NRF24L01 - MASTER

// SimpleTx - the master or the transmitter

#include <SPI.h>
#include <nRF24L01.h>
#include <RF24.h>
#define CE_PIN   9
#define CSN_PIN 10

const byte slaveAddress[][5] = {{'R', 'x', 'A', 'A', 'A'}, {'R', 'x', 'A', 'A', 'B'},
    {'R', 'x', 'A', 'A', 'C'}, {'R', 'x', 'A', 'A', 'D'}};

RF24 radio(CE_PIN, CSN_PIN); // Create a Radio

char dataToSend[10] = "Message 0";
char txNum = '0';

unsigned long currentMillis;
unsigned long prevMillis;
unsigned long txIntervalMillis = 1000; // send once per second


void setup() {

    Serial.begin(9600);
    Serial.println("SimpleTx Starting");
    radio.begin();
    radio.setDataRate( RF24_250KBPS );
    radio.setRetries(3, 5); // delay, count
    int numSlaves=4;


    radio.enableDynamicPayloads() ;
    radio.setAutoAck( true ) ;
}

//====================

void loop() {
    currentMillis = millis();
    if (currentMillis - prevMillis >= txIntervalMillis) {
    send();
    prevMillis = millis();
    }
}

//====================

void send() {

    bool rslt;
    for (byte n = 1; n < 3; n++) {
        radio.openWritingPipe(slaveAddress[n]);
        rslt = radio.write( &dataToSend, sizeof(dataToSend) );
            // Always use sizeof() as it gives the size as the number of bytes.
            // For example if dataToSend was an int sizeof() would correctly return 2

        Serial.print("Data Sent to Slave ");
        Serial,println(n);
        Serial.print(dataToSend);
        if (rslt) {
        Serial.print("  Acknowledge received from Slave ");
        Serial.println(n);
        updateMessage();
        }
    }
    else {
    Serial.println("  Tx failed");
    }
}

//================

void updateMessage() {
    // so you can see that new data is being sent
    txNum += 1;
    if (txNum > '9') {
    txNum = '0';
    }
    dataToSend[8] = txNum;
}