TM109902 (5/03) 3-13
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C OMMAND S YSTEMS D IVISION
C. Digital Integrated Circuit Testing
Digital circuit element output logic levels change with changes in the input logic level. By
checking the input versus output logic states faults can be isolated to a defective IC. In
operation, the logic levels at the input or output of the digital integrated circuits will be a logic 1
or logic 0 depending on the function in progress. A logic 1 voltage is between +2.4 to +5.5 volts
and a logic 0 is +0.8 volts or less.
Exact voltage measurements are not necessary in testing digital components other than to verify
that the voltage is at a logic 1 (HI) or logic 0 (LOW) level. When the input or output levels are
toggling HI and LOW at short durations (example 10 microseconds) an oscilloscope should be
used, since these level changes could not been seen otherwise. Table 3-5 is a summary of
possible IC failure.
Table 3-4.
54/74 Series IC Alpha Character Significance
ALPHA CHARACTER SIGNIFICANCE
None Standard Transistor-Transistor-Logic (TTL).
S
Schottky-clamped TTL device. Contains the integrated Schottky-
Barrier diode-clamped transistor circuitry.
LS Low-power Schottky TTL device.
AS Advanced Schottky TTL device.
ALS Advanced Low-power Schottky TTL device.
HC High-speed CMOS device. Requires special ESDS handling.
HCT
High-speed CMOS with TTL input voltage level compatibility.
Requires special ESDS handling.
AC Advanced CMOS device. Requires special ESDS handling.
ACT
Advanced CMOS with input TTL compatibility. Requires special
ESDS handling.
Table 3-5.
Summary of Possible IC Failures
FAILURE SYMPTOM
Inputs Shorted Together (Quite Common)
Identical signals/changes in signal level
between normally isolated inputs.
Ground Open No LO can be obtained.
Vcc Open No HI can be obtained.
Inputs or Output Open Constant Incorrect Logic Level
Inputs or Output Shorted to Vcc Constant HI
Inputs to Ground or Output Shorted Constant LO
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