def start_experiment(exp_controller):    
    """Send a message to experiment controller to begin 
    executing an experiment protocol.

    Parameters
    ----------
    exp_controller : serial.serialposix.Serial
        The serial interface to the experiment controller.

    Returns
    -------
    response : str
        The first line of data received from the 
        experiment controller verifying that the protocol
        has been initiated.

    """
    exp_controller.write(b"b")
    while not exp_controller.in_waiting:
        pass
    response = exp_controller.readline().decode('utf-8').split()
    exp_controller.flushInput()  
    print('trials completed:')
    print((response[0]))
    return response