Service Manuals, User Guides, Schematic Diagrams or docs for : HP Publikacje HP-Bench-Briefs-1977-09-10
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SERVICE INFORMATION FROM HEWLETT-PACKARD
SEPTEMBER-OCTOBER 1977
TROUBLESHOOTER3
IN
ACTION
Flgun 1. TIL S t g d . In Un digital world,
Unnktiwvalwofa [email protected]
faepect to th.thfeshold voH.g# G9w-
mines tho op#.tlon of the cimuk A 8lg-
nal abornth. ht@ thre8hokl k I th. n
hlghMato nb whether it I 2.8V o 3.0V I
r r s
unhparsrwtt to th. operation of Un cir-
cuit
the digital signal and this digital state
determines the operation of the IC, not
absolute leveis. In Figure 1, if the sig-
nal is greater than 2.4 vdts, it Ts a high
state and it is unimportant whether the
by Jim Bechtold, Editor The traditional equipment can still be level is 2.8 or 3.0volts. Simitarly
used, but an important characteristic low state the voltage must be below
of dgital IC's is that when they fail, 0.4 volts. It is not important what the
ANALOG CIRCUIT TESTING absolute level is as long as it is below
they usually fail catastrophically. This
means that timing parameters rarely this threshold. Thus when using an
When troubleshooting circuits built oscilloscope, the troubleshooter must
with discrete components (resistors, degrade or become marginal. Thus,
observing timing parameters on an over and over again determine if the
capacitors, transistors, etc.), the task signal meets the threshold require-
is one of verifying relatively simple oscilloscope and making repeated de-
cisions on their validity (a time con- ment for the desired digital state.
characteristics such as resistance,
capacitance, or bias voltages of com- suming ordeal that contributes very lit-
tle to the troubleshooting process), Figure 2 shows a problem created by
ponents. While the function of the total the TTL logic family. The output stage
circuit may be quite complex, each can all but be eliminated. The fact that
pulse activity exists is usually enough of a TTL device is a transistor totem
component in that circuit performs a pole. In either the high or low state, it is
relatively simple task, and proper op- indication of proper IC operation with-
out further observation of pulse width, a low impedance. In the low state it is
eration is easily verified using a signal a saturated transistor to ground. lt thus
generator and a voltmeter, ohmmeter, repetition rate, rise time o fall time.
r
appears as 5-10 ohms to ground.
diode checker, or oscilloscope - the
traditional troubleshoding tools. But Figure 1 shows a typical TTL
(Transistor-Transistor-Logic) signal;
when this circuit is built in integrated one of the most popular IC logic fam-
circuit form, these components are no ily. This might as well be any analog
longer accessible. It now becomes
necessary to test the operation of the signal when viewed on an oscillo-
scope. The oscilloscope displays ab-
complete circuit function using
specialized IC troubleshooters. solute voltage with respect to time, but
in the digital world absolute values are
unimportant. A digital signal exists in
"Node" as used in this article and throughout one of two or three states -high, low,
the digital troubleshooting world means the and undefined or in-between level -
pOini at which a cdlectbn of component termi- each determined by a threshold volt-
rials end interconnections come together; or the
interseGtion or@nclh of two or more PC traces age. It is the relative value of the signal
01 wires, o an e y e M wlth a component lead
r voltage with respect to these
W e f e d in It. -Ed&. thresholds that determines the state of
WWW.HPARCHIVE.COM
job. However, in a production rmvi- 0 A short between an input or output
.
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