Why Arduino not receiving RS485 data?

Question

I am trying to send data from Arduino mega (message "ping") to Arduino nano, then send back (message pong) to the mega board, I am not receiving "pong" back to the mega board, why is that? I have used uart tx complete interrupt to switch rs485 DE and RE pins as shown here in both Arduino boards

mega code

//Atmega2560
#define SerialTxControl 52 // DE RE
#define RS485Transmit HIGH
#define RS485Receive LOW
char buffer [100];
byte state = 0;
void setup (void) {
    UCSR1B |= (1<<TXCIE1);     // TX Complete Interrupt Enable serial 1
    Serial.begin (9600); // set the serial port to 9600baud speed

    Serial1.begin (9600); // set the serial port to 9600baud speed
    pinMode (SerialTxControl, OUTPUT);
    digitalWrite (SerialTxControl, RS485Transmit); // put the device into transmitter mode
}

ISR (USART_TX_vect)
{
   digitalWrite(SerialTxControl, RS485Receive);
}
void loop (void) {

//transmit rs485
digitalWrite (SerialTxControl, RS485Transmit);
Serial1.print ("ping"); // send text rs485


//receive rs485
int i = 0;
if (Serial1.available ()) {// if any data came to the port
    delay (5); // wait a bit for the entire data packet to be accepted by the port
    while (Serial1.available ()) {
        buffer [i ++] = Serial1.read (); // read data and write it to the buffer
    }
}
if (i> 0) {// if there is something in the buffer
    buffer [i ++] = '\ 0'; // convert the buffer contents to a string by adding a null character
        if (strcmp (buffer, "pong")) {// if the accepted string is equal to the ping text
        digitalWrite (13, state); // blink LED
        state =! state;
        Serial.println("pong"); // print serial monitor
        }
}     
}

nano code

//Nano    
#define SerialTxControl 3 // DE RE
#define RS485Transmit HIGH
#define RS485Receive LOW

char buffer [100];
byte state = 0;

void setup (void) {
    UCSR0B |= (1<<TXCIE0);     // TX Complete Interrupt Enable serial 0
    Serial.begin (9600);
    pinMode (13, OUTPUT); 
    pinMode (SerialTxControl, OUTPUT); 
    digitalWrite (SerialTxControl, RS485Receive); // put the device into receiver mode
}

ISR (USART_TX_vect)
{
    digitalWrite(SerialTxControl, RS485Receive);
}

void loop (void) {

//receive rs485
int i = 0;
if (Serial.available ()) {// if any data came to the port
    delay (5); // wait a bit for the entire data packet to be accepted by the port
    while (Serial.available ()) {
        buffer [i ++] = Serial.read (); // read data and write it to the buffer
    }
}
if (i> 0) {// if there is something in the buffer
    buffer [i ++] = '\ 0'; // convert the buffer contents to a string by adding a null character
    if (strcmp (buffer, "ping")) {// if the accepted string is equal to the ping text
        digitalWrite (13, state); // blink LED
        state =! state;
        digitalWrite (SerialTxControl, RS485Transmit);
        Serial.print ("pong");    //transmit rs485

    }
}
}