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DESCRIPTION
The Load 0V is caused by the input pin being connected to VCC with a low resistance to
Ground (3.5Ohm) The drive channel of the system attempts to pull the pin to a high logic
level (>2.4V for this device) but cannot obtain a valid logic High and so indicates that it is
loaded to 0V.
The system will try to drive the pin high for each successive loop. The Truth Table test will
be applied taking into account that the pin will remain in a Low state.
The Load 5V is a similar case where the tester attempts to take the pin to a Logic Low (<0.5V
for this device). If it cannot drive the pin to a valid state it will indicate the LOAD and amend
the following Truth Table test.
COMMENT
Loads can be caused by high current devices driving the inputs of the DUT. Large capacitors
used in timing circuits could also give a load as they cannot change quickly enough when the
tester drives a voltage across it. Because they are input loads the tester will adapt the truth
table test to take the condition into account.
There is no operator intervention required other than to confirm the presence of the load is
acceptable due to the circuit design and components used. It is possible that an internal IC
static protection diode can give a LOAD indication.
EXERCISE 17 : OUTPUT MID LEVELS
ACTION
Press START on IC Tester. Operate MID HIGH FAULT switch and observe indication on Pin 6.
NOTE: Test Fails. Operate MID LOW FAULT switch and observe indication.
STOP the test and read the description.
DESCRIPTION
The input threshold levels on IC Tester can be changed in the SETUP dialogue box.
For TTL logic, the default settings are as follows:LOW: 0.5V, SWITCHING: 1.2V, HIGH: 2.4V
When a functional test is executed, the outputs of the IC under test are interpreted by
comparing them with the preset threshold-voltages. They would normally be either HIGH
(above the HIGH threshold) or LOW (below the LOW threshold). When the output voltage
however is between the LOW threshold and the SWITCHING threshold, a MID LOW message
will appear indicating a fault. Similarly when the output voltage is between the SWITCHING
threshold and the HIGH threshold, a MID HIGH fault message will appear.
On the training board, the fault has been simulated by connecting the output of a NAND gate
through relays to GND and VCC via suitable resistors. When the relay is open, no MID LEVEL
fault will be seen but when the relay is closed the IC cannot drive the resistor to a valid logic
level. The voltage is between the SWITCHING threshold but not at the valid level, thus
displaying a MID HI or MID LO or both depending on the test result the system expected.
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.
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