Character Generators; Decimal Point Logic And Character Position Counter - Tektronix 7623A Instruction Manual

Theory of Operation--7623A/R 7623A Service
CHARACTER GENERATORS
The Character Generator stage consists of five similar integrated circuits (U2270 through U2278),
which generate the X (horizontal) and Y (vertical) outputs at pin 16 and 1, respectively, to produce the
character display on the crt. Each integrated circuit can produce 10 individual characters. U2270 (desig-
nated 'Numerals'} can produce the numerals 0 through 9 shown in row 1 of the Character Selection Matrix
(Fig. 3-25). U2272 can produce the symbols shown in row 2 of the Character Selection Matrix and U2274
produces the prefixes and someletters, used as prefixes, shown in row 4. U2276 and U2278 produce the
remaining letters shown in rows 5 and 6 of the Character Selection Matrix.
All of the Character Generator stages receive the Column Digital Data fram the Column Decoder
U2244in parallel. However, only one of the Character Generators receives row data at a particular time and
only the stage receiving this row data is activated. For example, if column 2 is encoded, the five Character
Generators are enabled so that either a 1,<, x, V, or an N can be produced. If row 4 has been encoded at the
same time, only the Prefix Character Generator U2274 will produce an outputto result in a 'u' being dis-
played. The activated Character Generator provides current outputs for the Format Generator to produce
the selected character on the ert. Ina similar manner, any of the characters shown in the Character Selec-
tion Matrix can be displayed by correct addressing of the row and column,
DECIMAL POINT LOGIC AND CHARACTER POSITION COUNTER
Decimal! Point Logic and Character Position Counter U2260 performs two functions. The first func-
tion is to add a staircase current to the X (horizontal) signal to space the characters horizontally on the crt.
After each character is generated, the negative-going edge of the Ready signal at pin 5 advances the Charac-
ter Position Counter. This produces a current step output at pin 3 which, when added to the X signal,
causes the next character to be displayed one character space to the right. This stage can also be advanced
when a Space instruction is encoded so a spaceis left between the displayed characters on the crt. Row 10
information from the Row Decoder is connected to pin 4 of U2260. When row 10 and column 0 are en-
coded, the output of this stage advances one step to move the next character another space to the right.
However, under this condition, no display is produced on the crt during this time-slot, since the Character
Generators are not activated.
Time-slot pulses 1, 2, and 3 are also connected to pin 4 of U2260 through VR2262, VR2263, and
VR2264 respectively and R2262-R2265. This configuration adds a space to the displayed word during
time-slots 1, 2, and 3 even if information is not encoded for display during these time-slots. With this fea-
ture, the information displayed during time-slot 4 (scaling data) always starts in the fourth character position
whether data has been displayed in the previous time-slots or not. Therefore, the resultant ert display does
not shift position as normal-invert or cal-uncal information is encoded. The Word Trigger pulse connected
to pin 8 resets the Character Position Counter to the first character position at the end of each word.
The Decimal Point Logic portion of this stage allows decimal points to be added to the crt display.
With the Standard Readout Format, row 7, encoded coincident with columns 3 through 7, addresses a
decimal at one of the five locations identified in row 7 of the Character Selection Matrix (Fig. 3-25). This
instruction refers to the decimal point location in relation to the total number of characters possible in one
3-64