Modbus is an industrial communication protocol. The RTU variant communicates over serial lines such as UART, RS-232, or RS-485. The full details of the Modbus protocol can be found at modbus.org. A good summary can also be found on Wikipedia.

This is an Arduino library that implements the master/client logic of the Modbus RTU protocol. It enables an Arduino or Arduino compatible board to send Modbus RTU requests to Modbus slave/server devices, and to process their responses.
This library implements the following function codes:

• 1 (Read Coils)
• 2 (Read Discrete Inputs)
• 3 (Read Holding Registers)
• 4 (Read Input Registers)
• 5 (Write Single Coil)
• 6 (Write Single Holding Register)
• 15 (Write Multiple Coils)
• 16 (Write Multiple Holding Registers).

This library will work with any Stream object, like Serial. A driver enable pin can be set up, enabling a half-duplex RS-485 transceiver to be used. Only SERIAL_8N1, SERIAL_8E1, SERIAL_8O1, SERIAL_8N2, SERIAL_8E2, and SERIAL_8O2 configurations are supported; attempting to use any other configuration will cause the library to default to timings for SERIAL_8N1.

There are some significant changes going from version 1.x.x to version 2.x.x of this library.

• begin() for the Serial object used needs to be run before running begin() for the library itself, e.g.:

  Serial1.begin(38400);
  modbus.begin(38400);

• The read and write functions return an error value instead of a pass/fail bool.
• The getTimeoutFlag(), clearTimeoutFlag(), and clearExceptionResponse() functions have been removed.
• This library is also now dependent on ModbusADU and ModbusRTUComm.

This library has been tested with the following boards and cores:

Board Name	Core	Works
Arduino Due	Arduino SAM Boards (32-bits ARM Cortex-M3) by Arduino 1.6.12	Yes
Arduino Giga	Arduino Mbed OS GIGA Boards by Arduino 4.1.5	Yes
Arduino Leonardo	Arduino AVR Boards by Arduino 1.8.6	Yes
Arduino Make Your UNO	Arduino AVR Boards by Arduino 1.8.6	Yes
Arduino Mega 2560	Arduino AVR Boards by Arduino 1.8.6	Yes
Arduino Nano	Arduino AVR Boards by Arduino 1.8.6	Yes
Arduino Nano 33 BLE	Arduino Mbed OS Nano Boards by Arduino 4.1.5	Yes
Arduino Nano 33 IoT	Arduino SAMD Boards (32-bits ARM Cortex-M0+) by Arduino 1.8.14	Yes
Arduino Nano ESP32	Arduino ESP32 Boards by Arduino 2.0.13	Yes
Arduino Nano ESP32	esp32 by Espressif Systems 3.0.4	Yes
Arduino Nano Every	Arduino megaAVR Boards by Arduino 1.8.8	Yes
Arduino Nano Matter	Silicon Labs by Silicon Labs 2.1.0	No 1
Arduino Nano RP2040 Connect	Arduino Mbed OS Nano Boards by Arduino 4.1.5	No 2
Arduino Nano RP2040 Connect	Raspberry Pi Pico/RP2040 by Earle F. Philhower, III 4.0.1	Yes
Arduino UNO R3 SMD	Arduino AVR Boards by Arduino 1.8.6	Yes
Arduino UNO R4 Minima	Arduino UNO R4 Boards by Arduino 1.2.0	Yes

• ModbusRTUMasterExample
• ModbusRTUMasterProbe

Several functions return error codes. Here is what those error codes mean.

• 0: Success
• 1: Invalid unit ID
• 2: Invalid buffer
• 3: Invalid quntity
• 4: Response timeout
• 5: Frame error
• 6: CRC error
• 7: Unknown communication error
• 8: Unexpected unit ID in response
• 9: Exception response
• 10: Unexpected function code in response
• 11: Unexpected response length
• 12: Unexpected byte count in response
• 13: Unexpected data address in response
• 14: Unexpected data value in response
• 15: Unexpected quantity in response

ModbusRTUMaster()

Description

Creates a ModbusRTUMaster object and sets the serial port to use for data transmission. Optionally sets a driver enable pin. This pin will go HIGH when the library is transmitting. This is primarily intended for use with an RS-485 transceiver, but it can also be a handy diagnostic when connected to an LED.

