onWrite : function(evt) {
        eventData = evt.data;
      }
    }
  }
},
{ advertise: [ 'EDCB' ]});

That way you can tell it that it can take more data.