LOGIC PROBES

2-30 Figure 2-25.—Logic comparator. Q-22. What does a lighted LED indicate on a logic comparator? LOGIC PROBES Logic probes, as shown in figure 2-26, are extremely simple and useful devices that are designed to help you detect the logic state of an IC. Logic probes can show you immediately whether a specific point in the circuit is low, high, open, or pulsing. A high is indicated when the light at the end of the probe is lit and a low is indicated when the light is extinguished. Some probes have a feature that detects and displays high-speed transient pulses as small as 5 nanoseconds wide. These probes are usually connected directly to the power supply of the device being tested, although a few also have internal batteries. Since most IC failures show up as a point in the circuit stuck either at a high or low level, these probes provide a quick, inexpensive way for you to locate the fault. They can also display that single, short-duration pulse that is so hard to catch on an oscilloscope. The ideal logic probe will have the following characteristics: Figure 2-26.—Logic probe. 1. Be able to detect a steady logic level 2. Be able to detect a train of logic levels 3. Be able to detect an open circuit 4. Be able to detect a high-speed transient pulse