been
stored, the processor uses a shorter routine to deter-
mine
if
any
pot setting changes. To do this the
DAC
out-
put
is
set to the last known value of the pot (plus
and
minus a small drift
value),
and
the status bit
is
read
to
see
that a
HI
and
LO
occurs. If within the limits, the processor
assumes that the pot setting has not changed
and
scans
the next pot.
When
the processor detects that a pot set-
ting has changed, it does another binary search routine to
find the new value of that pot.
Analog Control
The operating mode
and
status of the instrument
requires that various analog voltages (for controlling instru-
ment functions)
be
set
and
updated.
The
digital values of
the controlling voltages are generated
by
the Microproces-
sor
and
converted
by
the
DAC.
Analog multiplexers
U2521
and
U2530
(on
diagram
2)
and
U170
(on
diagram
4)
route
the
DAC
voltages to sample-and-hold circuits that maintain
the control voltages between updates.
The
Microprocessor writes three selection bits to regis-
ter
U2301
that directs the
DAC
output to the appropriate
sample-and-hold circuit
and
charges a capacitor (or capac-
itors) to the
level
of the
DAC.
When
the processor discon-
nects the
DAC
voltage from the sample-and-hold circuit
(by
disabling the multiplexer) the capacitor(s) remains
charged
and
holds the control voltage
near
the
level
set
by
the
DAC.
Due
to the extremely high input impedance of
the associated operational amplifiers, the charge
on
the
capacitor(s) remains nearly constant between updates.
FRONT-PANEL CONTROLS
The Front
Panel
is the operator's interface for control-
ling the user-selectable oscilloscope functions. Along with
the crt,
it
provides visual feedback to the user about the
present operating state of the instrument.
Most of the Front-Panel controls (diagram
3)
are
·cold"
controls;
i.e.,
they
are
not connected directly into the sig-
nal
path. Therefore, associated circuits
are
not influenced
by
the physical parameters
(such
as
capacitance, resis-
tance,
and
inductance) of the controls.
In
addition,
translating the analog output levels of most of the poten-
tiometers to digital equivalents allows the processor to
handle the data
in
ways that result
in
a variety of
enhanced control features.
To maintain the front-panel operating setup between
uses of the instrument, the digitized values of the poten-
tiometers
and
front-panel switch settings
are
stored
in
bat-
tery backed up
RAM
so that
when
the instrument power
is
turned off, these control settings are not lost. Then, when
Theory
of
Operation-2445A/2455A Service
power
is
next applied, the instrument will power up to the
same configuration
as
when the power was last removed
(assuming the settings of the non-digitized pots
and
switches remain the
same).
The Front-Panel Controls also allow the user to initiate
and
direct the diagnostic routines
(and
when enabled, the
calibration routines) programmed into the read-only
memory (ROM).
These
routines
are
explained
in
the
Maintenance section of this manual.
Front-Panel Switches
The Front
Panel
Switches
are
arranged
in
a ten-row-
by-five-column matrix, with each switch assigned a unique
location within the matrix (see Figure 3-3). A closed switch
connects a row
and
a column together through
an
isolat-
ing
diode. To detect a switch closure, the switch matrix
is
scanned once every
32
ms (every tenth Microprocessor
interrupt cycle).
When
scanning, the Microprocessor
sequentially sets
each
individual row line
LO
. A closed
switch enables the
LO
to be passed through the associ-
ated diode to a column line.
When
the processor checks
each
of the five column lines associated with the selected
row, the
LO
column
is
detected. The intersection of the
selected row
and
the detected column uniquely identifies
the switch that
is
closed. Further information about switch
scanning is found
in
the "Front-Panel Scanning" descrip-
tion located
in
the • Analog Control" discussion.
As
each
switch
is
read, the processor compares .the
present state of the switch to its last-known state (stored
in
memory)
and,
if the same, advances to check the next
switch.
When
a switch
is
detected
as
having changed, the
processor immediately reconfigures the setup conditions to
reflect the mode change and stores the
new
state of the
switch
in
memory.
The
detected status of the switch
on
each
of the following scan cycles
is
then compared against
the new stored data to determine if the switch changes
again.
The
32-ms delay between the time a switch
is
detected
as
having changed
and
the next time it
is
read
effectively eliminates the effects of switching noise (switch
bounce) that
may
occur after the switch is actuated.
Front-Panel Pots
The
thirteen Front-Panel Potentiometers, READOUT
INTENSITY,
and
INTENSITY
are
"cold" controls that con-
trol the linear functions of the instrument. (SCALE ILLUM
and
FOCUS
are not considered part of the Front-Panel
Control circuitry for the purposes of this description.)
All
are
digitized
and
control their functions indirectly. Data
Selectors U2401, U2501, and
U2601
in
the Analog Control
circuitry (diagram
2)
route the wiper arm voltage of the pot
being
read
to comparator U2510 where it
is
compared
3-11
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