Specifications

CONSTRUCTION
space-moving characters in the standard
typewriters of the Indian languages. Par-
ticularly in Hindi, there are multiple
hooks, such as in ‘hoom’. In the type-
writer, the hook characters do not advance
(move space) after they are typed. The
program checks the code, and if it is a
hook code, it does not write immediately
the dots corresponding to that hook into
the video memory, but waits for the suc-
ceeding keyboard stroke(s) for a non-hook
character to follow before shifting the cur-
sor. Thereupon, the program combines the
dot pattern of the hook characters with
that of the following main character, and
then places the net dot pattern into the
video memory.
3. Since memory contains only 8 bits
per location, one-and-a-half memory lo-
cations are assigned for each character
shown on screen, thus providing 12 dots
per horizontal row in TV format. (This
is more like the computer format.) In
this way, even characters start at a
memory byte and extend up to the next
byte (its higher order nibble). Odd num-
bered characters start at the right nibble
(lower order nibble) of a byte and extend
to the next complete byte (refer Fig. 4).
With 64 bytes on each horizontal row,
up to 43 characters can be shown per
row. The hardware caters to a 64 x 16
character display comprising 512 x 256
pixels.
The control software in the 8085 board
for the entire unit does the job of reading
the keyboard, selecting the language, writ-
ing the key code into video RAM, and
doing minor editing as well.
A double-sided PTH PCB is required
for assembling the circuit. The actual-size
component-side and solder-side track lay-
outs for the PCB are shown in Figs 5 and
6, respectively. Fig. 7 shows the compo-
nent layout.
Testing the board
The board may be tested by a sequence of
small programs written into the control
EEROM. Verifications are done as per the
guidelines given below:
1. The first thing to test is whether
the data bus and address lines are func-
tional and the output port 80H is also
functional. Here is a simple program for
the same:
MVI A,55H
OUT 80H
HLT
fixed on the board.
Conclusion
The two designs, the first one based on a
simple PC and the second one based on
dedicated hardware/software using com-
puter keyboard, for display of Indian lan-
guage text on a monitor and TV screen
respectively are illustrative of the tech-
niques of video display and software pro-
gramming for Indian languages. The
former is useful in an industrial or office
environment, while the latter can be used
in public display systems.
The main intention of this article
is not merely to show the design of either
the dedicated display unit or the program
on PC for typing multilingual text, but to
demonstrate the coding scheme for Indian
languages with just 128 8-bit codes in-
stead of the currently talked about 16-bit
codes. Further, the coding scheme sug-
gested here does not disturb existing typ-
ists of the 11 Indian languages, for which
typewriters already exist.
The memory saving is a vital factor
when one uses such codes for the Indian
languages like English. Presently, all such
Indian text is treated on a computer or on
the Internet as graphic patterns only and
consumes large memory space. If 1k of
memory is taken for one page of screen
with coding like this, it would take 8k in
an ordinary graphics mode. When archives
of text are to be kept in databases, the
ASCII-like coding is the best.
With program PIXEL.bas or with the
dedicated display unit using IBM PC-
compatible keyboard, one can type in
three languages using the ASCII-like
codes.
Note: The following softwares pertain-
ing to this project, which could not be
issued with September EFY-CD due to
unavoidable circumstances, will now be
included in October EFY-CD:
1. Pixel6.BAS
2. Pixel6.EXE
3. Chtamil2
(The above files pertain to computer
based display scheme).
4. Tam.LST
5. Tam.EPR
6. Chtamil3
(The files at sl. no. 4 and 5 pertain to
control program and its hex dump for con-
trol EPROM while file at sl. no. 6 con-
tains hex code for character generator
EPROM.
This can be written into an EEROM
(using any 8085 kit or the one published
in Nov. ’99 issue of EFY) and is fixed into
the board, and then the LEDs on the left
bottom of the board wired at the 7475
(IC9) outputs would indicate the No. 5 as
they glow.
If this is not observed, one has to
check for proper connections from the
data lines to the 7475, connections to the
74156 address decoder, and the gate sig-
nals to pins 4.13 of the 7475 as per
Fig. 1.
Further, the connections to the video
RAM 62256 through the buffer IC 74244
and 74157 (pair of ICs 17 and 18) should
be checked for their correctness. When
the CPU 8085 is writing to the video RAM,
the 74157 (pair) connects A0-A5 address
lines of the 8085 to those of the video
RAM. Then, pin 1 of the 74157 ICs should
be pulsing low.
Thus, the following program to write
to 1800H in a loop would check for pulse
at pin 1 of 74157 and a high pulse at pins
1 and 19 of 74244. When the IC 74244 is
passing the 6845 signals, the IC 74373 is
in tri-state condition because its pin 1 is
then high.
P: MVI A,55
STA 1800H
LDA 1800H
OUT 80H
HLT
Or, in place of HLT, a loop may be
executed as under:
JMP P
The above short programs will enable
the checks to be made.
2. Another program to initialise
the 6845 as per the routine given in the
listing is to be entered in the EEROM
and then tested for proper H sync and
V sync signals from pins 39 and 40 of
6845.
3. The video clock signals and the
video output should be checked for proper
random display raster.
4. Another program for checking
ERASE memory should be entered into
the EEROM and then tested for the era-
sure of clear screen of the raster.
5. The keyboard program should be
tested as per the KBD routine.
Only after successful testing of
the board as per above-mentioned guide-
lines, the full program as per the listing
given in Appendix 1 should be pro-
grammed into control EPROM at its high-
est 2k address range (1800-1FFF) and
140