# include <ModbusRTUSlave.h>

const uint8_t coilPins[2] = {4, 5};
const uint8_t discreteInputPins[2] = {2, 3};

ModbusRTUSlave modbus(Serial);

bool coils[2];
bool discreteInputs[2];

void setup() {
  pinMode(coilPins[0], OUTPUT);
    pinMode(coilPins[1], OUTPUT);
      pinMode(discreteInputPins[0], INPUT);
        pinMode(discreteInputPins[1], INPUT);

          modbus.configureCoils(coils, 2);
            modbus.configureDiscreteInputs(discreteInputs, 2);
              modbus.begin(1, 38400);
              }

              void loop() {
                discreteInputs[0] = digitalRead(discreteInputPins[0]);
                  discreteInputs[1] = digitalRead(discreteInputPins[1]);

                    modbus.poll();

                      digitalWrite(coilPins[0], coils[0]);
                        digitalWrite(coilPins[1], coils[1]);
                        }