I have a chinese vernier caliper with BLE interface. It normally connects to a device as HID keyboard and this works fine. But I need the data as RS232, because it has to fit to an old machine. With Android BLE analyzer I fond out, that I can maket the caliper transfer data if I use this sequence:

scan
connect to 'Bluetooth Keyboard' service 0x1812 (HID)
just for information read characteristic 0x2a4e (Protocol Mode) with result 0x01
Write 0x00 to characteristic 0x2a4e (Protocol Mode). This is mandatory, if not there is no notification.
change to characteristic 0x2a22 (Boot Keyboard Input Report)
enable 'Read on Notify' I get data then after each press of the transmit button.

So I tried to adapt the ESP32 Client example in Arduino, but this did not work. I left everything as it was, just added the characteristic for the Protocol Mode. But when I read from this characteristic, result is always 0x00, never 0x01 what it shows in any BLE analyzer. Subsequently I also cannot write 0x00 to this characteristic. Any ideas or suggestions? Thanks Uli

`/**

A BLE client example that is rich in capabilities.
There is a lot new capabilities implemented.
author unknown
updated by chegewara */

include "BLEDevice.h"

//#include "BLEScan.h"

// The remote service we wish to connect to. static BLEUUID serviceUUID("1812"); // The characteristic of the remote service we are interested in. static BLEUUID charUUID("2a22"); static BLEUUID modUUID("2a4e");

static boolean doConnect = false; static boolean connected = false; static boolean doScan = false; static BLERemoteCharacteristic pRemoteCharacteristic; static BLEAdvertisedDevice myDevice;

static void notifyCallback( BLERemoteCharacteristic pBLERemoteCharacteristic, uint8_t pData, size_t length, bool isNotify) { Serial.print("Notify callback for characteristic "); Serial.print(pBLERemoteCharacteristic->getUUID().toString().c_str()); Serial.print(" of data length "); Serial.println(length); Serial.print("data: "); Serial.println((char*)pData); }

class MyClientCallback : public BLEClientCallbacks { void onConnect(BLEClient* pclient) { }

void onDisconnect(BLEClient* pclient) { connected = false; Serial.println("onDisconnect"); } };

bool connectToServer() { Serial.print("Forming a connection to "); Serial.println(myDevice->getAddress().toString().c_str());

BLEClient*  pClient  = BLEDevice::createClient();
Serial.println(" - Created client");

pClient->setClientCallbacks(new MyClientCallback());

// Connect to the remove BLE Server.
pClient->connect(myDevice);  // if you pass BLEAdvertisedDevice instead of address, it will be recognized type of peer device address (public or private)
Serial.println(" - Connected to server");
pClient->setMTU(517); //set client to request maximum MTU from server (default is 23 otherwise)

// Obtain a reference to the service we are after in the remote BLE server.
BLERemoteService* pRemoteService = pClient->getService(serviceUUID);
if (pRemoteService == nullptr) {
  Serial.print("Failed to find our service UUID: ");
  Serial.println(serviceUUID.toString().c_str());
  pClient->disconnect();
  return false;
}
Serial.println(" - Found our service");

// Obtain a reference to the Protocol Mode characteristic in the service of the remote BLE server.
pRemoteCharacteristic = pRemoteService->getCharacteristic(modUUID);
if (pRemoteCharacteristic == nullptr) 
  {
  Serial.print("Failed to find our characteristic modUUID: ");
  Serial.println(charUUID.toString().c_str());
  pClient->disconnect();
  return false;
  }
Serial.println("Found our protocol mode characteristic");  
Serial.println(pRemoteCharacteristic->getUUID().toString().c_str());
if(pRemoteCharacteristic->canRead()) 
  {  
  Serial.printf("Mode: %D\n", pRemoteCharacteristic->readUInt8());
  if(pRemoteCharacteristic->canWriteNoResponse())
    {
      pRemoteCharacteristic->writeValue((uint16_t)0x00,true);
      Serial.println("Can write to protocol mode characteristic");
      delay(1000);
      Serial.printf("Mode changed to : %D\n", pRemoteCharacteristic->readUInt8());
    }
  }  

