Star Tech Journal

S*TJ
MARCH 1981
6
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MICROPROCESSOR TROUBLESHOOTING
(Part 3)
0
This is the final installment of this series covering Logic
Probes, Logic Probe Applications, Signal Injection, Current
Tracing, Clips and Comparators, and Dynamic Logic
Analyzers.
CUPS AND COMPARATORS
The logic probe provides a means of conveniently examining the
logic level at each of the individual pins on an integrated circuit.
Quite often, when dealing with an analysis of more complex
functions, it is helpful to simultaneously observe the input and
output signals on a given IC. This is conveniently accomplished by
utilizing one of the many Logic Clips available. A typical logic clip
is similar to the one shown in Figure 12. A series of LED indicators
on the unit indicate the logic level at each of the individual
integrated circuit pins. A slot in the base of the unit permits the
logic clip to be snapped over the integrated circuit to be tested.
Connecting pins located in the slot provide an electrical connection
to each one of the integrated circuit pins. The unit is generally self-
powered and it contains circuitry which automatically senses the
pin connected to the power source and makes the appropriate
internal connections within the logic clip. By observing the LED
indicators, you can determine if each IC pin has the correct output.
It's a simple matter to place the logic clip on each of the integrated
circuits in the system and then observe operation in order to isolate
a fault. The simultaneous display of inputs and outputs permits
easy observation of the actions of multiple gates in a single
package as well as that of devices such as multiplexers and
decoders. Most logic clips provide a means for detecting the
presence of short pulses so that dynamic operation of an integrated
circuit can also be observed.
As mentioned earlier, substitution is often a time-saving and
convenient means of pinpointing trouble within a given system. To
avoid the need to unsolder an integrated circuit from a circuit board
where no socket is provided and where there is the attendant risk of
damage to the board, a device referred to as a Logic Comparator
is used as shown in Figure 13. A test clip is used to connect the
logic comparator to the integrated circuit being tested on its circuit
board. A test reference integrated circuit is plugged into the
comparator. Circuitry within the logic comparator compares the IC
known to be good, which serves as a standard, against the in-circuit
performance of the IC being tested. The logic comparator checks
the output responses of the reference IC against those of the in-
circuit test IC, and indicates any malfunctions present on the LED
indicators on a pin-by-pin basis. Circuitry is included to detect
pulses of as short a duration as 200 nsec. By using the comparator,
the performance of the integrated circuits on a board can be readily
checked in both static and dynamic modes and the risk of
damaging the circuit board is greatly reduced.
DYNAMIC LOGIC ANALmRS
Logic probes and current tracers are used primarily for static
checks and simple pulse analyses of integrated circuits and
associated systems. Great care must be exercised in the
application of logic probes and tracers so that error conditions
encountered will not be misinterpreted. In complex digital systems,
circuit action is often of a dynamic nature. That is, the proper bit
patterns (ls and Os) must be developed and transferred from place
to place on the buses. A logic probe or a simple pulse detecting
device is not adequate for evaluating circuit operation.
FIGUR£l2
ERROR INDICATORS
CIRCUIT BOARO
FIGURf/3