diff --git a/samples/subsys/modbus/rtu_client/README.rst b/samples/subsys/modbus/rtu_client/README.rst
index 129616dfc3d..1159cfa771f 100644
--- a/samples/subsys/modbus/rtu_client/README.rst
+++ b/samples/subsys/modbus/rtu_client/README.rst
@@ -19,12 +19,15 @@ RTU client example is running on an evaluation board and communicates
 with another board that has been prepared according to the description
 `modbus-rtu-server-sample`.
 
-In addition to the evaluation board, an USB to RS-485 bus adapter and
-a RS-485 shield are required. Shield converts the UART TX, RX signals to
-RS-485 and does not need a specific overlay, an Arduino header compatible
-shield like `joy-it RS-485 shield for Arduino`_ can be used.
+In addition to the evaluation board a RS-485 shield may be used.
+The shield converts UART TX, RX signals to RS-485.
+An Arduino header compatible shield like `joy-it RS-485 shield for Arduino`_
+can be used. This example uses DE signal, which is controlled by pin D9
+on the JOY-IT shield. For other shields, ``de-gpios`` property must be adapted
+or removed in the application overlay file
+:zephyr_file:`samples/subsys/modbus/rtu_client/app.overlay`
 
-The A+, B- lines of the RS-485 shield should be connected together.
+The A+, B- lines of the RS-485 shields should be connected together.
 
 Alternatively UART RX,TX signals of two boards can be connected crosswise.
 
diff --git a/samples/subsys/modbus/rtu_client/app.overlay b/samples/subsys/modbus/rtu_client/app.overlay
index 840434e4917..124708a18c3 100644
--- a/samples/subsys/modbus/rtu_client/app.overlay
+++ b/samples/subsys/modbus/rtu_client/app.overlay
@@ -11,5 +11,6 @@
 		compatible = "zephyr,modbus-serial";
 		label = "MODBUS";
 		status = "okay";
+		de-gpios = <&arduino_header 15 GPIO_ACTIVE_LOW>;	/* D9 */
 	};
 };
diff --git a/samples/subsys/modbus/rtu_server/README.rst b/samples/subsys/modbus/rtu_server/README.rst
index b7eb5b16e04..9367c8bdd40 100644
--- a/samples/subsys/modbus/rtu_server/README.rst
+++ b/samples/subsys/modbus/rtu_server/README.rst
@@ -19,9 +19,12 @@ Additionally the board should have three LEDs.
 RTU server example is running on an evaluation board. Client is running
 on a PC or laptop.
 In addition to the evaluation board, an USB to RS-485 bus adapter and
-a RS-485 shield are required. Shield converts the UART TX, RX signals to
-RS-485 and does not need a specific overlay, an Arduino header compatible
-shield like `joy-it RS-485 shield for Arduino`_ can be used.
+a RS-485 shield are required. The shield converts UART TX, RX signals to RS-485.
+An Arduino header compatible shield like `joy-it RS-485 shield for Arduino`_
+can be used. This example uses DE signal, which is controlled by pin D9
+on the JOY-IT shield. For other shields, ``de-gpios`` property must be adapted
+or removed in the application overlay file
+:zephyr_file:`samples/subsys/modbus/rtu_server/app.overlay`
 
 The USB to RS-485 adapter connects to the USB port of a computer.
 The two A+, B- lines should be connected to the RS-485 shield.
diff --git a/samples/subsys/modbus/rtu_server/app.overlay b/samples/subsys/modbus/rtu_server/app.overlay
index 840434e4917..124708a18c3 100644
--- a/samples/subsys/modbus/rtu_server/app.overlay
+++ b/samples/subsys/modbus/rtu_server/app.overlay
@@ -11,5 +11,6 @@
 		compatible = "zephyr,modbus-serial";
 		label = "MODBUS";
 		status = "okay";
+		de-gpios = <&arduino_header 15 GPIO_ACTIVE_LOW>;	/* D9 */
 	};
 };
diff --git a/tests/subsys/modbus_rtu/boards/frdm_k64f.overlay b/tests/subsys/modbus_rtu/boards/frdm_k64f.overlay
index 9974d570874..581336fe344 100644
--- a/tests/subsys/modbus_rtu/boards/frdm_k64f.overlay
+++ b/tests/subsys/modbus_rtu/boards/frdm_k64f.overlay
@@ -11,6 +11,8 @@
 		compatible = "zephyr,modbus-serial";
 		label = "MODBUS";
 		status = "okay";
+		de-gpios = <&gpiob 22 GPIO_ACTIVE_LOW>;	/* red LED */
+		re-gpios = <&gpioe 26 GPIO_ACTIVE_LOW>;	/* green LED */
 	};
 };