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Theory of Operation—2246 1Y and 2246 Mod A Service
READOUT DOT. A Readout dot is refreshed in this
way:
1. RO REQ is unasserted (this causes RO HORIZ
and RO VERT to control the crt horizontal and
vertical) briefly to show the dot.
2. RO BLANK is unasserted then asserted (this
unblanks then blanks the crt beam).
3. DOT CLK is asserted and unasserted (this in
crements the dot counter lines DOT4-DOTO).
FAST REFRESH.
Fast refresh occurs when the
Processor asserts FAST (whenever the refresh rate
is too low) or when A GATE is unasserted (the
sweep is in holdoff). In this mode, RO REQ is
asserted at the start of a character, and unasserted
at the end. Whenever RO REQ is asserted, the
Readout system controls the crt beam intensity and
the vertical and horizontal position of the beam.
Dots are refreshed every 1.6
jj
.
s
during fast refresh.
SLOW REFRESH.
Slow refresh occurs when the
Processor unasserts FAST (when the refresh rate is
not falling behind in refreshing the readout) and
A GATE is asserted. In this mode, RO REQ is
asserted before each dot in a character, and
unasserted after each dot.
Data flow for the dots in a character is roughly this:
1. FLD2-FLD0 give the current field being
refreshed.
2. CH7-CH0 give the position of the character
within that field. CH7-CH5 gives the row
within the Readout (row 0 at the bottom, and
7 at the to p ), and CH4-CH0 gives the column
(column 0 at the left, column I f hex at the
rig h t).
3. Given the field and character position, the
RAM (U2406) outputs the character code
(the code for the character that is to be dis
played at that position) on R7-R0.
4. DOT4-DOTO gives the dot that is being re
freshed within the character.
5. Given the character code and dot number,
ROM U2408 outputs the position of the dot
within the character. There are up to 31 dots
in a character, in an array of 128 possible dot
positions
(16 vertical by 8 horizontal).
DD6-DD3 gives the vertical position of the
dot, and DD2-DD0 gives the horizontal
position.
6. Given the row and column containing the
character, and the vertical and horizontal
position of the dot, U2412 generates the
vertical analog current for the dot, and U2413
the horizontal analog current.
7. U2414 sets up the mixing for the vertical out
put signal (see Readout Position Mixer).
8. U2415 sets up the mixing for the horizontal
output signal.
Interrupt Request Latch
When the Measurement Processor wants to write
new display data to the Readout Processor or
Character Codes RAM (U2406), it latches the new
data into the Readout Write Latch (U2401) from the
D0-D7 bus lines by issuing the RO BUF WR (readout
buffer write) strobe to the Interrupt Request Latch
(U2417). The output of U2417D (pin 11) is latched
low and the Readout Processor is interrupted from
its display processes ( RO INTR goes low ). The
Readout Processor enables the Readout Write Latch
and reads in the new data. When the character is
received, the Readout Processor transfers the byte
to the Character Code RAM and resets the Interrupt
Request Latch (U2417D) to let RO INTR go high
again.
Communication Latches
Communication from the Measurement Processor
and the Readout Processor is done via the Readout
Write Latch (U2401). The Readout Read Latch
(U2402) is used only for diagnostics communication.
Character Position Address Counter
The starting address of a readout field to be
displayed is loaded into the Character Position
Address Counter (U2404 and U2405). The counter
then sequences through the addresses of the
characters loaded in Character Code RAM U2406.
The vertical and horizontal position of the character
being displayed is also defined by the output of the
counter and is supplied to the Vertical and Horizontal
DACS on the CH0-CH7 bus lines.
Character Codes RAM
The ASCII codes needed to display a field of readout
are loaded into the Character Codes RAM (U2406)
from the Measurement Processor via the Readout
Writer Buffer (U2401) on the R0-R7 bus lines. When
the field is displayed, the RAM is addressed in
sequence by the Character Position Address
Counter to output those codes for a display refresh.
The field of codes accessed by the FLD0-FLD2
3-47
Содержание 2246 1Y
Страница 13: ...2246A Service 7062 01 X The 2246 1Y or 2246 Mod A Portable Oscilloscope ...
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Страница 354: ...A L B i C 1 D 1 E i F i G 1 2 3 4 5 6 7 8 2246 1Y AND 2246 MOD A _________A fc A A REV JUNE 1991 7062 19 ...
Страница 355: ...I BOARD LOCATION 8K 8J 8H 7H 8H 8H 7J 8D 7H 8H 8J 8J 7K 8K 10D 6L 6K 6L 6K 6G 5H I ...
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