BLE Serial

A tool to connect Bluetooth 4.0+ Low Energy to UART modules and normal PCs/laptops/RaspberryPi.

It fulfills the same purpose as rfcomm bind for the old Bluetooth 2.0, creating a virtual serial port in /dev/pts/x, which makes it usable with any terminal or application.

On Windows it provides a COM port, similar to the Microsoft "Standard Serial over Bluetooth" (a driver which exists since Windows XP and unsurprisingly also does not support BLE standards).

Installation

Standard (via Python Package Index)

The software is written completely in Python and packaged as module, so it can be easily installed with pip:

$ pip install ble-serial

Now you should have 2 new scripts: ble-scan and the main ble-serial.

On Linux/Mac you are ready now and can directly jump to the usage section! For Windows follow the additional steps below.

From source (for developers)

You can clone the repository with:

$ git clone https://github.com/Jakeler/ble-serial.git

Then switch branches, make changes etc... Make sure the dependencies are installed, I recommend to use a virtualenv like this:

$ python -m venv ble-venv
$ source ble-venv/bin/activate
$ pip install -r requirements.txt

The package can be either started directly with -m:

$ python -m ble_serial ARGUMENTS # Main tool = ble-serial
$ python -m ble_serial.scan # BLE scan = ble-scan
$ python -m ble_serial.setup_com0com # Windows only setup = ble-com-setup

Or installed with pip from the current directory:

$ pip install .

and started as usual.

Additional steps for Windows

Windows does not have a builtin feature to create virtual serial ports (like Linux does), so it is required to install a additional driver. I decided to use the open source com0com Null-modem emulator, downloaded from here as signed version. This is required because unsigned drivers can not be installed anymore.

Note: on Windows 10+ you still have to disable secure boot for it to work. Otherwise you might get a Unexpected Error: SerialException when starting ble-serial.

ble-serial includes the ble-com-setup script to make the com0com configuration easier:

> ble-com-setup.exe -h
usage: ble-com-setup [-h] [--install-path INSTALL_PATH]

Setup required COM port pair

optional arguments:
  -h, --help            show this help message and exit
  --install-path INSTALL_PATH
                        Installation directory of the null modem emulator (default: C:/Program Files (x86)/com0com/)

It will request administrator privileges (if it does not already have it) and setup the port in another CMD window:

> ble-com-setup.exe
OK

[New Window]
Changing into C:/Program Files (x86)/com0com/

> Checking port list for BLE
       CNCA0 PortName=-
       CNCB0 PortName=-

BLE port does not exist

> Checking port list for COM9
       CNCA0 PortName=-
       CNCB0 PortName=-

> Trying to create port pair
       CNCA1 PortName=COM9
       CNCB1 PortName=BLE
ComDB: COM9 - logged as "in use"

Setup done!

Hit any key to close

As you can see it created the BLE<->COM9 pair. ble-serial will internally connect to BLE, users can then send/receive the data on COM9.

Otherwise there exist multiple proprietary serial port emulators, these should work too. Just manually create a pair that includes a port named BLE.

Usage

Finding devices

First make sure the bluetooth adapter is enabled, for example with bluetoothctl power on, then the scan function can be used:

$ ble-scan

Started BLE scan

20:91:48:4C:4C:54 (RSSI=-56): UT61E -  JK
...

Finished BLE scan

The output is a list of the recognized nearby devices. After the MAC address and signal strength it prints out the device name, if it can be resolved.

On macOS it displays a system specific device ID instead of a MAC address:

55AEB150-5AF1-4318-A02E-19A46223F572 (RSSI=-70): nRF52 Accelerometer

It can be used simply in place of the MAC to specify a device to connect in the following section.

If there are no devices found it might help to increase the scan time. All discoverable devices must actively send advertisements, the interval of this can be quite long to save power, try for example 30 seconds in this case.

$ ble-scan -h
usage: ble-scan [-h] [-t SEC] [-d ADDR]

Scanner for BLE devices and service/characteristics.

options:
  -h, --help            show this help message and exit
  -t SEC, --scan-time SEC
                        Duration of the scan in seconds (default: 5.0)
  -i ADAPTER, --interface ADAPTER
                        BLE host adapter number to use (default: hci0)
  -d ADDR, --deep-scan ADDR
                        Try to connect to device and read out service/characteristic UUIDs (default: None)
  -s SERVICE_UUID, --service-uuid SERVICE_UUID
                        The service used for scanning of potential devices (default: None)

On Bluetooth 2.0 there was a "serial port profile", with 4.0 - 5.2 (BLE) there is unfortunately no standardized mode anymore, every chip manufacturer chooses their own UUIDs to implement the features.

