BleakCharacteristicNotFoundError

[Skorpeo]

I have a bluetooth relay that I am trying to control. I sniffed the packets from my iphone and and the "on" command trace shows the following commands

ATT Send         0x0057  00:00:00:00:00:00  Write Command - Handle:0x001D - Value: 6E20 0000 2001 0101 0100 6F
	Write Command - Handle:0x001D - Value: 6E20 0000 2001 0101 0100 6F
	Opcode: 0x52
	Attribute Handle: 0x001D (29)
	Value: 6E20 0000 2001 0101 0100 6F

    Handle Value Notification - Handle:0x0019 - Value: 6E20 0000 2001 0100 006F
	Opcode: 0x1B
	Attribute Handle: 0x0019 (25)

ATT Send         0x0057  00:00:00:00:00:00  Write Command - Handle:0x001D - Value: 6E20 0000 2002 0101 026F
	Write Command - Handle:0x001D - Value: 6E20 0000 2002 0101 026F
	Opcode: 0x52
	Attribute Handle: 0x001D (29)
	Value: 6E20 0000 2002 0101 026F

ATT Receive      0x0057  00:00:00:00:00:00  Handle Value Notification - Handle:0x0019 - Value: 6E20 0000 2002 0101 026F
	Handle Value Notification - Handle:0x0019 - Value: 6E20 0000 2002 0101 026F
	Opcode: 0x1B
	Attribute Handle: 0x0019 (25)

The service discovery shows:

  [Characteristic] 00002a50-0000-1000-8000-00805f9b34fb (Handle: 13): PnP ID (read), Value: bytearray(b'\x02\x8a$f\x82\x01\x00')
[Service] 0000180f-0000-1000-8000-00805f9b34fb (Handle: 15): Battery Service
  [Characteristic] 00002a19-0000-1000-8000-00805f9b34fb (Handle: 16): Battery Level (read,notify), Value: bytearray(b'c')
    [Descriptor] 00002902-0000-1000-8000-00805f9b34fb (Handle: 18): Client Characteristic Configuration, Value: bytearray(b'')
[Service] 00010203-0405-0607-0809-0a0b0c0d1912 (Handle: 19): Unknown
  [Characteristic] 00010203-0405-0607-0809-0a0b0c0d2b12 (Handle: 20): Unknown (read,write-without-response), Value: bytearray(b'\x00'), Max write w/o rsp size: 93
    [Descriptor] 00002901-0000-1000-8000-00805f9b34fb (Handle: 22): Characteristic User Description, Value: bytearray(b'OTA')
[Service] 00010203-0405-0607-0809-0a0b0c0d1910 (Handle: 23): Unknown
  [Characteristic] 00010203-0405-0607-0809-0a0b0c0d2b10 (Handle: 24): Unknown (notify)
   [Descriptor] 00002902-0000-1000-8000-00805f9b34fb (Handle: 26): Client Characteristic Configuration, Value: bytearray(b'\x00')
    [Descriptor] 00002901-0000-1000-8000-00805f9b34fb (Handle: 27): Characteristic User Description, Value: bytearray(b'Telink SPP: Module->Phone')
  [Characteristic] 00010203-0405-0607-0809-0a0b0c0d2b11 (Handle: 28): Unknown (write-without-response), Max write w/o rsp size: 93
    [Descriptor] 00002901-0000-1000-8000-00805f9b34fb (Handle: 30): Characteristic User Description, Value: bytearray(b'Telink SPP: Phone->Module')

I am trying to reproduce the command in bleak:

async with BleakClient(address) as client:
        await client.write_gatt_char(0x001D, bytes.fromhex("6E2000002001010101006F"), response=False)
        await client.write_gatt_char(0x001D, bytes.fromhex("6E20000020020101026F"), response=False)

but I get error:

bleak.exc.BleakCharacteristicNotFoundError: Characteristic 29 was not found!

I am a little confused because the service explorer doesn't show a characteristic with handle 29 but the packet trace shows the app sending them yet I can't send them with bleak.

Any pointers/input/assistance is greatly appreciated. This is all done using a Mac and packets sniffed using an iphone.

[dlech]

To use short UUIDs, you need to turn them into a full UUID with normalize_uuid_16 or drop the 0x and put it in quotes ("001D").

[dlech]

Ah, I see now you are trying to use a handle instead of a UUID. This is not recommended because it doesn't work cross platform or if a device attribute database changes.

Instead, if there is only one characteristic with a given UUID, use the UUID to look it up. Otherwise use the BleakGATTServiceCollection object to find the characteristic you are looking for, e.g. the nth characteristic with some UUID.

[Skorpeo]

Thanks for the reply. I see now that handles seem to be dynamic. I got it to work by using a characteristic uuid - and the magic piece was subscribing to a notification.