The ModbusRTU library is a simple to use library that allows an arduino to act as a Modbus slave. It attempts to obscure all unnecessary parts of the protocol to provide a clean interface.

The library pre-allocates all necessary memory upon instantiation, which improves the performance of the library and allows for simpler diagnostics.

Slave ID's are supported, it is therefore possible to have multiple slaves on the same bus.

• Discrete Inputs
• Coils
• Input Registers
• Holding Registers

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

• Exception Response Codes
  • 1 - Illegal Function
  • 2 - Illegal Data Address
  • 3 - Illegal Data Value

Whole libary is enclosed in a namespace called ModbusRTU.

The class is a template class with one parameter called registerCount. The parameter specifies the size of the register array or maximum register count.

   void begin(unsigned long baud, HardwareSerial *pHardwareSerial = &Serial, unsigned char slaveId = 1)

The begin function sets up variables and configures the hardware serial class.

• Parameters
  • baud: The baud rate the serial interface should run at.
  • pHardwareSerial: Pointer to a HardwareSerial instance. Defaults to &Serial.
  • slaveId: Slave ID to be used by this slave. Defaults to 1.

   void update()

The function checks for incoming frames from master, attempts to parse the frame upon receiving, thereafter returns the result to the master. The parsing includes reading/writing to the modbus entites. If parsing a frame fails, the slave will attempt to send an exception.

   long addCoil(bool *coil, unsigned short _register)

The function adds a coil to the register array.

• Parameters
  • coil: Pointer to a read/write boolean value.
  • _register: The register number to be assigned to the coil.
• Return value
  • Returns the register number upon success.
  • Returns -1 upon failure.

   long addDiscreteInput(const bool *discreteInput, unsigned short _register)

The function adds a discrete input to the register array.

• Parameters
  • discreteInput: Pointer to a read-only boolean value.
  • _register: The register number to be assigned to the discrete input.
• Return value
  • Returns the register number upon success.
  • Returns -1 upon failure.

   long addInputRegister(const short *inputRegister, unsigned short _register)

The function adds an input register to the register array.

• Parameters
  • inputRegister: Pointer to a read-only 16-bit value.
  • _register: The register number to be assigned to the input register.
• Return value
  • Returns the register number upon success.
  • Returns -1 upon failure.

   long addHoldingRegister(short *holdingRegister, unsigned short _register)

The function adds an input register to the register array.

• Parameters
  • holdingRegister: Pointer to a read/write 16-bit value.
  • _register: The register number to be assigned to the holding register.
• Return value
  • Returns the register number upon success.
  • Returns -1 upon failure.