source: rtos_arduino/trunk/arduino_lib/libraries/Firmata/examples/SimpleDigitalFirmata/SimpleDigitalFirmata.ino@ 136

/*
 * Firmata is a generic protocol for communicating with microcontrollers
 * from software on a host computer. It is intended to work with
 * any host computer software package.
 *
 * To download a host software package, please clink on the following link
 * to open the download page in your default browser.
 *
 * http://firmata.org/wiki/Download
 */

/* Supports as many digital inputs and outputs as possible.
 *
 * This example code is in the public domain.
 */
#include <Firmata.h>

byte previousPIN[TOTAL_PORTS];  // PIN means PORT for input
byte previousPORT[TOTAL_PORTS];

void outputPort(byte portNumber, byte portValue)
{
  // only send the data when it changes, otherwise you get too many messages!
  if (previousPIN[portNumber] != portValue) {
    Firmata.sendDigitalPort(portNumber, portValue);
    previousPIN[portNumber] = portValue;
  }
}

void setPinModeCallback(byte pin, int mode) {
  if (IS_PIN_DIGITAL(pin)) {
    pinMode(PIN_TO_DIGITAL(pin), mode);
  }
}

void digitalWriteCallback(byte port, int value)
{
  byte i;
  byte currentPinValue, previousPinValue;

  if (port < TOTAL_PORTS && value != previousPORT[port]) {
    for (i = 0; i < 8; i++) {
      currentPinValue = (byte) value & (1 << i);
      previousPinValue = previousPORT[port] & (1 << i);
      if (currentPinValue != previousPinValue) {
        digitalWrite(i + (port * 8), currentPinValue);
      }
    }
    previousPORT[port] = value;
  }
}

void setup()
{
  Firmata.setFirmwareVersion(0, 1);
  Firmata.attach(DIGITAL_MESSAGE, digitalWriteCallback);
  Firmata.attach(SET_PIN_MODE, setPinModeCallback);
  Firmata.begin(57600);
}

void loop()
{
  byte i;

  for (i = 0; i < TOTAL_PORTS; i++) {
    outputPort(i, readPort(i, 0xff));
  }

  while (Firmata.available()) {
    Firmata.processInput();
  }
}