File content as of revision 0:b90c11323d98:

/*Program Example 7.5: I2C Master, transfers switch state to second mbed acting as
slave, and displays state of slave’s switches on its leds.
*/
#include "mbed.h"
I2C i2c_port(p9, p10); //Configure a serial port, pins 9 and 10 are sda, scl

DigitalOut red_led(p25); //red led
DigitalOut green_led(p26); //green led
DigitalIn switch_ip1(p5); //input switch
DigitalIn switch_ip2(p6);

char switch_word ; //word we will send
char recd_val; //value received from slave

const int addr = 0x52; //the I2C slave address, an arbitrary even number <-- ojo con esto

int main() 
{
    while(1) {
        switch_word=0xa0; //set up a recognizable output pattern
        if (switch_ip1==1)
            switch_word=switch_word | 0x01; //OR in lsb
        if (switch_ip2==1)
            switch_word=switch_word | 0x02; //OR in next lsb
            
        //send a single byte of data, in correct I2C package
        i2c_port.start(); //force a start condition
        i2c_port.write(addr); //send the address
        i2c_port.write(switch_word); //send one byte of data, ie switch_word
        i2c_port.stop(); //force a stop condition
        wait(0.002);
        
        //receive a single byte of data, in correct I2C package 
        i2c_port.start();
        i2c_port.write(addr | 0x01); //send address, with R/W bit set to Read  <-- ojo con esto
        recd_val=i2c_port.read(addr); //Read and save the received byte
        i2c_port.stop(); //force a stop condition
        
        //set leds according to incoming word from slave
        red_led=0; //preset both to 0
        green_led=0;
        recd_val=recd_val&0x03; //AND out unwanted bits
        if (recd_val==1)
            red_led=1;
        if (recd_val==2)
            green_led=1;    
        if (recd_val==3){
            red_led=1;
            green_led=1;
            }
        }
}