#include <ModbusSerial.h>

#define BAUD_RATE 57600
#define SLAVE_ID 1
#define COIL_COUNT 4
#define COIL_START 1

#define COIL_0 4
#define COIL_1 5
#define COIL_2 6
#define COIL_3 7

#define STATUS_0 12
#define STATUS_1 11

ModbusSerial mb (Serial, SLAVE_ID);

void setup() {
  Serial.begin (BAUD_RATE);
  while (! Serial);
  mb.config(BAUD_RATE);

  mb.addCoil (0);
  mb.addCoil (1);
  mb.addCoil (2);
  mb.addCoil (3);

  mb.addIsts(0);
  mb.addIsts(1);

  pinMode(COIL_0, OUTPUT);
  pinMode(COIL_1, OUTPUT);
  pinMode(COIL_2, OUTPUT);
  pinMode(COIL_3, OUTPUT);

  pinMode(STATUS_0, INPUT_PULLUP);
  pinMode(STATUS_1, INPUT_PULLUP);
}

void loop() {
  mb.task();
  readCoilValues();
  writeStatusValues();
}

void readCoilValues() {
  digitalWrite(COIL_0, mb.coil(0));
  digitalWrite(COIL_1, mb.coil(1));
  digitalWrite(COIL_2, mb.coil(2));
  digitalWrite(COIL_3, mb.coil(3));
}

void writeStatusValues() {
  mb.setIsts(0, !digitalRead(STATUS_0));
  mb.setIsts(1, !digitalRead(STATUS_1));
}