Horizontal Character Position Counter; Decimal Position Logic - Tektronix 7140 Instruction Manual

02204 is an octal D-type flip-flop used as a latch to stabilize
to-BCD Converter's outputs to address all the EPROM lo
and synchronize the Character Generator EPROM output . It
cations necessary to form the readout character .
is reset by the same signal that starts the oscillator and is
clocked at pin 11 by the oscillator output from 02153.
02204 will be considered to be part of the Character Gen-
Suppose the next address produces a Character Generator
erator in the discussion that follows.
output of "01111010" or octal "172". The octal digit "1 " indi-
cates binary bit 7 is high which will turn on 02132 and the Z
Readout output to the instrument . The instrument will now
The Character Generator 02203 is a 4k X 8-bit EPROM
provide a trace from the previous vertical and horizontal co-
which contains the binary words used by the output stages
ordinates to the new ones, vertical "7" and horizontal "2".
in creating the signals necessary to form readout charac-
Thus the character is formed by a series of binary words
ters . There are twelve address inputs, with the lower four
causing the instrument CRT beam to move or draw between
coming from the Lower Order Address Generator, the cen-
points .
ter four from the Column Decimal-to-BCD Converter, and
the upper four from the Row Decimal-to-BCD Converter. As
previously mentioned, each character is developed on an 8-
HORIZONTAL CHARACTER POSITION COUNTER
point by 8-point grid (see Fig. 3-22 for a typical character) .
The Horizontal Character Position Counter U2202A is a 4-
The Character Generator's 8-bit output provides the in-
formation necessary to move the instrument beam around
bit binary counter. Its output is converted to current by
within the grid, to turn the beam on and off, and to indicate
82266 through 82269 and added to the X (horizontal) signal
when a character is complete .
for spacing readout characters horizontally on the CRT. The
counter is reset to "0000" with a Word Trigger pulse from
U2127B and is advanced with inputs from two possible
The row and column data cause a 4-bit binary code to be
sources. The first is a HI End-of-Character signal from pin
generated at the outputs of the Row and Column Decimal-
19 of 02204 . The counter can also be advanced when a
to-BCD Converters when a readout character is to be dis-
SPace instruction is encoded by the plug-in unit to cause a
played . The Lower Order Address Generator is enabled and
space to be left between two characters on the CRT. A
also provides a 4-bit binary code . These twelve bits are
SPace instruction occurs when row 10 from the Row De-
combined to form the EPROM address containing the 8-bit
coder goes LO and is inverted by U2157D to advance the
Horizontal Character Position Counter. No character could
binary word which will locate the instrument beam at the
be displayed in this situation as no character information is
character's starting grid location .
stored at the Character Generator addresses formed using
row 10 .
The 8-bit word can be broken down into four parts. The
lower three bits are the horizontal grid coordinate, bits 4
through 6 are the vertical coordinate, bit 7 turns the Z Read=
out on and off, and bit 8 indicates whether or not the char-
acter is complete .
The character grid (Fig . 3-22) can be thought of as having
vertical and horizontal coordinates numbered 0 through 7,
with location "0,0" in the lower left corner . The 8-bit binary
word from the Character Generator is converted to octal to
easier recognize the vertical and horizontal coordinates . A
binary "00001010" becomes octal "012". This number
would cause the instrument CRT beam to point at grid co-
ordinates vertical "1 " and horizontal "2". The fact that the
first octal digit is "0" indicates two things . First it shows that
bit 7 of the binary word is LO which turns off 02132 and the
Z Readout signal to the instrument . It also shows that bit 8
is LO so the character is not complete . When bit 7 is HI, it
advances the Horizontal Character Position Counter for the
next character within the readout word .
The 4-bit outputs from the Row and Column Decimal-to-
BCD Converters remain the same until the character is com-
plete. However, the Lower Order Address Generator keeps
counting and combines with the Row and Column Decimal-
REV NOV 1985
DECIMAL POSITION LOGIC
Theory of Operation-7104
Time slots 1, 2, and 3 are also connected to the Space
instruction through VR2185, VR2186, and VR2187 respec-
tively . This configuration adds a space to the displayed
word during time slots 1, 2, and 3, even if information is not
encoded during these time slots. With this feature, the in-
formation which is displayed during time-slot 4 (1-2-5 data)
always starts in the fourth character position whether data
has been displayed in the previous time-slots or not. There-
fore, the resultant CRT display does not shift positions as
normal/invert or cal/uncal information is encoded by the
plug-in.
The Decimal Position Logic stage allows decimal points to
be displayed at five possible locations within a readout word
(see Fig. 3-18). The decimal location encoded by a plug-in
during time-slot one is achieved by adding positioning cur-
rent to the X (horizontal) readout signal . Circuitry for this
stage includes five 2-input NOR gates in 02157 and 02251
with precision resistors connected to their outputs. One in-
put of each NOR gate is connected to row 7 on the Row
Decoder and the other to one of columns 3 through 7 on the
Column Decoder. When a decimal is to be displayed, row 7
goes LO and disables the Horizontal Character Position
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