Modbus TCP Slave with Arduino IDE

The Modbus TCP Slave module implements the Modbus TCP Slave functionality.

#include <ModbusTCPSlave.h>

ModbusTCPSlave slave;

The default TCP port is the 502, but you can change it with:

// Set the TCP listening port to 510 instead of 502
ModbusTCPSlave slave(510);

To map the coils, discrete inputs, holding registers and input registers addresses with the desired variables values, the module uses four variables arrays:

bool coils[NUM_COILS];
bool discreteInputs[NUM_DISCRETE_INPUTS];
uint16_t holdingRegistesr[NUM_HOLDING_REGISTERS];
uint16_t inputRegisters[NUM_INPUT_REGISTERS];

The lengths of these arrays depend on the application and the register's usages. Obviously, the names of the arrays also depend on your preferences.

To associate the registers arrays with the library, it is possible to use their functions in the setup:

slave.setCoils(coils, NUM_COILS);
slave.setDiscreteInputs(discreteInputs, NUM_DISCRETE_INPUTS);
slave.setHoldingRegisters(holdingRegisters, NUM_HOLDING_REGISTERS);
slave.setInputRegisters(inputRegisters, NUM_INPUT_REGISTERS);

It is not required to have all kinds of registers mapping to work, only the ones used by the application.

// Init the Ethernet
Ethernet.begin(mac, ip);

// Init the ModbusTCPSlave object
slave.begin();

// Update discrete inputs and input registers values
discreteInputs[0] = digitalRead(I0_7);
inputRegisters[0] = analogRead(I0_0);
// ...

// Update the ModbusTCPSlave object
slave.update();

// Update coils and holding registers
digitalWrite(Q0_0, coils[0]);
// ...