Using The Logic Probe - HP 545A Operating And Service Manual

Model 545A
Page 7
USING THE LOGIC PROBE
Probe Power Supply. The Logic Probe can be powered from the supply of the circuit under test or from a regulated
DC power supply. If a separate power supply is used, the power supply and circuit-under-test grounds should be connec-
ted together. The power supply voltage range for TTL operation is 4.5-15 Vdc, and for CMOS the range is 3-18 Vdc.
In the TTL mode the thresholds change slightly as the supply voltage is increased (see Figure 4). In the CMOS mode
the threshold levels are a function of the supply voltage (see Figure 5). A ground wire (provided with the Probe)
may be connected just behind the Probe indicator window. The ground wire is a convenient means of connecting
grounds when using external regulated power supplies. It also improves pulse width sensitivity and noise immu-
nity. However, its use is optional and not required for all applications.
Pulse Detection. The Logic Probe is ideal for detecting short duration and low repetition rate pulses that would be
difficult to observe on an oscilloscope. Positive pulses to ?.10 nsec or more in width trigger the indicator on for
?.50 msec. Negative pulses cause the indicator to go off momentarily.
Troubleshooting. The bad level feature of the Model 545A Logic Probe is useful for testing three-state logic out-
puts. The logic HIGH and the logic LOW states are detected as described under Pulse Detection and the third state
(i.e., high impedance output) is detected as an open circuit (or bad level) condition, which leaves the probe indi-
cator dim. It is also useful for detecting floating or unconnected TTL or CMOS gate inputs which look like a bad
level to the logic probe.
Several logic circuit analysis techniques are useful with the Logic Probe. One technique is to run the circuit under
test at its normal clock rate while monitoring for various control signals such as reset, start, stop shift, transfer, or
clock signals. Questions such as "is the counter operating?" are quickly resolved by noting if the probe indicator is
flashing on and off, indicating that pulse train activity is present.