Logic probe


A logic probe is a hand-held test probe used for analyzing and troubleshooting the logical states of a digital circuit.

Overview

While most logic probes are powered by the circuit under test, some devices use batteries. They can be used on either TTL or CMOS logic integrated circuit devices, such as 7400-series, 4000 series, and newer logic families that support similar voltages.
Most modern logic probes typically have one or more LEDs on the body of the probe:
A control on the logic probe allows either the capture and storage of a single event or continuous running.
When the logic probe is either connected to an invalid logic level or not connected at all, none of the LEDs light up.
Another control on the logic probe allow selection of either TTL or CMOS family logic. This is required as these families have different thresholds for the logic-high and logic-low circuit voltages.
Some logic probes have an audible tone, of which vary across models. 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. When many logic levels need to be observed or recorded simultaneously, a logic analyzer is used.

History

The Logic Probe was invented by Gary Gordon in 1968 while he was employed by Hewlett-Packard.