Tektronix 2467B

Theory of Operation—2465B/2467B Service
Table 3-5
Blanking and Intensity Control Selection
Control Inputs
TXY
X
a
X
X
L
H
HSA
H
H
L
L
L
HSB
H
L
H
L
L
Intensity
Affected
By
BRIGHT (RO level)
BRIGHT, Z EXT
BRIGHT, SGBZ.Z EXT
BRIGHT, SGBZ, Z EXT
BRIGHT, SGBZ, Z EXT
Blanking
Affected
By
BLANK
BLANK, SGAZ, SGBZ
BLANK, SGAZ
BLANK
BLANK, SGAZ
Typical Display
Readout
Delayed Sweep
Main Sweep
X-Y
X-Y
a
X = State doesn't matter.
Dynamic Centering adjustment pots R3401 and R3407
set the gain and polarity of the signals at their related out-
puts by varying the current in the emitter circuit of one of
two emitter-coupled pairs of transistors. Adjusting the
bias level, at either pin 4, above ^ -10.6 volts (deter-
mined by R3410 and R3411 at the complementary inputs,
pins 1) will generate an inverted signal, while adjusting the
bias levels below -10.6 volts will cause a noninverted sig-
nal. Amplitude of the resulting signal is dependent on how
far from the -10.6-volt reference the bias is set. The out-
put signal is added or subtracted from the position voltage
applied to the Vertical and Horizontal Output Amplifiers.
Both pots are adjusted so that position shifts due to
display intensity variations are minimized.
READOUT
The Readout circuitry (diagram 7) is responsible for
displaying the alphanumeric readout characters in the crt.
An eight-bit character code specifying each character (or
cursor segment) to be displayed is written from the
Microprocessor to a corresponding location in the Charac-
ter RAM U2920 (a 8K-x-8-bit, random access memory
integrated circuit). Each of the following 128 locations in
the RAM, address locations 0 through 63 for the first and
fourth readout lines and 128 through 191 for the second
and third readout lines, corresponds to one of the 128
possible character locations in the crt readout display (see
Figure 3-6). The next 128 RAM locations, address loca-
tions 64 through 127 for the first and fourth readout lines
and 192 through 255 for the second and third readout
lines, are used to store cursor segment information for the
display of the AV and At measurement cursors. The eight-
bit character code written to each location in RAM points
to a block of addresses in Character ROM U2930. This
block in the ROM contains the dot-position information for
the specific character to be displayed at the associated crt
position.
Each character is made up of zero (for a space charac-
ter) or more dots displayed in an eight-wide by sixteen-
high dot matrix. Specific blocks of ROM addresses contain
all the X-Y offset coordinates for the dots in a particular
character in the readout. The coordinates are referenced
to the lower-left corner of the character dot matrix. Each
individual data byte in the block of ROM addresses con-
tains both the X and the Y coordinates for one dot of the
associated character.
To display a character, a combination of the character
position on the crt (the RAM address) and the byte of X-Y
position data from Character ROM U2930 (relative to that
character position) is applied to Horizontal and Vertical
DAC (digital-to-analog converters) circuits, U2910 and
U2905 respectively. In these circuits, the X-Y position data
is converted to analog deflection signals used to position
each dot in the crt readout display. Each of the position
bytes are read from the block of ROM defining the charac-
ter under control of the readout timing and sequencing cir-
cuitry. The resulting dots, when displayed in sequence,
form the character at the proper location on the crt.
Readout I/O
The Readout I/O circuitry, composed of U2860, U2865,
U2960, and associated components, provides the interface
between the Microprocessor and the Readout board. Two
types of data, Readout mode data and character data, are
written to the Readout board serially via data bus line
BDO.
3a-27