'0000ff01-0000-1000-8000-00805f9b34fb', # LithiumBatteryPCB adapter: read/notify
'0000ff02-0000-1000-8000-00805f9b34fb', # LithiumBatteryPCB adapter: write
'0000ffe1-0000-1000-8000-00805f9b34fb', # TI CC245x (HM-10, HM-11)

Some usual read/write UUIDs are included in ble-serial, these will be tried automatically if nothing is specified. You might skip this part and start directly with the connection.

Otherwise to find the correct UUID, use the deep scan option. It expects a device MAC/ID, connects to it and reads out all services/characteristic/descriptors:

$ ble-scan -d 20:91:48:4C:4C:54
Started deep scan of 20:91:48:4C:4C:54

SERVICE 00001801-0000-1000-8000-00805f9b34fb (Handle: 12): Generic Attribute Profile
     CHARACTERISTIC 00002a05-0000-1000-8000-00805f9b34fb (Handle: 13): Service Changed ['indicate']
         DESCRIPTOR 00002902-0000-1000-8000-00805f9b34fb (Handle: 15): Client Characteristic Configuration
SERVICE 0000ffe0-0000-1000-8000-00805f9b34fb (Handle: 16): Vendor specific
     CHARACTERISTIC 0000ffe1-0000-1000-8000-00805f9b34fb (Handle: 17): Vendor specific ['read', 'write-without-response', 'notify']
         DESCRIPTOR 00002902-0000-1000-8000-00805f9b34fb (Handle: 19): Client Characteristic Configuration
         DESCRIPTOR 00002901-0000-1000-8000-00805f9b34fb (Handle: 20): Characteristic User Description

Completed deep scan of 20:91:48:4C:4C:54

Now the interesting parts are the characteristics, grouped into services. The ones belows the first service starting with 00002 are not relevant in this case, because they are standard values (for example the device name), if you want to know more look at this list.

After the UUID, handle and type the permissions are listed in []. We are searching for a characteristic that allows writing = sending to the device, the only candidate in here is 0000ffe1-0000-1000-8000-00805f9b34fb (spoiler: a HM-11 module again). Same procedure with the read characteristic, here you have to actually look for notify or indicate, that is how the receiving side is informed about new data in BLE. This module handles both directions through the same characteristic.

Some other chips split it up, for example (this time on macOS with device ID):

$ ble-scan -d 55AEB150-5AF1-4318-A02E-19A46223F572
Started deep scan of 55AEB150-5AF1-4318-A02E-19A46223F572

SERVICE 00000001-af0e-4c28-95a4-4509fd91e0bb (Handle: 10): SDP
     CHARACTERISTIC 00000006-af0e-4c28-95a4-4509fd91e0bb (Handle: 11): Unknown ['read', 'notify']
         DESCRIPTOR 00002902-0000-1000-8000-00805f9b34fb (Handle: 13): Client Characteristic Configuration
     CHARACTERISTIC 00000005-af0e-4c28-95a4-4509fd91e0bb (Handle: 14): TCS-BIN ['write-without-response', 'notify']
         DESCRIPTOR 00002902-0000-1000-8000-00805f9b34fb (Handle: 16): Client Characteristic Configuration

Completed deep scan of 55AEB150-5AF1-4318-A02E-19A46223F572

As you can see, here the read/notify UUID is 00000006-af0e-4c28-95a4-4509fd91e0b and write UUID 00000005-af0e-4c28-95a4-4509fd91e0bb.

Connecting a device

The ble-serial tool itself has a few more options:

$ ble_serial -h
usage: __main__.py [-h] [-v] -d DEVICE [-t {public,random}] [-i ADAPTER] [-m MTU] [-w WRITE_UUID] [-l FILENAME] [-b]
                   [-p PORT] [-r READ_UUID]

Create virtual serial ports from BLE devices.

