home-assistant · emontnemery · Jul 1, 2025 · Apr 13, 2025 · Apr 15, 2025 · Apr 15, 2025
diff --git a/homeassistant/components/modbus/modbus.py b/homeassistant/components/modbus/modbus.py
@@ -172,7 +172,7 @@ async def async_stop_modbus(event: Event) -> None:
 
     async def async_write_register(service: ServiceCall) -> None:
         """Write Modbus registers."""
-        slave = 0
+        slave = 1
         if ATTR_UNIT in service.data:
             slave = int(float(service.data[ATTR_UNIT]))
 
@@ -195,7 +195,7 @@ async def async_write_register(service: ServiceCall) -> None:
 
     async def async_write_coil(service: ServiceCall) -> None:
         """Write Modbus coil."""
-        slave = 0
+        slave = 1
         if ATTR_UNIT in service.data:
             slave = int(float(service.data[ATTR_UNIT]))
         if ATTR_SLAVE in service.data:

diff --git a/tests/components/modbus/test_init.py b/tests/components/modbus/test_init.py
@@ -1327,3 +1327,89 @@ async def test_check_default_slave(
     assert mock_modbus.read_holding_registers.mock_calls
     first_call = mock_modbus.read_holding_registers.mock_calls[0]
     assert first_call.kwargs["slave"] == expected_slave_value
+
+
+@pytest.mark.parametrize(
+    "do_config",
+    [
+        {
+            CONF_NAME: TEST_MODBUS_NAME,
+            CONF_TYPE: SERIAL,
+            CONF_BAUDRATE: 9600,
+            CONF_BYTESIZE: 8,
+            CONF_METHOD: "rtu",
+            CONF_PORT: TEST_PORT_SERIAL,
+            CONF_PARITY: "E",
+            CONF_STOPBITS: 1,
+            CONF_SENSORS: [
+                {
+                    CONF_NAME: "dummy_noslave",
+                    CONF_ADDRESS: 8888,
+                }
+            ],
+        },
+    ],
+)
+@pytest.mark.parametrize(
+    "do_write",
+    [
+        {
+            DATA: ATTR_VALUE,
+            VALUE: 15,
+            SERVICE: SERVICE_WRITE_REGISTER,
+            FUNC: CALL_TYPE_WRITE_REGISTER,
+        },
+        {
+            DATA: ATTR_STATE,
+            VALUE: False,
+            SERVICE: SERVICE_WRITE_COIL,
+            FUNC: CALL_TYPE_WRITE_COIL,
+        },
+    ],
+)
+@pytest.mark.parametrize(
+    "do_return",
+    [
+        {VALUE: ReadResult([0x0001]), DATA: ""},
+        {VALUE: ExceptionResponse(0x06), DATA: "Pymodbus:"},
+        {VALUE: ModbusException("fail write_"), DATA: "Pymodbus:"},
+    ],
+)
+async def test_pb_service_write_no_slave(
+    hass: HomeAssistant,
+    do_write,
+    do_return,
+    caplog: pytest.LogCaptureFixture,
+    mock_modbus_with_pymodbus,
+) -> None:
+    """Run test for service write_register in case of missing slave/unit parameter."""
+
+    func_name = {
+        CALL_TYPE_WRITE_COIL: mock_modbus_with_pymodbus.write_coil,
+        CALL_TYPE_WRITE_REGISTER: mock_modbus_with_pymodbus.write_register,
+    }
+
+    value_arg_name = {
+        CALL_TYPE_WRITE_COIL: "value",
+        CALL_TYPE_WRITE_REGISTER: "value",
+    }
+
+    data = {
+        ATTR_HUB: TEST_MODBUS_NAME,
+        ATTR_ADDRESS: 16,
+        do_write[DATA]: do_write[VALUE],
+    }
+    mock_modbus_with_pymodbus.reset_mock()
+    caplog.clear()
+    caplog.set_level(logging.DEBUG)
+    func_name[do_write[FUNC]].return_value = do_return[VALUE]
+    await hass.services.async_call(DOMAIN, do_write[SERVICE], data, blocking=True)
+    assert func_name[do_write[FUNC]].called
+    assert func_name[do_write[FUNC]].call_args.args == (data[ATTR_ADDRESS],)
+    assert func_name[do_write[FUNC]].call_args.kwargs == {
+        "slave": 1,
+        value_arg_name[do_write[FUNC]]: data[do_write[DATA]],
+    }
+
+    if do_return[DATA]:
+        assert any(message.startswith("Pymodbus:") for message in caplog.messages)