Tektronix DC 501 Instruction Manual page 11

Theory
of
Operation—
DC
501
Fig. 2-2.
Time
Base generator normal
gating
mode
ladder diagram.
U222B,
causing
its
Q
output
to
go
LO,
closing
the
GATE.
Simultaneously, the
Q
output goes HI, removing current
from
the
GATE
indicator
LED.
When
the
GATE
output goes LO, the negative transition
toggles
U220A,
switching
Q
LO
and
Q
HI.
Now
NAND
gate
U230D
has
two
HI inputs, placing a
LO
at
the input of
OR
gate
U230B
and
activating
the
LATCH
control
signal
(HI
state).
One
microsecond
later (T.^), a
negative edge
from
the
1-MHz
clock toggles
U222A,
switching
its
outputs and
placing a
LO
on
the input of
NAND
gate
U230D.
U230D
reverts to
its
original condition, terminating the
LATCH
signal.
The
display time
begins
when
the
GATE
signal
ends
(Tg).
When 0230
turns off,
C235
begins to charge through
R232-R235 toward
the
Vcc
supply.
R235,
DISPLAY
TIME,
provides
an
adjustable
time
constant to vary the
display
time
from about
0.1
second
to
about 10
seconds.
When
the
DISPLAY TIME
control
is
fully
clockwise
(HOLD
detent
position),
S235
opens, and
C235
stops
charging.
When
S235
is
closed and
C235
charges
sufficiently to bring
Q238
to
its
firing
potential
(T.|),
the
display
time ends and
the
next
GATE-opening
sequence
begins.
Manual
Gate.
The manual
mode
of operation
is
selected
by
placing the
MEASUREMENT
INTERVAL
switch
in
the
MANUAL
position.
The
switch closure to ground (cam 5 of
the switch) places a
LO
on
the
set
inputs of
U220B
and
U222A,
and
a
LO
on
the
clear
input of
U220A.
This forces
the
Q
outputs of
U222A
and
U220B
HI,
and
the
Q
output
of
U220A
LO. With both inputs of
U230D
held HI, the
LATCH
output
is
held HI, allowing the counter to
update
the display continuously.
The
GATE
is
opened
when
the
front-panel
START
button
is
pushed
in,
opening
S210
and
applying
a
HI to the
clear
input of
U222B. As
before, the
GATE-open
condition
is
HI
at
the
Q
output of
U222B. The
GATE
is
then closed
when S210
is
set
to
STOP
(button
out).
To
reset
the counters
in
the
manual mode,
the
RESET
butt
on
mus
t
be pushed
to activate the
CLEAR, CLEAR,
and
RESET
control
signals.
Automatic Gate (For Instruments Having Option
2).
The
automatic gate
mode
is
selected
by
placing the
MEASURE-
MENT INTERVAL
switch
in
the
AUTO
position.
The
output
of the
automatic time base
circuit
is
connected
to
the gate generator
via
contact
1
of the switch.
Contact 2
opens
to enable the readout-scaling
circuit.
The
automatic
gating cycle begins
with
the
CLEAR
pulse,
which
occurs
when Q238
reaches
its
firing potential, as
discussed for the
© 2-4