public void sendInterrupt (byte buf [])
    throws IOException
    {
	if ("interrupt" != getType () || buf.length > getMaxPacketSize ())
	    throw new IllegalArgumentException ();
	if (spi == null)
	    spi = iface.getDevice ().getSPI ();
	// FIXME getInterval() ms timeout (caller guarantees periodicity)
	spi.writeIntr (getU8 (2), buf);
    }