Logic probe basics

- a summary or tutorial about the basics of logic probes - what a logic probe is, how a logic probe works and how one may be used.

A logic probe is a low cost item of test equipment. As the name indicates, a logic probe is used for probing and analyzing logic circuits.

A logic probe is normally a small item of test equipment, typically handheld in the form of what could be thought of as a large oscilloscope probe, but without the lead.

This makes logic probes very portable and easy to use for troubleshooting and general simple maintenance work on low complexity logic circuitry.

For most applications, a logic probe would not be used within a laboratory test environment - here more sophisticated test equipment would be used because much closer investigation of problems is required.

Logic probe basics

A logic probe is able to give an indication of the logic state of a line carrying a digital signal The logic probe indicates whether there is a logic state "1" or "0", normally using an LED as the indicator. Often the LED on the logic probe will use different colours to indicate different states.

A logic probe normally may be capable of indicating up to four different states:

• Logic high :   If the logic circuit is at a logic or digital high voltage, the logic probe will indicate this on its interface - typically this will be a colour red.
• Logic low:   Again the logic probe will indicate a logic or digital low. The most common colour for this is green.
• Pulses:   The logic probe is likely to incorporate a pulse detection circuit. When the line is active a third colour, possibly amber will be indicated. The logic probe may well incorporate circuitry to detect very short pulses and in this way indicate when the line is active.
• Line tri-stated :   Often it is possible for lines to be tri-stated, i.e. the output device has its output turned off and no real state is defined. Many logic probes are able to indicate this state by having all indicators turned off.

Some logic probes may have a control to select the logic family being tested - different logic families have slightly different high and low voltage levels. Some logic probes are able to detect when they are operating within their specified ranges. This could be the key to determining some faults.

Another facility that some logic probes may include is an audible indication of the logic state. This feature is particularly useful when using a probe as eyes may need to be trained on the circuit and not on the logic probe itself.

A logic probe is a cheap and relatively simple item of test equipment. It is versatile and very transportable, and it also is able to provide a quick test for many circuits. However it is not nearly as flexible as an oscilloscope or a logic analyzer. A logic probe can be used for quick testing, whereas for more in-depth testing more sophisticated test equipment is needed.