Reading RS485 Modbus displays

[pereplex]

Hi everybody.
I'm trying to read a Modbus RS485 display.
I'm using a MEGA 2560, a RS485 to TTL circuit based on MAX485.
I'm using Serial1 for the Max485. RE/DE both connected to pin 2
The final target is to read 6 displays (node 1 to 6) but I'm not there yet .
I'm trying to read 1 display for the moment and I want to get
Voltage, frequency, Amperage.
I'm using a sample programme found on internet and it works, I get the reading but.
1- If I off the display (power lost) the last value is still displayed.
2- I tried to force to 0 the data before to send the request but this doesn't work, the result in this case is always 0.

This is the code

#include <SimpleModbusMaster.h>
#define baud 19200 
#define timeout 100 
#define polling 100 
#define retry_count 10 
#define TxEnablePin 2 
#define TOTAL_NO_OF_REGISTERS 13 
float V[7], A[7], F[7];

enum //Keep the number of registers in batches.
{
  PACKET1,
  PACKET2,
  PACKET3,
  PACKET4,
  TOTAL_NO_OF_PACKETS //leave this last entry.
};

Packet packets[TOTAL_NO_OF_PACKETS]; //Bringing data together
//unsigned int regs[TOTAL_NO_OF_REGISTERS];
int regs[TOTAL_NO_OF_REGISTERS];
float Result;
void setup() {
  Serial.begin(9600);
  modbus_configure(&Serial1, baud, SERIAL_8N1, timeout, polling, retry_count, TxEnablePin, packets, TOTAL_NO_OF_PACKETS, regs);   //กำหนดค่าตามเครื่องที่จะใช้อ่านค่า
  modbus_construct(&packets[PACKET1], 1, READ_INPUT_REGISTERS,0,1, 1);
}

void loop() {
	  delay(500);

Serial.print("Voltage 1: ");
Serial.println(readVolt(1));
Serial.print("Frequency: ");
Serial.print(readFreq(1));;
Serial.println(" Hz");


  }
float readVolt(int8_t Nd){
	//regs[0]=0; This doesn't work,
	modbus_construct(&packets[PACKET1], 1, READ_INPUT_REGISTERS,0,Nd, 0);
	modbus_update();
	Result=regs[0]*0.01;
	return Result;
}
float readFreq(int8_t Nd){
	//regs[12]=0;
	modbus_construct(&packets[PACKET2], 1, READ_INPUT_REGISTERS,12,Nd, 12);
	modbus_update();
	modbus_update();	
	Result=regs[12]*.1;
	return Result;
}	

This is the result

Voltage 1: 0.00
Frequency: 0.00 Hz
Voltage 1: 229.66
Frequency: 0.00 Hz
Voltage 1: 229.66
Frequency: 0.00 Hz
Voltage 1: 229.66
Frequency: 49.90 Hz
Voltage 1: 229.66
Frequency: 49.90 Hz
Voltage 1: 229.81
Frequency: 49.90 Hz

Voltage get one time 0 before to get the value
Frequency up to 3 times 0 before to get the value

I try to understand why I have those 0 at the beginning and how to solve this.

Thank you for your help

Thank you for your help.