Theory of Operation—2465B/2467B Service
Delta time cursor displays are similar in that the CUR-
SOR 0 and DLY REF 1 signals are used to position the
cursors. In this case, however, analog switch U2805
selects the CURSOR 0 and DLY REF 1 signals alternately
to position the cursors horizontally, and the Horizontal
DAC output is routed via analog switch U2800 and buffer
U2820C to vertically position the dots within each cursor
segment.
Refresh Prioritizer
The Refresh Prioritizer circuitry consists of U2850A and
B, U2950B, U2990A, and U2985. It keeps track of how
well the Readout circuitry is doing in displaying all the
required readout information and maintains the overall
refresh rate. Since the readout display must remain
flicker-free and at a constant intensity over the entire
sweep rate range, various modes of displaying readout
information are provided. The Refresh Prioritizer keeps
track of the display status and enables the various
readout-display modes as required to produce minimal
interference with the displayed waveform trace(s).
The next most desirable time for dots to be displayed is
during "triggerable" time: that time between sweeps when
the oscilloscope is waiting for a sweep trigger event to
occur. This is designated priority two and may cause slight
interference on the leading edge of the displayed trace if a
dot is being displayed when the actual trigger occurs.
Finally, the least desirable dot display time is during a
waveform trace display. This display time is designated
either priority three or priority four. (Priority four indicates
a higher demand of display time.) In priorities three and
four, dot displays occur during the main portion of the
waveform display. However, the waveform blanking asso-
ciated with these displays is relatively random in nature
and is usually not noticeable.
To start a readout display, the ROSFRAME (readout
subframe) request from the Timing Logic (diagram 1)
clocks the Q output of flip-flop U2850A HI. ROSFRAME
is a periodic clocking signal used to hold the overall
refresh rate constant and occurs at regular intervals,
regardless of the state of the display.
Ideally, readout information should be displayed only
when the oscilloscope is not trying to display waveform
traces. These times occur before a trace commences,
after a trace is completed, or between consecutive traces.
Displaying in this mode corresponds to "priority one" in
Figure 3-7 and causes no interference with the displayed
waveforms. If the Readout circuitry is able to display all
the required readout dots during the holdoff time between
sweeps, the prioritizer U2985 will turn off the Dot Start
Governor until the next subframe of readout information is
to be displayed. When the sweep times are either too fast
to finish a readout display during holdoff (at 5 ns per
divi-
sion no identifiable holdoff time exists) or too slow to allow
flicker-free readout, readout display modes other than
priority one are initiated.
As the Dot Cycle Generator runs, it resets half of
U2830 in the Dot Timer at somewhat irregular intervals
with the STARTDOT signal (via inverter U2890A). The Dot
Timer then starts a timing sequence, and the rising edge
of the REFRESH signal from U2830 pin 4 clocks the
latched ROSFRAME request from U2850A pin 5 to the Q
output (pin 9) of flip-flop U2850B. This HI, applied to the
S1 input (pin 10) of prioritizer U2985, sets it up to incre-
ment with the next REFRESH clock applied to its clock
input (pin 11). The LOO" output of U2850B (pin 8) applied
to the reset input of U2850A resets the latched ROS-
FRAME request. See Figure 3-8 for an illustration of the
timing sequence involved.
DISPLAY '
PRIORITY |
SWEEP
GATE
3 OR A
TRIGGERABLE
TIME
3831-05
Table 3-7
Operation of Prioritizer Shift Register
Figure 3-7. Readout display priorities.
Select Inputs
SO
H
H
L
L
S1
H
L
H
L
Mode
Parallel Load
L - Q
A
(decrease priority)
H
->
Q
D
(increase priority)
Hold Data
3a-32
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