options:
  -h, --help            show this help message and exit
  -v, --verbose         Increase verbosity, can be specified multiple times for connection/DBus debugging (default: 0)
  -p PORT, --port PORT  Symlink to virtual serial port (default: /tmp/ttyBLE)

connection parameters:
  -t SEC, --timeout SEC
                        BLE connect/discover timeout in seconds (default: 5.0)
  -i ADAPTER, --interface ADAPTER
                        BLE host adapter number to use (default: hci0)
  -m MTU, --mtu MTU     Max. bluetooth packet data size in bytes used for sending (default: 20)

device parameters:
  -d DEVICE, --dev DEVICE
                        BLE device address to connect (hex format, can be separated by colons) (default: None)
  -a {public,random}, --address-type {public,random}
                        BLE address type, only relevant on Windows, ignored otherwise (default: public)
  -s SERVICE_UUID, --service-uuid SERVICE_UUID
                        The service used for scanning of potential devices (default: None)
  -w WRITE_UUID, --write-uuid WRITE_UUID
                        The GATT characteristic to write the serial data, you might use "ble-scan -d" to find it out (default: None)
  -r READ_UUID, --read-uuid READ_UUID
                        The GATT characteristic to subscribe to notifications to read the serial data (default: None)
  --permit {ro,rw,wo}   Restrict transfer direction on bluetooth: read only (ro), read+write (rw), write only (wo) (default: rw)

logging options:
  -l FILENAME, --log FILENAME
                        Enable optional logging of all bluetooth traffic to file (default: None)
  -b, --binary          Log data as raw binary, disable transformation to hex. Works only in combination with -l (default: False)

In any case it needs to know which device to connect, the simple and most reliable way to specify this is by device address/id:

$ ble-serial -d 20:91:48:4c:4c:54
20:38:31.271 | INFO | linux_pty.py: Slave created on /tmp/ttyBLE -> /dev/pts/5
20:38:31.271 | INFO | ble_interface.py: Receiver set up
20:38:31.485 | INFO | ble_interface.py: Trying to connect with 20:91:48:4C:4C:54: UT61E -  JK
20:38:32.844 | INFO | ble_interface.py: Device 20:91:48:4C:4C:54 connected
20:38:32.844 | INFO | ble_interface.py: Found write characteristic 0000ffe1-0000-1000-8000-00805f9b34fb (H. 17)
20:38:32.844 | INFO | ble_interface.py: Found notify characteristic 0000ffe1-0000-1000-8000-00805f9b34fb (H. 17)
20:38:33.128 | INFO | main.py: Running main loop!

This log shows a successful start on Linux, the virtual serial port was opened on /dev/pts/8, the number at the end changes, depending on how many pseudo terminals are already open on the system. It uses the same mechanism on macOS, just the path is slightly different, in the format /dev/ttys000. In addition it automatically creates a symlink to /tmp/ttyBLE, so you can easily access it always on the same file, the default can be changed with the -p/--port option.

On Windows it uses the port pair created in the setup described above, this does not dynamically change and endpoint is always COM9 if you use the default script.

Now it is possible to use any serial monitor program, just connect to that port, baud rate etc. does not matter, it will work with any value (settings are ignored, because it is only virtual). The software acts as transparent bridge, everything that is sent to that virtual port gets transmitted to the BLE module and comes out of the TX pin there. Same in the other direction, everything that the BLE module receives on the RX pin gets transmitted to the PC and shows up in the virtual serial port. This makes it also possible to add ble module to create a wireless serial connection with existing hard/software.

Another way to find a device is by service uuid:

$ ble-serial -s 0000ffe0-0000-1000-8000-00805f9b34fb
20:35:41.964 | INFO | linux_pty.py: Slave created on /tmp/ttyBLE -> /dev/pts/5
20:35:41.964 | INFO | ble_interface.py: Receiver set up
20:35:41.964 | WARNING | ble_interface.py: Picking first device with matching service, consider passing a specific device address, especially if there could be multiple devices
20:35:42.308 | INFO | ble_interface.py: Trying to connect with 20:91:48:4C:4C:54: UT61E -  JK
20:35:43.020 | INFO | ble_interface.py: Device 20:91:48:4C:4C:54 connected
...

This has the advantage that you can replace the device/chip without changing anything in the host script, it will automatically pick the first device that provides the service. It is also possible to add both the service and address, then it will only connect if both match. On early versions of macOS 12 (Monterey) it is always required to filter with the service uuid, otherwise scanning/connecting won't work.

