Tektronix 2445A - Page 45

With all thirteen Front-Panel Potentiometers, READOUT
INTENSITY,
and INTENSITY controls, the processor reads
the magnitude
and direction of pot rotation and produces
variable-resolution control voltages. If a pot's direction of
rotation changes, the magnitude of the change from the
last-set position remains
small, or if it was not the last pot
moved, a fine-resolution control voltage results.
In the
fine-resolution
range, a given rotation displacement will
cause a small control voltage change.
The same displace-
ment farther
away from the last-set reference will cause a
proportionally larger control voltage change, producing a
coarse-resolution effect. If the changing pot is the last one
moved and the direction of rotation remains the same, the
algorithm continues from where it left off during the
preceding scan; producing control voltage changes with
the
same increment as it was last using.
The delta reference controls þÿ(” REF OR DL Y POS and
þÿ”) are continuous-rotation potentiometers. They each con-
sist of two pots ganged together with their wiper arms
electrically oriented at 180[degree] apart. As the wiper of
one pot
is leaving its resistive element, the wiper of the
other pot comes onto its element. The Microprocessor has
the ability to watch the output voltage from
each wiper
and
when it detects that the controlling wiper is nearing
the
end of its range, it will switch control over to the other
wiper.
The routi ne the processor uses to watch these pots
sets the associated control voltage
on the basis of relative
voltage changes
þÿ(”V) that occur. Switching between the
pots to change control to the opposite wiper arm
is based
on specific voltage levels being sensed.
Sensing specific voltage levels is also used
when read-
ing the VOLTS/DIV
VAR , SEC/DIV VAR , and HOLDOFF
controls. These pots have both a mechanical detent and a
processor-generated electrical detent. As one of these
controls
is moved out of the mechanical detent position,
the processor watches the analog voltage changes that
occur; but the associated control voltage will not change
until a specific voltage
level (the electrical detent level) is
reached.
Once the electrical detent value is exceeded, the
processor begins to vary the associated control voltage
in
response to further pot rotation. When returning to the
mechanical position, the electrical detent
level is reached
first,
and the variable voltage action is stopped before the
mechanical detent is entered.
Front-Panel Status LEDs
Light-emitting diodes (LEDs) are used to provide visual
feedback to the operator about the oscilloscope status
and operating mode by backlighting front-panel nomencla-
ture. A 48-bit status word, defining the diodes to
be
illuminated, is generated by the processor and then serially
clocked into the six LED-Status Registers (U3001, U3002,
U3003, U3004, U3005,
and U3006). The registers hold the
selected diodes
on until the next update. Whenever the
processor detects that a front-panel control has changed
Theory of Operation-2445A/2455A Service
(and a new different status display is required), a new
status word is generated
and applied to pin 1 of U3002.
As
each of the bits is clocked into the QA position of
U3002, the preceding bit
is shifted to the next register
position. After
48 bits have been clocked into (and 40 bits
through) U3002,
all six LED-Status registers are full and
contain the LED illumination pattern to be displayed to the
user. A
LO at any Q output of the registers illuminates the
corresponding front-panel
LED.
The TRIG'D LED is not driven by the LED-Status
Register. It
is driven by the Analog Control circuitry and
illuminated whenever a triggered sweep is in progress.
ATTENUATORS AND PREAMP$
The Attenuators and Preamps circuitry (diagram 4)
allows the operator to select the vertical deflection factors.
The Microprocessor reads the Channel VOL TS/DIV
switches and VOL TS/DIV
VAR controls and then digitally
switches the attenuator
and sets the preamplifier gains
accordingly.
CHANNEL 1 AND CHANNEL 2 ATTENUATORS
The Channel 1 and Channel 2 Attenuators are identical
in operation, with corresponding circuitry in each channel
performing the
same function. Therefore, only the Chan-
nel 1 circuitry is described.
Input signals from the Channel 1 input connector are
routed through
an attenuator network by four pairs of
magnetic-latch
relay contacts. The position of the relays is
set by Microprocessor data placed into Auxiliary Control
Register U140.
Relay buffer U110 provides the necessary
drive current to the relays.
Four input coupling modes
(1 þÿM© AC, GND, 1 þÿM© DC ,
and 50 þÿ© DC) and three attenuation factors (1 X, /10, and
/ 1 00) may be selected by closing different combinations
or relay contacts.
The three attenuation factors, along with
the variable gain factors of the Vertical Preamplifier, are
used together to obtain the crt deflection factors.
The
relays are magnetically latched and once set, remain in
position until new attenuator-relay-setting data and
strobes are generated. (See the "Auxiliary Control Regis-
ter" description for a discussion of the relay-latching pro-
cedure.)
The 50 þÿ©termination resistor has a thermal sensor
associated with it that produces a
de voltage (CH 1 OVL)
proportional to the input power. Should the input power
exceed the normal safe-operating
level for the 50 þÿ© DC
input, the termination resistor temperature will exceed the
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