Tektronix 2445, Instruction Manual

Page: 73 / 351

Theory o f Operation—2445 Service
B HI. The HI outputs of U2900B and C applied to the
select input pins of analog switch U2805 cause the Hori­
zontal DAC output signal applied to U2805 pin 11 to be
routed to the Horizontal A m plifier (diagram 6) via buffer
U2820B. The same HI logic levels cause NOR-gates U2810C
and D to produce a LO at their outputs. This causes analog
switch U2800 to route the Vertical DAC output signal
applied to pin 12 to the Vertical O utput A m plifier (also
diagram 6) via buffer U2820A.
For cursor displays, cursor select bit CA6 goes HI.
This HI is routed through analog switch U2800 and latched
into U2905 when GETDOT next goes HI. This produces a
HI at U2905 pin 5, enabling the Mode Select Logic to
decode output bits DD3, DD4, and DD5 (from U2905) to
determine which of the four possible cursor modes is
selected (see Table 3-6). Once one o f the cursor modes is
entered, analog switch U2800 routes a fixed HI from pin 5,
pin 2, or pin 4 to U2905 to keep the Mode Select Logic
enabled. Character display mode is reentered only when
return-to-character-mode data is decoded (DD4 and DD5
both LO). When that occurs, U2800 routes the CA6 bit to
U2905 and, if the b it is LO, the cursor display mode is
halted.
CURSOR DEVELOPMENT. Cursors are displayed in
short sections, alternating between both vertical positions
(for the delta voltage cursors) or both horizontal positions
(for the delta time cursors). When displaying delta voltage
cursors, the DLY REF 0 level is routed to the Vertical
A m plifier by analog switch U2800. This level determines
the vertical position of one o f the voltage cursors.
Horizontal-positioning voltages fo r one segment o f the
cursor are routed from Horizontal DAC through analog
switch U2805 and buffer U2820B to horizontally position
each o f the dots making up the cursor segment. DLY
REF 1 is then used to vertically position the second cursor,
and the Horizontal DAC positions each o f the dots fo r that
cursor segment. The cycle is repeated until all segments of
both cursors are displayed.
Delta time cursor displays are similar in that the D LY
REF 0 and D LY REF 1 signals are used to position the
cursors. In this case, however, analog switch U2805 selects
the D LY REFO and D LY REF 1 signals alternately to
position the cursors horizontally, and the Horizontal DAC
output is routed via analog switch U2800 and buffer
U2820A to vertically position the dots w ithin each cursor
segment.
Refresh Prioritizer
The Refresh Prioritizer circuitry consists of U2850A
and B, U2950A, U2990A, and U2985. It keeps track o f
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 o f displaying readout information are
provided. The Refresh Prioritizer keeps track o f the display
status and enables the various readout-display modes as
required to produce minimal interference w ith the dis­
played waveform trace(s).
Table 3-6
Readout Display Mode Selection
Control Bits
Mode
Selected
Horizontal
Signal
Vertical
Signal
CA6
(Cursor Select)
DD5 DD4 DD3
L Xa X
X
Character Display Horiz DAC Vert DAC
H L H L Vert Cursor 1 Horiz DAC
D LY REF 1
H l_
H
H
Horiz Cursor 1 D LY REF 1 Horiz DAC
H H
L
L Vert Cursor 0 Horiz DAC DLY REF 0
H H
L
H
Horiz Cursor 0 D LY REF 0 Horiz DAC
H
L L
X Return to Character Display Mode
aX = State doesn't matter.
3-31