// Obtain a reference to the characteristic in the service of the remote BLE server.
pRemoteCharacteristic = pRemoteService->getCharacteristic(charUUID);
if (pRemoteCharacteristic == nullptr) {
  Serial.print("Failed to find our characteristic UUID: ");
  Serial.println(charUUID.toString().c_str());
  pClient->disconnect();
  return false;
}
Serial.println(" - Found our characteristic");
Serial.println(pRemoteCharacteristic->getUUID().toString().c_str());

// Read the value of the characteristic.
if(pRemoteCharacteristic->canRead()) {
  std::string value = pRemoteCharacteristic->readValue();
  Serial.print("The characteristic value was: ");
  Serial.println(value.c_str());
}

if(pRemoteCharacteristic->canNotify())
  {
  const uint8_t notifyOn[] = {0x1, 0x0};
  pRemoteCharacteristic->getDescriptor(BLEUUID((uint16_t)0x2902))->writeValue((uint8_t*)notifyOn, 2, true);
  pRemoteCharacteristic->registerForNotify(notifyCallback);
  }
connected = true;
return true;

} /**

Scan for BLE servers and find the first one that advertises the service we are looking for. */ class MyAdvertisedDeviceCallbacks: public BLEAdvertisedDeviceCallbacks { /**
- Called for each advertising BLE server. */ void onResult(BLEAdvertisedDevice advertisedDevice) { Serial.print("BLE Advertised Device found: "); Serial.println(advertisedDevice.toString().c_str());
// We have found a device, let us now see if it contains the service we are looking for. if (advertisedDevice.haveServiceUUID() && advertisedDevice.isAdvertisingService(serviceUUID)) {

BLEDevice::getScan()->stop(); myDevice = new BLEAdvertisedDevice(advertisedDevice); doConnect = true; doScan = true;

} // Found our server } // onResult }; // MyAdvertisedDeviceCallbacks

void setup() { Serial.begin(115200); Serial.println("Starting Arduino BLE Client application..."); BLEDevice::init("");

// Retrieve a Scanner and set the callback we want to use to be informed when we // have detected a new device. Specify that we want active scanning and start the // scan to run for 5 seconds. BLEScan* pBLEScan = BLEDevice::getScan(); pBLEScan->setAdvertisedDeviceCallbacks(new MyAdvertisedDeviceCallbacks()); pBLEScan->setInterval(1349); pBLEScan->setWindow(449); pBLEScan->setActiveScan(true); pBLEScan->start(5, false); } // End of setup.

// This is the Arduino main loop function. void loop() {

// If the flag "doConnect" is true then we have scanned for and found the desired // BLE Server with which we wish to connect. Now we connect to it. Once we are // connected we set the connected flag to be true. if (doConnect == true) { if (connectToServer()) { Serial.println("We are now connected to the BLE Server."); } else { Serial.println("We have failed to connect to the server; there is nothin more we will do."); } doConnect = false; }

// If we are connected to a peer BLE Server, update the characteristic each time we are reached // with the current time since boot. if (connected) { String newValue = "Time since boot: " + String(millis()/1000); Serial.print("."); //ln("Setting new characteristic value to \"" + newValue + "\"");

// Set the characteristic's value to be the array of bytes that is actually a string.
//pRemoteCharacteristic->writeValue(newValue.c_str(), newValue.length());

}else if(doScan){ BLEDevice::getScan()->start(0); // this is just example to start scan after disconnect, most likely there is better way to do it in arduino }

delay(1000); // Delay a second between loops. } // End of `loop``

nkolban / esp32-snippets

Can't read or write characteristic without descriptor #1101

include "BLEDevice.h"