File content as of revision 0:addc96275da2:

/*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

int main() {
  while(1) {
    switch_word=0xa0;                    //set up a recognisable 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
    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;
    }
  }
}