Syntax

• ModbusRTUMaster(serial)
• ModbusRTUMaster(serial, dePin)
• ModbusRTUMaster(serial, dePin, rePin)

Parameters

• serial: the Stream object to use for Modbus communication. Usually something like Serial1.
• dePin: the driver enable pin. This pin is set HIGH when transmitting. If this parameter is set to -1, this feature will be disabled. The default value is -1. Allowed data types: int.
• rePin: the read enable pin. This pin is always set LOW. If this parameter is set to -1, this feature will be disabled. The default value is -1. Allowed data types: int.

Example

# include <ModbusRTUMaster.h>

const int dePin = A6;
const int rePin = A5;

ModbusRTUMaster modbus(Serial1, dePin, rePin);

setTimeout()

Description

Sets the maximum timeout in milliseconds to wait for a response after sending a request. The default is 500 milliseconds.

Syntax

modbus.setTimeout(timeout)

Parameters

• modbus: a ModbusRTUMaster object.

• timeout: the timeout duration in milliseconds. Allowed data types: unsigned long.

begin()

Description

Sets the data rate in bits per second (baud) for serial transmission. Optionally it also sets the data configuration. Note, there must be 8 data bits for Modbus RTU communication. The default configuration is 8 data bits, no parity, and one stop bit.

Syntax

• modbus.begin(baud)
• modbus.begin(baud, config)

Parameters

• modbus: a ModbusRTUMaster object.
• baud: the baud rate to use for Modbus communication. Common values are: 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200. Allowed data types: unsigned long.
• config: the serial port configuration to use. Valid values are:
  SERIAL_8N1: no parity (default)
  SERIAL_8N2
  SERIAL_8E1: even parity
  SERIAL_8E2
  SERIAL_8O1: odd parity
  SERIAL_8O2

In the 2.0.0+ version of this library, begin() for the serial port used with the modbus object must be run seperately.

Example

void setup() {
  Serial1.begin(38400, SERIAL_8E1);
  modbus.begin(38400, SERIAL_8E1)
}

readCoils()

Description

reads coil values from a slave/server device.

Syntax

modbus.readCoils(unitId, startAddress, buffer, quantity)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 1 - 246.
• startAddress: the address of the first coil to read. Allowed data types: uint16_t.
• buffer: an array in which to place the read coil values. Allowed data types: array of bool.
• quantity: the number of coil values to read. This value must not be larger than the size of the array. Allowed data types: uint16_t.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

Example

bool coils[8];
uint8_t error = modbus.readCoils(1, 0, coils, 8);

readDiscreteInputs()

Description

reads discrete input values from a slave/server device.

Syntax

modbus.readDiscreteInputs(unitId, startAddress, buffer, quantity)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 1 - 246.
• startAddress: the address of the first discrete input to read. Allowed data types: uint16_t.
• buffer: an array in which to place the read discrete input values. Allowed data types: array of bool.
• quantity: the number of discrete input values to read. This value must not be larger than the size of the array. Allowed data types: uint16_t.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

Example

bool discreteInputs[8];
uint8_t error = modbus.readDiscreteInputs(1, 0, discreteInputs, 8);

readHoldingRegisters()

Description

reads holding register values from a slave/server device.

Syntax

modbus.readHoldingRegisters(unitId, startAddress, buffer, quantity)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 1 - 246.
• startAddress: the address of the first holding register to read. Allowed data types: uint16_t.
• buffer: an array in which to place the read holding register values. Allowed data types: array of uint16_t.
• quantity: the number of holding register values to read. This value must not be larger than the size of the array. Allowed data types: uint16_t.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

Example

uint16_t holdingRegisters[8];
uint8_t error = modbus.readHoldingRegisters(1, 0, holdingRegisters, 8);

readInputRegisters()

Description

reads input register values from a slave/server device.

