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

#define MySerial Serial // define serial port used, Serial most of the time, or Serial1, Serial2 ... if available
 
bool coils[10]; 
int pinToRead = 2;  
 
ModbusSerial mb (MySerial, 1);

void setup() {

  MySerial.begin (9600, MB_PARITY_NONE); // prefer this line in accordance with the modbus standard.
  while (! MySerial)
    ;

  mb.config (9600);
  mb.addIsts(0);
}
 
void loop() { 
  mb.setIsts(0, digitalRead(pinToRead)); 
  mb.task();   
  // int r = Serial.read();
  // if (r >= 0){
  //    Serial.write(r);
  //    Serial.write(r);
  // }
}