[modbus] Only apply turnaround delay after broadcasts (#17209)

tests/integration/fixtures/uart_mock_modbus_broadcast.yaml

@@ -0,0 +1,56 @@
+esphome:
+  name: uart-mock-modbus-bcast
+
+host:
+api:
+logger:
+  level: VERBOSE
+
+external_components:
+  - source:
+      type: local
+      path: EXTERNAL_COMPONENT_PATH
+
+# Dummy uart entry to satisfy modbus's DEPENDENCIES = ["uart"]
+# The actual UART bus used is the uart_mock component below
+uart:
+  baud_rate: 115200
+  port: /dev/null
+
+# No on_tx injection: a broadcast (address 0) gets no reply on a real bus.
+uart_mock:
+  - id: virtual_uart
+    baud_rate: 9600
+    auto_start: true
+    debug:
+
+modbus:
+  - uart_id: virtual_uart
+    id: virtual_modbus
+    role: client
+    send_wait_time: 200ms
+    turnaround_time: 10ms
+
+modbus_controller:
+  - address: 0
+    modbus_id: virtual_modbus
+    update_interval: 60s
+    id: modbus_controller_bcast
+
+number:
+  - platform: modbus_controller
+    modbus_controller_id: modbus_controller_bcast
+    id: bcast_write
+    name: "bcast_write"
+    address: 0x01
+    register_type: holding
+    value_type: U_WORD
+    min_value: 0
+    max_value: 65535
+
+interval:
+  - interval: 400ms
+    then:
+      - number.set:
+          id: bcast_write
+          value: 42

tests/integration/test_uart_mock_modbus.py

@@ -330,3 +330,28 @@ async def test_uart_mock_modbus_server_controller_multiple(
         await tracker.setup_and_start_scenario(client)
         await tracker.await_all(futures)
         _assert_no_modbus_errors(error_log_lines, warning_log_lines)
+
+
+@pytest.mark.asyncio
+async def test_uart_mock_modbus_broadcast(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test that broadcast writes (address 0) don't wait for a response.
+
+    A controller at address 0 sends broadcast writes that get no reply. The
+    client must not arm the response timeout for them: otherwise every write
+    blocks for send_wait_time and logs a spurious "no response from 0" warning.
+    """
+
+    line_callback, error_log_lines, warning_log_lines = _make_modbus_line_callback()
+
+    async with (
+        run_compiled(yaml_config, line_callback=line_callback),
+        api_client_connected(),
+    ):
+        # Several broadcast writes fire on the 400ms interval; send_wait_time is
+        # 200ms, so the old behaviour would have warned on each one by now.
+        await asyncio.sleep(3.0)
+        _assert_no_modbus_errors(error_log_lines, warning_log_lines)