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EXERCISE 18 : OUTPUT MID LEVEL WITH LOGIC TRACE
ACTION
Go to SETUP and select the LOGIC TRACE then exit SETUP.
Press START on the IC Tester and note the LOGIC TRACE window.
Press the MID-LEVEL switches again and observe the changes to the monitored values on Pin
6 by clicking on the LOGIC TRACE display at the relevant step.
DESCRIPTION
The LOGIC TRACE is an ideal tool for examining the behaviour of an output. In this case, the
outputs of Pin 6 seems to be giving conflicting information.
Being Mid-High and Mid-Low at the same is not exactly what it means. The Result shows all
the conditions seen on the pin for the whole test. Examining the LOGIC TRACE shows that
for a particular part of the test the result was Mid-High when a High was expected.
At another part of the test the result was Mid-Low when a LOW was expected. Examining the
LOGIC TRACE in will highlight the exact nature of the results because the trace is derived
from the actual TRUTH TABLE applied to the IC.
COMMENT
This type of fault could be seen on the output of an IC that is genuinely faulty. It can also be
seen if the output is driving an input of another gate or device that was applying a loading
effect to such an extent that the driving stage could not reach its normal output voltage. This
shows that a MID LEVEL fault shown on the output of one IC may in fact indicate a problem
elsewhere in the circuit, as in the example on the training board.
EXERCISE 19 : OUTPUT SHORTS
ACTION
Press START on the IC Tester.
ONLY APPLY A SHORT FOR SHORT PERIODS OF TIME TO AVOID DAMAGE.
Press OUTPUT SHORT 0V switch and Observe device failure and indication on Pin 8. When
satisfied remove SH0V. Press OUTPUT SHORT 5V switch and observe device failure due to
OUTPUT SHORT 5V on Pin 3.
DESCRIPTION
The maximum resistance to GND that will indicate a SHORT 0V is approximately 1.5 Ohms. If
a pin of an IC has a resistance less than 1.5 Ohms to GND then the SHORT 0V message will
appear. If the pin is an input, this may be a valid circuit configuration that is recognised by
AUTOMATIC CIRCUIT COMPENSATION and allowed for in the test. If however an output is
shorted to ground, this is regarded as a fault and a FAIL result occurs.
TRI-STATE and OPEN-COLLECTOR outputs however can be shorted to ground in some
designs, therefore a FAIL may not be indicated in these situations although the short is still
shown on the display.
