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components. A test instrument of value for any technician who works on digital equipment is the LOGIC
PROBE, which is an integrated circuit-testing device.
INTEGRATED CIRCUIT-TESTING DEVICES
Digital integrated circuits are relatively easy to troubleshoot and test because of the limited numbers
of input and output combinations involved in circuits. The two-state conditions in logic circuits are often
referred to as (1) low or high, (2) on or off, or (3) one or zero (1 or 0).
Other terminology may also be used. Any particular integrated circuit (IC) can be tested by simply
comparing it to a known good one. The LOGIC PROBE is a device that can be of great value in
troubleshooting digital integrated logic circuits.
The ideal logic probe has the following characteristics:
It will detect a steady logic level.
It will detect a train of logic levels.
It will detect an open circuit.
It will detect a high-speed transient pulse.
It will have over-voltage protection.
It will be small, light, and easy to handle.
The use of a suitable logic probe can greatly simplify your troubleshooting of logic levels through
digital integrated logic circuitry. It can immediately show you whether a specific point in the circuit is
low, high, open, or pulsing. Some probes have a feature that detects and displays high-speed transient
pulses as small as 10 nanoseconds wide. These probes are usually connected directly to the power supply
of the device being tested, although a few have internal batteries.
Most IC failures show up in a circuit as a constant high or low level. Because of this, logic probes
provide a quick, inexpensive way of locating the fault. They can also display the single, short-duration
pulse that is hard to detect on an oscilloscope. Figure 5-17 shows a basic logic probe.
