Why should you consider an IC as a black box?
Digital ICs are relatively easy for you to troubleshoot and test because of the limited numbers of
input/output combinations involved. When using positive logic, the logic state of the inputs and outputs of
a digital IC can only be represented as either a high (also referred to as a 1 state) or as a low (also referred
to as a 0 state). In most digital circuitry, a high is a steady 5-vdc level, and a low is a 0-vdc level. You can
readily determine the logic state of an IC by using high-input-impedence measuring devices, such as an
oscilloscope. Because of the increased use of ICs in recent years, numerous pieces of test equipment have
been designed specifically for testing ICs. They are described in the following paragraphs.
What are the two logic states of an IC?
Logic clips, as shown in figure 2-24, are spring-loaded devices that are designed to clip onto a dual-
in-line package IC while the IC is mounted in its circuit. It is a simple device that usually has 16 light
emitting diodes (LEDs) mounted at the top of the clips. The LEDs correspond to the individual pins of the
IC, and any lit LED represents a high logic state. An unlit LED represents a low logic state. Logic clips
require no external power connections, and they are small and lightweight. Their ability to simultaneously
monitor the input and output of an IC is very helpful when you are troubleshooting a logic circuit.
Figure 2-24.Logic clip.
A lighted LED on a logic clip represents what logic level?
The logic comparator, as shown in figure 2-25, is designed to detect faulty, in-circuit-DIP ICs by
comparing them with ICs that are known to be good (reference ICs). The reference IC is mounted on a
small printed-circuit board and inserted into the logic comparator. You then attach the logic comparator to
the IC under test by a test lead, which is connected to a spring-loaded device similar in appearance to a
logic clip. The logic comparator is designed to detect differences in logic states of the reference IC and
the IC being tested. If any difference in logic states does exist on any pin, an LED corresponding to the
pin in question will be lit on the logic comparator. The logic comparator is powered by the IC under test.