Syntax

modbus.readInputRegisters(unitId, startAddress, buffer, quantity)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 1 - 246.
• startAddress: the address of the first input register to read. Allowed data types: uint16_t.
• buffer: an array in which to place the read input register values. Allowed data types: array of uint16_t.
• quantity: the number of input register values to read. This value must not be larger than the size of the array. Allowed data types: uint16_t.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

Example

uint16_t inputRegisters[8];
uint8_t error = modbus.readInputRegisters(1, 0, inputRegisters, 8);

writeSingleCoil()

Description

writes a single coil value to a slave/server device.

Syntax

modbus.writeSingleCoil(unitId, address, value)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 0 - 246. 0 indicates a broadcast message.
• address: the address of the coil to write to. Allowed data types: uint16_t.
• value: the value to write to the coil. Allowed data types: bool.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

writeSingleHoldingRegister()

Description

writes a single holding register value to a slave/server device.

Syntax

modbus.writeSingleHoldingRegister(unitId, address, value)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 0 - 246. 0 indicates a broadcast message.
• address: the address of the holding register to write to. Allowed data types: uint16_t.
• value: the value to write to the holding register. Allowed data types: uint16_t.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

writeMultipleCoils()

Description

writes multiple coil values to a slave/server device.

Syntax

modbus.writeMultipleCoils(unitId, startingAddress, buffer, quantity)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 0 - 246. 0 indicates a broadcast message.
• startAddress: the address of the first coil to write to. Allowed data types: uint16_t.
• buffer: an array of coil values. Allowed data types: array of bool.
• quantity: the number of coil values to write. This value must not be larger than the size of the array. Allowed data types: uint16_t.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

Example

bool coils[2] = {true, false};
uint8_t error = modbus.writeMultipleCoils(1, 0, coils, 2);

writeMultipleHoldingRegisters()

Description

writes multiple holding register values to a slave/server device.

Syntax

modbus.writeMultipleHoldingRegisters(unitId, startingAddress, buffer, quantity)

Parameters

• modbus: a ModbusRTUMaster object.
• unitId: the id number of the device to send this request to. Valid values are 0 - 246. 0 indicates a broadcast message.
• startAddress: the address of the first holding register to write to. Allowed data types: uint16_t.
• buffer: an array of holding register values. Allowed data types: array of uint16_t.
• quantity: the number of holding register values to write. This value must not be larger than the size of the array. Allowed data types: uint16_t.

Returns

Error code. Data type: ModbusRTUMasterError or uint8_t.
See error codes.

Example

uint16_t holdingRegisters[2] = {42, 328};
uint8_t error = modbus.writeMultipleHoldingRegisters(1, 0, holdingRegisters, 2);

getExceptionResponse()

Description

Gets the last exception response that has occured.

When a valid Modbus request is sent, and the recipient slave/server device cannot process it, it responds with an exception code. This response is called an exception resonse, and it can be helpful in diagnosing issues.

Syntax

modbus.getExceptionResponse()

Parameters

• modbus: a ModbusRTUMaster object.

Returns

Exception code. Data type: uint8_t.

• 0: None
• 1: Illegal function
• 2: Illegal data address
• 3: Illegal data value
• 4: Server device failure

Details on exeption responses can be found in the Modbus Application Protocol Specification. More exeption responses exist than are listed here, but these are the most common.

Footnotes

1. Arduino Nano RP2040 Connect
   This board has trouble receiving Modbus messages when using the Arduino Mbed OS Nano Boards core by Arduino.
   It seems that there is some sort of timing issue.
   It can technically be made to work if you tell the library that it is operating at a lower baud rate than the serial port assigned to the library is actually operating at. However, this would cause the library to operate with unknown timing tolerances, possibly well outside the Modbus specification. ↩

2. Arduino Nano Matter
   As of this writing (2024-09-07), flush() is not properly implemented with Serial on this board.
   ModbusRTUMaster depends on flush() to know when to set the DE and RE pins LOW after a message is sent. ↩