 //////////////// registers of your slave ///////////////////
 enum 
 {
-  ADC_VAL,     
-  PWM_VAL,        
+  reg0_VAL,     
+  reg1_VAL,        
   HOLDING_REGS_SIZE
 };
 

 // holdingRegs[ADC_VAL] = analogRead(A0);
  
 digitalWrite(LED, HIGH);
 delay(holdingRegs[PWM_VAL]);
 digitalWrite(LED, LOW);
 delay(holdingRegs[PWM_VAL]);
 
}

void setup()
{
  modbus_configure(&Serial, 19200, SERIAL_8N1, 1, 2, HOLDING_REGS_SIZE, holdingRegs);
  modbus_update_comms(19200, SERIAL_8N1, 229);
  
  pinMode(LED, OUTPUT);
}
