title: iOS Bluetooth Low Energy (BLE) not discovering TX characteristic
body: <p>I am using an an arduino feather BLE board and trying to create an iOS app that can send data to the board over BLE. I can connect but I can't get access to the txCharacteristic</p>
  
  <pre><code>func peripheral(_ peripheral: CBPeripheral, didDiscoverCharacteristicsFor service: CBService, error: Error?) {
      guard let characteristics = service.characteristics else {
          return
      }
  
      for characteristic in characteristics {
          if characteristic.properties.contains(.write) || characteristic.properties.contains(.writeWithoutResponse) {
              writableCharacteristic = characteristic
          }
  
          //            if(characteristic.uuid == CBUUID(string: "6e400002-b5a3-f393-e0a9-e50e24dcca9e"))
          //            {
          //                txCharacteristic = characteristic
          //            }
  
          var txUUID = CBUUID(string: "6e400002-b5a3-f393-e0a9-e50e24dcca9e")
          let temp = characteristic.uuid.uuidString;
          switch characteristic.uuid {
          case txUUID:
              txCharacteristic = characteristic;
              break;
          default:
              break;
          }
  
          peripheral.setNotifyValue(true, for: characteristic)
      }
  }
  </code></pre>
  
  <p>This code works, but only discovers the following UUIDs:</p>
  
  <pre><code>temp    String  "00001532-1212-EFDE-1523-785FEABCD123"  
  temp    String  "00001531-1212-EFDE-1523-785FEABCD123"  
  temp    String  "00001534-1212-EFDE-1523-785FEABCD123"  
  </code></pre>
  
  <p>I have figured out that these UUID's are DFU UUIDs. How can I discover the txCharacteristic instead?</p>
  
  <hr>
  
  <p>Added for more information how I am calling discoverCharacteristics():</p>
  
  <pre><code>extension SimpleBluetoothIO: CBPeripheralDelegate {
  func peripheral(_ peripheral: CBPeripheral, didDiscoverServices error: Error?) {
      guard let services = peripheral.services else {
          return
      }
  
      targetService = services.first
      if let service = services.first {
          targetService = service
          peripheral.discoverCharacteristics(nil, for: service)
      }
  }
  </code></pre>