Wire-And Node; Gate-To-Gate Faults - HP LOGIC PULSER 546A Operating And Service Manual

Model 546A Page 13
Wire-AND Node
One of the most difficult problems encountered in troubleshooting integrated circuits is a stuck wire-
AND node. Typically, one of the open-collector gates may still continue sinking current after it has
been turned off. The Logic Pulser and Current Tracer provides an easy method of identifying the fault
gate.
Referring to Figure 7 place the Current Tracer on the gate side of the pull-up resistor. Align the mark
on the probe tip along the length of the printed-circuit trace and adjust the probe's sensitivity control
until the indicator is just fully lighted.
If
the indicator will not light - use a logic Pulser programmed
to the 100 Hz continuous mode to excite the line. Place the tracer tip on the output pin of each gate;
only the faulty gate will cause the indicator to light.
Gate-to-Gate Faults
When a low-impedance fault exists between two gates, the Current Tracer and logic Pulser combine
to quickly pinpoint the defect. In Figure 8 gate A's output is shorted to ground. Place the pulse mid-
way between the two gates and place the Current Tracer's tip on the Pulser's pin. Pulse the line in the
100 Hz continuous mode and adjust the Current Tracer's sensitivity control until the indicator just
lights. First place the Current Tracer tip next to gate A and then gate B while continuing to excite the
trace. The tracer will light only on the gate A side, since gate A, (the defect in this example), is sink-
ing the majority of the current.