Applications
—
7A42
APPLICATION
2
SOLVING
ABNORMAL
PULSE PROBLEMS
WITH
THE
7A42
Defective
gates
or
timing
errors
in
a
logic
system
can
produce
low
amplitude
pulses which
are not
recognizable
as
valid logic
levels. These abnormalities
can
introduce errors
in
the
performance of
a
digital
system
which
are
very
difficult-to
trace
and
locate.
To
an
ordinary
oscilloscope
triggering
system,
one pulse
in
a pulse
train
looks like
any other.
The
oscilloscope
can
only
recognize a pulse
which
has
a
higher-than-
normal
amplitude.
A
low-amplitude
pulse
can
be
difficult,
if
not
impossible,
to
detect.
Low
amplitude
pulses,
or
half-logic
levels,
can
be
detected
by
means
of
the
7A42
variable
threshold
feature.
Each
of
the four
channels
can
have a distinct
threshold
recognition
level
for
a high- or
low-logic
level.
The
7A42
features
an
automatic
preset
TTL
or
ECL
threshold
level.
If
a
variable
threshold
is
needed,
enter
the
Program
Channel
mode, turn
the
threshold
monitor
on,
and select
the
channel whose
threshold
is
to
be
varied.
The
LEVEL
↑
and
LEVEL
↓
controls
can
then be used to
set
the desired
threshold
voltage.
The
threshold voltage
for each
channel
can,
in
turn,
be
set
individually
in
this
manner.
MEASUREMENT
TECHNIQUE
When
one
wants
to trigger
on
a
pulse
that
is
somewhere
in
the
TTL
threshold
region
(+0.8V
and
+2.0V),
a problem arises
because
the
pulse
is
too low
to
be
recognized
as a
TTL high
level
and
too
high
to
be
recognized
as a
TTL
low
level;
see
Figure
3-4.
To
locate
and
trigger
on
a
TTL
low amplitude
pulse,
connect
the
CH1 and CH2
probes
to
the
same
point
on
the
signal under
test
and establish a
dual
threshold
to
bracket
the
suspected
amplitude
of
the
pulse
as
follows:
1.
Set the
CH1
SWITCHING
THRESHOLD
voltage
for
+2.0V
and
the
CH2
SWITCHING
THRESHOLD
readout voltage
for
+0.8V.
2.
Enter
keystroke
sequence
NOT
CH1
AND
CH2.
The
7A42
will
monitor
the
data
line
looking
for
a
pulse
which
is
less
than
+2.0V
(NOT
CH1) and
greater
than
+0.8V
(AND
CH2)
in
amplitude.
The
time
base
should
be
set
to
Auto
trigger
and
+Slope
so that
it
will
trigger
on the
positive
slope
of the signal
from
the
de
coupled
internal source.
Because
the
same
signal
is
present
on
CH1
and
CH2,
only
one
channel
need
be
displayed.
Even
though
only
one
channel
is
displayed,
both
CH1
and CH2
will
contribute
to
the
trigger
function.
Because
valid
logic
signal
transitions
also
cross
the
voltage window
between
the
CH1
and CH2
thresholds,
trigger
outputs
may
occur
when
the
signals
pass
through
this
transition
region.
The
7A42
TRIGGER
FILTER
control
can
be
used
to
3-4
