int btnTara = 4;
int btnBruto = 3;
int btnOk = 2;

int ledTara = 5;
int ledBruto = 6;
int ledOk = 7;

char state;
char prev_state = 1 | 2 | 4;
int incomingByte = 0;

void setup()
{

Serial.begin(9600);

pinMode(btnTara, INPUT);
pinMode(btnBruto, INPUT);
pinMode(btnOk, INPUT);
pinMode(ledTara, OUTPUT);
pinMode(ledBruto, OUTPUT);
pinMode(ledOk, OUTPUT);
digitalWrite(ledTara, LOW);
digitalWrite(ledBruto, LOW);
digitalWrite(ledOk, LOW);
}

void loop()
{
if (Serial.available() > 0)
{ incomingByte = Serial.read();
Serial.print("I received: ");
Serial.println(incomingByte, DEC);
if (incomingByte & 1)
digitalWrite(ledTara, HIGH);
else
digitalWrite(ledTara, LOW);

if (incomingByte & 2)
digitalWrite(ledBruto, HIGH);
else
digitalWrite(ledBruto, LOW);

if (incomingByte & 4)
digitalWrite(ledOk, HIGH);
else
digitalWrite(ledOk, LOW);

}

state = 0;
if (digitalRead(btnTara)==HIGH)
state |= 1;
if (digitalRead(btnBruto)==HIGH)
state |= 2;
if (digitalRead(btnOk)==HIGH)
state |= 4;

if (state != prev_state)
{
Serial.write(0x80);
Serial.write(state);
Serial.write(0x7F);
prev_state = state;
}
delay(100);
}