Introduction; Logic Level Lamp Indications - HP 545A Operating And Service Manual

Page 2 Model 545A
INTRODUCTION
The Hewlett-Packard Model 545A Logic Probe will detect and indicate HIGH and LOW logic levels. The probe will
also indicate intermediate or "bad" Logic levels, including an open circuit on a TTL or CMOS gate, and open cir-
cuits such as an open collector output without pull-up resistors. The presence and polarity of single pulses of
~10
nsec in duration and pulse trains up to 40 MHz CMOS and 80 MHz TTL will also be indicated. The probe is com-
patible with virtually all TTL, DTL, RTL, HTL, MOS, CMOS, and HiNIL integrated circuits. A memory indicator
lamp is located under the MEM-CLR button. This indicator turns on when 1) either HIGH or LOW level logic
change is detected; 2) when the probe is first powered; 3) when the probe tip is touched to any circuit. The
memory indicator remains on until the MEM-CLR button is depressed.
LOGIC LEVEL LAMP INDICATIONS
The logic level indicator lamp, near the probe tip, will give an immediate indication of the logic states, either static
or dynamic, existing in the circuit under test. Figures 2 and 3 show how the logic level indicator lamp responds to
voltage levels and pulses for TTL and CMOS operation respectively. The indicator lamp can give any of four indi-
cations: 1) off; 2) dim (about half brilliance; 3) bright (full brilliance); 4) flashing on and off. The lamp is normally
in its dim state and must be driven to one of the other three states by voltage levels at the probe tip. The lamp is
bright for inputs above the logic 1 threshold and off for inputs below the logic 9 threshold. The lamp is dim for
voltages between the logic 1 and the logic 9 thresholds and for open circuits. Pulsating inputs will cause the lamp
to flash at about a 10 Hz rate. More information about using the 545A is on page 7.
MATCHING INSTRUMENTS
Hewlett-Packard makes several digital troubleshooting instruments that complement the Logic Probe. These are
the Model 546A or 10526T Logic Pulser, Model 548A or 10528A Logic Clip, Model 547A Current Tracer, and Model
10529A Logic Comparator. The Logic Pulser can be used to stimulate the input of a logic element while the Logic
Probe is touching the output to sense the activity of the element.