こちらのページを参考にしました。
BlueZ の example-gatt-server と example-gatt-client の使い方

RaspberryPi （サーバー) の環境構築

sudo apt install bluez
sudo apt install bluez-test-tools
sudo apt install bluez-test-scripts
sudo apt install python3-dbus

Ubuntu 24.04 （クライアント) の環境構築

sudo apt install bluez
sudo apt install bluez-test-tools
sudo apt install bluez-test-scripts
sudo apt install python3-dbus

Bluetoothアダプターの MAC アドレスの調べ方

hciconfig

実行結果

$ hciconfig
hci0:	Type: Primary  Bus: UART
	BD Address: DC:A6:32:70:45:45  ACL MTU: 1021:8  SCO MTU: 64:1
	UP RUNNING PSCAN 
	RX bytes:3008 acl:21 sco:0 events:151 errors:0
	TX bytes:5494 acl:23 sco:0 commands:119 errors:0

RaspberryPi （サーバ）の設定

$ bluetoothctl
# power on
# agent on
# default-agent
# discoverable on
# advertise on

実行結果

$ bluetoothctl
Agent registered
[CHG] Controller DC:A6:32:70:45:45 Pairable: yes
[bluetooth]# power on
Changing power on succeeded
[bluetooth]# agent on
Agent is already registered
[bluetooth]# default-agent
Default agent request successful
[bluetooth]# discoverable on
Changing discoverable on succeeded
[CHG] Controller DC:A6:32:70:45:45 Discoverable: yes
[bluetooth]# advertise on
[CHG] Controller DC:A6:32:70:45:45 SupportedInstances: 0x04 (4)
[CHG] Controller DC:A6:32:70:45:45 ActiveInstances: 0x01 (1)
Advertising object registered
Tx Power: off
Name: off
Appearance: off
Discoverable: on
[bluetooth]#

Ubuntu 24.04 （クライアント) の設定

RaspberryPi の bluetooth の MAC アドレスを
DC:A6:32:70:45:45 とする。

$ bluetoothctl
# power on
# scan on
# pair DC:A6:32:70:45:45
# connect DC:A6:32:70:45:45

双方で、y を入力する

Client

Server

サーバーの起動

python /usr/share/doc/bluez-test-scripts/examples/example-gatt-server

実行結果

Updating value: [dbus.Byte(6), dbus.Byte(105)]
Updating value: [dbus.Byte(6), dbus.Byte(122)]
Updating value: [dbus.Byte(6), dbus.Byte(99)]
Updating value: [dbus.Byte(6), dbus.Byte(128)]
Updating value: [dbus.Byte(6), dbus.Byte(119)]
Updating value: [dbus.Byte(6), dbus.Byte(113)]
Updating value: [dbus.Byte(6), dbus.Byte(106)]
Updating value: [dbus.Byte(6), dbus.Byte(102)]

クライアントの起動

python /usr/share/doc/bluez-test-scripts/examples/example-gatt-client

実行結果

Getting objects...
Heart Rate Service found: /org/bluez/hci0/dev_DC_A6_32_70_45_45/service004b
Body sensor location value: Chest
HR Measurement notifications enabled
New HR Measurement
	HR: 130
	Sensor Contact status: contact detected
	Energy Expended: 0
New HR Measurement
	HR: 106
	Sensor Contact status: contact detected
New HR Measurement
	HR: 102
	Sensor Contact status: contact detected
New HR Measurement
	HR: 125
	Sensor Contact status: contact detected

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