Electronics: Principles and Electronics: Principles and ApplicationsApplications

Eighth EditionEighth Edition(Instrumentation Labs)(Instrumentation Labs)

Charles A. Schuler

Lab 3Introduction to the

Logic Probe

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Lamp OFF … LOGIC LOW

Lamp ON … LOGIC HIGH

Lamp DIM … OPEN CIRCUIT (FLOATING) OR BAD LEVEL

(Other brands of probes may differ)

0

10

60

50

80

70

40

30

20

100

90HIGH

LOW

Digital logic levels as % of VSUPPLY

HIGH

TTL CMOS(5 V supply) (3 to 18 V supply)

% of VSUPPLY

UNDEFINED

UNDEFINED

LOW

= 1 = ON

= ? = FLOATING

= 0 = OFF

Common faults that can be detected with logic probes

Open bond(floating output)

Internal short(stuck high)

Solder bridge(stuck low)

Defective input

Logic probe with pulse memory(often used to catch “glitches”)

1. Set TTL/CMOS switchto family under test.2. Place tip on circuitunder test.3. Press MEM/CLR(light goes out).4. Light comes on whena single pulse (“glitch”)occurs.

Pulse trains cause the probe to flash at less than a 10 Hz rateeven if the pulse frequency is much higher. (up to 80 MHz)

This probe will “stretch” pulses asshort as 10 ns and the lamp will flash.

Logic Probe Quiz

When the lamp is off, the logic level is LOW

When the lamp is on, the logic level is HIGH

When the lamp is dim, the logic level is bad or floating

When the lamp is flashing, the logic level is a pulse train

A probe with pulse memory is useful whenlooking for glitches