This library provides some core functions for implementing Modbus RTU communication. It is not a full implementation of Modbus RTU. Other libraries are available for that purpose: see ModbusRTUSlave and ModbusRTUMaster.

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

Description

Creates a ModbusRTUComm 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. A read enable pin can also be specified. This pin will be set LOW, always enabling reading.

Syntax

• ModbusRTUComm(serial)
• ModbusRTUComm(serial, dePin)
• ModbusRTUComm(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 <ModbusRTUComm.h>

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

ModbusRTUComm rtuComm(Serial1, dePin, rePin);

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

rtuComm.begin(baud);
rtuComm.begin(baud, config);

Parameters

• rtuComm: a ModbusRTUComm 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

begin() for the serial port used with the modbus object must be run seperately.

Example

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

Description

Sets the maximum time in milliseconds that readAdu() will wait for a data to be received. The default value is 0.

Syntax

• rtuComm.setTimeout(timeout)

Parameters

• rtuComm: a ModbusRTUComm object.

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

Description

Reads serial data to a ModbusADU object.

Syntax

• rtuComm.readAdu(adu)

Parameters

• rtuComm: a ModbusRTUComm object.
• adu: a ModbusADU object.

Returns

Error code. Data type: ModbusRTUCommError or uint8_t.

• 0: Success
• 1: Timeout
• 2: Frame error
• 3: CRC error

Example

ModbusADU adu;
uint8_t error = rtuComm.readAdu(adu);

Description

Writes serial data from a ModbusADU object.

Syntax

• rtuComm.writeAdu(adu)

Parameters

• rtuComm: a ModbusRTUComm object.
• adu: a ModbusADU object.

Returns

• true when the transmitted message has been verified as having been sent.
• false if the transmission could not be verified.

This will only return true if some sort of loopback mechanism is in place where all the transmitted data is also received. This can easily be done with an RS-485 transceiver by connecting the RE pin to GND.

It is not essential that this value be read.

Example

ModbusADU adu;
adu.setUnitId(1);
adu.setFunctionCode(1);
adu.setDataRegister(0, 0);
adu.setDataRegister(2, 2);
adu.setDataLen(4);
bool success = rtuComm.writeAdu(adu);

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 issue with how the timing of Serial.read() works with this core. ↩