Tektronix 2445, Instruction Manual

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Theory of Operation—2445 Service
idea!!'/, 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. Dis­
playing in this mode corresponds to "p rio rity one" in
Figure 3-8 and causes no interference w ith the displayed
waveforms. If the Readout circuitry is able to display all
the required readout dots during the h oldo ff time between
sweeps, the prioritizer U2985 w ill turn o ff the Dot Start
Governor until the next subframe o f 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
division no identifiable holdoff time exists) or too slow to
allow flicker-free readout, readout display modes other
than p rio rity one are initiated.
The next most desirable time fo r dots to be displayed
is during "triggerable" time; that time between sweeps
when the oscilloscope is waiting fo r a sweep trigger event
to occur. This is designated priority tw o and may cause
slight interference on the leading edge of the displayed
trace if a dot is being displayed when the actual trigger
occurs.
Ac the Dot Cycle Generator runs, it resets U2830B in
the Dot Timer at somewhat irregular intervals w ith the
STARTDOT signal (via inverter U2890A). The Dot Timer
then starts a tim ing sequence, and the rising edge o f the
REFRESH signal from U2830A pin 4 clocks the latched
ROSFRAME request from U2850A pin 5 to the Q output
(pin 9) o f flip-flop U2850B. This HI, applied to the SI
input (pin 10) o f prioritizer U2985, sets it up to increment
w ith the next REFRESH clock applied to its clock input
(pin 11). The LO Q output o f U2850B (pin 8) applied to
the reset input of U2850A resets the latched ROSFRAME
request. See Figure 3-9 fo r an illustration of the timing
sequence involved.
The next REFRESH clock increments the display
p rio rity to one by clocking a HI to the QD output (pin 12)
o f prioritizer shift register U2985. (Table 3-7 illustrates
the operation o f U2985. The same clock latches the now
LO ROSFRAME request at U2850B pin 12 to the Q
output (pin 9), where it is applied to the SI input (pin 10)
of prioritizer U2985. The LO on the SI input o f the
prioritizer w ill remain until another ROSFRAME request
from the Timing Logic occurs, and the encoded priority
at the output pins o f U2985 w ill remain as it is presently
set.
Finally, the least desirable dot display time is during a
waveform trace display. This display time is designated
either p rio rity three or p riority 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
associated w ith 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.
As each o f the consecutive dots o f the readout frame are
displayed, the Dot and Character Counters increment until
all dots of the subframe have been displayed (eight char­
acters). As the Character Counter increments to address
the next character of the display (first character o f the
next frame), the fourth b it o f counter U2940B goes HI
and sets the SO input (pin 9) o f prioritizer U2985 HI via
exclusive-OR-gate U2990A. The Dot Timer then clocks the
prioritizer w ith a REFRESH clock on pin 11 o f U2985, and
the p riority is decremented back to zero (indicating that
the subframe is completed). The next ROSFRAME request
starts the process over again to display the next subframe o f
readout display. The sequence just described is the priority
one display mode and is used when holdoff time between
sweeps allows all dots of the subframe to be displayed
before the next ROSFRAME request occurs.
DISPLAY _
1
PRIORITY
1 1 2
|
3 OR 4
'
SWEEP
GATE 1
1
VARIABLE , VARIABLE
HOLDOFF 'TRIGGERABLE SWEEP HOLDOFF
H — TIME TIME —H 1-*— TIME—H 1-*— TIME —
3831-05
Figure 3-8. Readout display priorities.
Table 3-7
Operation o f Prioritizer Shift Register
Select Inputs
i
! Mode
SO
SI
H
H Parallel Load
i
H
L L
-+
Qa (decrease priority)
L H ■ H -> Q
d
(increase priority)
L L Hold Data
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