Connecting Various Devices Using Bluetooth as Serial Port

Question

I managed to connect a sensor via Bluetooth to a serial port on the Raspberry Pi 4 by doing the following:

First, I added the SP profile to the Raspberry by opening and editing the following file:

sudo nano /etc/systemd/system/dbus-org.bluez.service

I added the compatibility flag -C and a new line to add the SP profile lines:

ExecStart=/usr/lib/bluetooth/bluetoothd -C
ExecStartPost=/usr/bin/sdptool add SP

Then I paired and connected entering the following on the terminal:

bluetoothctl
pair <deviceAddress>
trust <deviceAddress>
exit

On a different terminal window, I entered:

   sudo rfcomm connect hci0 <deviceAddress> 

This connected the device, and then I opened a minicom window to see the data the sensor was sending by entering:

minicom -b 9600 -o -D /dev/rfcomm0

This worked fine, but now I want to connect another sensor to a different serial port (an eventually a third sensor), so that I can simultanously see the data the two sensors are sending on two different minicom windows. I managed to pair and trust the new sensor but when I execute the folling line it gives me an error:

   sudo rfcomm connect hci0 <deviceAddress> 

The error is: "Can't create RFCOMM TTY: Address already in use"

I guess the problem might be with the hci0 but I'm not sure, as I don't completely understand what that is. I tried to use channel 2 by adding a 2 to the prior line (I don't know if this makes sense) and it didn't work either ("Can't create RFCOMM TTY: Address already in use".)

I also tried to use another device, hci1 instead of hci0, and it says: "Can't connect RFCOMM socket: Host is down". When I run the folowing:

hcitootl dev

only hci0 appears, so I don't know if I should create another device to be able to connect another sensor or how it is done.

So, anyone knows a way to make several Bluetooth serial connections, so that I can see the data several sensors are sending at the same time on different minicom windows?

Thanks.

Answer 1

hci0 refers to the physical Bluetooth adapter/radio in your system. If you connect a Bluetooth USB dongle to your RPi, then you would see hci1. e.g.

pi@raspberrypi:~ $ bluetoothctl list
Controller 00:02:5B:33:44:07 RPi_UART [default]
Controller B8:27:EB:33:57:E0 SeeMe 

pi@raspberrypi:~ $ busctl tree 'org.bluez'
└─/org
  └─/org/bluez
    ├─/org/bluez/hci0
    └─/org/bluez/hci1

It would be best to avoid using bluetoothd -C, rfcomm and hcitool as they have been deprecated

As outlined in http://blog.kevindoran.co/bluetooth-programming-with-python-3/, a Serial Port Profile (SPP) Bluetooth client connection can be done with Python3 (after doing the pairing in bluetoothctl):

import socket

serverMACAddress = '00:1f:e1:dd:08:3d'
port = 3
s = socket.socket(socket.AF_BLUETOOTH, socket.SOCK_STREAM, socket.BTPROTO_RFCOMM)
s.connect((serverMACAddress,port))
while 1:
    text = input()
    if text == "quit":
        break
    s.send(bytes(text, 'UTF-8'))
s.close()

There is also a library like bluedot: https://bluedot.readthedocs.io/en/latest/btcommapi.html#bluetoothclient

There are a lot of pieces that come together to support a Bluetooth profile. There is the hardware, BlueZ stack etc and getting them all to line up can require effort. However, the specification for SPP is clear that while it is a point-to-point protocol, there is the expectation that there can be multiple SPP's running:

SPP is a serial cable replacement. So I think of it as that you have to create multiple cables connections. I don't have the hardware here to test this but does the following work:

s1.connect((serverMACAddress_1,port))
s2.connect((serverMACAddress_2,port))