 A model may 1) emit a tone for high logic state otherwise no tone, or 2) emit a higher frequency tone for a high logic state, lower frequency tone for a low logic state, and no tone for no connection or tri-state.An oscillating signal causes the probe to alternate between high-state and low-state tones.A logic probe is a low cost, versatile and convenient digital test instrument, but can test only a single signal at a time.