Specifications

CONSTRUCTION
are extended by buffer IC 74LS244 to the
RAM high address lines if CS1 is low. If
CS1 goes high, the buffer is tri-stated at
its output, allowing the latch (74LS373)
data to reach the address lines A6-A13 of
the video RAM chip. In this way, the video
RAM is addressable by both the CRTC
6845 circuitry as well as the 8085, when
a new key is typed.
The data bus lines are likewise con-
nected by a 74LS245 bidirectional buffer.
The pixel data for the typed-in character
must be written into the video RAM, after
reading the table of dots stored in the char-
acter code EPROM and writing the same
into the video RAM. The table of data (dots)
for each language occupies a 2k memory
area, and hence four languages can be se-
lected by address lines A11 and A12 of the
character generator EPROM. For each
character currently being entered, the set
of pixels are read byte-by-byte, stored as
nibbles temporarily in buffer memory by
the program, and then output into the
video RAM nibble-by-nibble.
If desired, four languages may be
typed on the same row using function keys
F1-F4 under software control. The se-
lected language is indicated by two of the
LEDs (LED1 and LED2) at the output of
7475. The same outputs are also wired to
the A11 and A12 address lines of the char-
acter EPROM. You may press F1 and
start typing in English, then press F2 and
start typing in Hindi, and so on.
The 74165 video shift register (com-
monly used with all CRT display-based
circuits) is used to shift the dot signals
loaded in parallel (8 bits) from the
memory into a serial form to get the ac-
tual video line signal.
In Fig. 1(b), the dot clock is generated
using a 74132 gate (N15) in conjunction
with a capacitor-resistor combination of
R23 (and preset VR1) and C2 to function
as an oscillator. The frequency is about
10 MHz, which is divided by 8 in 74190
divider/counter. This gives the character-
slot clock. The load command to the shift
register is obtained from pin 11/13
(shorted) of the 74190 IC which goes to
pin 2 of IC 74LS165. The 2764 EPROM
is filled with control program at its high-
est address range of 2k (i.e. 1800-1FFF),
because its pins A11 and A12 are pulled
high.
Basic principles
The basic principles of Indian language
display software are summarised below
while a flowchart for storage of pixel data
in the video RAM is given in Fig. 4.
1. Multiple language fonts are stored
in an 8k or bigger memory space, if nec-
essary. EPROM occupies 2k locations for
each font of a language. Thus, four lan-
guage fonts can be stored using 8k
Fig. 6: Actual-size, solder-side track layout
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