As mentioned before, the start might fail because the characteristic UUIDs are not in the list, then you can manually specify the correct write UUID like this:

$ ble-serial -d 20:91:48:4c:4c:54 -w 0000ffe1-0000-1000-8000-00805f9b34fb

Same for reading with -r/--read-uuid, for example:

$ ble-serial -d 20:91:48:4c:4c:54 -r 0000ffe1-0000-1000-8000-00805f9b34fb

Per default it always tries to setup reading and writing and searches for both characteristics. If none of your characteristics are in the builtin list then you have to specify both -w and -r.

Otherwise if your device has only a read/notify characteristic or you want to intentionally prevent writing then you can use --permit:

$ ble-serial -d 20:91:48:4c:4c:54 -r 0000ffe1-0000-1000-8000-00805f9b34fb --permit ro

Here -r is enough for ro = read only. This works also the other way around with wo = write only.

Logging

There is an option to log all traffic on the link to a text file:

$ ble-serial -d 20:91:48:4c:4c:54 -l demo.txt
...
$ cat demo.txt
2019-12-09 21:15:53.282805 <- BLE-OUT: 48 65 6c 6c 6f 20 77 6f 72 6c 64
2019-12-09 21:15:53.491681 -> BLE-IN: b0 b0 b0 b0 b0 b0 3b b0 b0 b0 ba b0 0d 8a
2019-12-09 21:15:53.999795 -> BLE-IN: b0 b0 b0 b0 b0 b0 3b b0 b0 b0 ba b0 0d 8a

Per default it is transformed to hex bytes, use -b/--binary to log raw data.

You can use -v to increase the log verbosity to DEBUG:

18:31:25.136 | DEBUG | ble_interface.py: Received notify from 17: bytearray(b'\xb0\xb0\xb0\xb0\xb0\xb0;\xb0\xb0\xb0\xba\xb0\r\x8a')
18:31:25.136 | DEBUG | linux_pty.py: Write: bytearray(b'\xb0\xb0\xb0\xb0\xb0\xb0;\xb0\xb0\xb0\xba\xb0\r\x8a')

18:31:25.373 | DEBUG | linux_pty.py: Read: b'hello world'
18:31:25.373 | DEBUG | ble_interface.py: Sending b'hello world'

This will log all traffic going through. Note that everything shows up two times, because it goes through the ble module and then into the serial port and vice versa. It also helps with figuring out how characteristics are selected:

14:32:47.589 | DEBUG | ble_interface.py: No write uuid specified, trying builtin list
14:32:47.589 | DEBUG | ble_interface.py: Characteristic candidates for write: 
        0000ffe1-0000-1000-8000-00805f9b34fb (Handle: 17): Vendor specific ['read', 'write-without-response', 'notify']
14:32:47.589 | INFO | ble_interface.py: Found write characteristic 0000ffe1-0000-1000-8000-00805f9b34fb (H. 17)
14:32:47.589 | DEBUG | ble_interface.py: No notify uuid specified, trying builtin list
14:32:47.589 | DEBUG | ble_interface.py: Characteristic candidates for notify: 
        0000ffe1-0000-1000-8000-00805f9b34fb (Handle: 17): Vendor specific ['read', 'write-without-response', 'notify']
14:32:47.589 | INFO | ble_interface.py: Found notify characteristic 0000ffe1-0000-1000-8000-00805f9b34fb (H. 17)

Always try the verbose option if something is not working properly.

Automated connections

The repo helper/ directory contains a ble-autoconnect.py script:

$ python helper/ble-autoconnect.py -h
usage: ble-autoconnect.py [-h] [-c CONFIG] [-v]

Service to automatically connect with devices that get available.

optional arguments:
  -h, --help            show this help message and exit
  -c CONFIG, --config CONFIG
                        Path to a INI file with device configs (default: autoconnect.ini)
  -v, --verbose         Increase log level from info to debug (default: False)

This continuously scans for devices and compares them with the configuration, it then automatically starts up ble-serial (or other tools) if a known device is detected. This should bring similar convenience like USB adapters, just turn the BLE device on and the serial port shows up on the PC. See the example autoconnect.ini for configuration.

On Linux you can also use the included systemd (user) service to auto start this on boot.

Known limitations

• Chromium 73+ based applications, including NW.js/electron desktop apps, for example current Betaflight/INAV Configurator: Connection to the virtual serial port (pty) fails. This is because of explicit whitelisting in chromium.

Closing remarks

If you encounter unexpected problems, please use the issue tracker. For general questions there is also the discussions tab. As always, I hope it was helpful, if you like it then I would appreciate if you could star it on GitHub.