#include <ModbusSerial.h>
const int GPIOCoil = 0;

bool coils[10];
const int pinToRead = 2;

ModbusSerial mb (Serial, 10);

void setup() {
  Serial.begin(9600);
  pinMode(pinToRead, INPUT);
  mb.addIsts(0);
}

void loop() {
  mb.setIsts(0 digitalRead(pinToRead));
  mb.task();
}