Modbus Master Arduino dengan Modbus Slave

#include <ModbusRTU.h>
ModbusRTU mb;
int a = 0;
unsigned long skr = 0;
void setup() {
  Serial.begin(9600, SERIAL_8N1);
  mb.begin(&Serial);
  mb.setBaudrate(9600);
  mb.master();
  pinMode(5, INPUT_PULLUP);
  pinMode(6, INPUT_PULLUP);
  pinMode(7, INPUT_PULLUP);
  pinMode(8, INPUT_PULLUP);
}
bool coils[3];
void loop() {
  if (millis() - skr > 100) {
    skr = millis();
    a++;
    if (a > 3) a = 0;
    switch (a) {
      case 0:
        mb.writeCoil(1, 1, digitalRead(5));
        break;
      case 1:
        mb.writeCoil(2, 1, digitalRead(6));
        break;
      case 2:
        mb.writeCoil(3, 1, digitalRead(7));
        break;
      case 3:
        mb.writeCoil(4, 1, digitalRead(8));
        break;
    }
  }
  mb.task();
}