Specifications

CONSTRUCTION
This involves a simple 8085 micropro-
cessor and an additional CRT controller.
The dedicated CRT controller chip 6845
has been popular ever since it was first
used by the IBM in its display controller
cards. The circuit of this board is shown
in Figs 1(a) and 1(b). It comprises:
1. Video generation circuitry includ-
ing dot and character clocks.
2. Pixel or video RAM.
3. Character dot pattern EPROM for
four languages
4. 8085 firmware on EPROM for four
languages
5. 6845 CRT controller IC.
Fig. 1(a) shows the 8085 microproces-
sor and its signals. Crystal of 4 MHz be-
tween its pins 1 and 2 provides the clock
for the processor to tick and work. Reset
pin 36 is connected to get itself reset upon
power on. Manual resetting is also pos-
sible using reset switch S1. The address-
cum-data signal lines AD0-AD7 are con-
nected to a 74LS373 latch to separate the
address signals A0-A7, using the ALE
pulse from pin 30 of 8085. The data-bus
connects to all devices such as EPROMs,
RAM, and the 74245 bidirectional trans-
ceiver. Some of the data lines are also
connected to output port (at I/O address
80) using a 7475 IC for providing four
bits of outputs (D0’ to D3’). The input
port (also at I/O address 80) employing a
74365 caters to six bits of input. The PC
keyboard data and clock signals are con-
nected to data bus via two of its input
lines.
The Address decoder is a 74156, which
has open collector outputs. It enables one
or two of the chip select decoded signals to
be combined by just joining them (in wired-
OR fashion). Using the address lines A12,
A11, and A10, the decoder provides eight
chip select signals for the address ranges
as shown in the figure. Each output cov-
ers a 1k memory range. Thus pins 9 and
10 (shorted) serve as the chip select signal
for EPROM1 covering a 2k memory ad-
dress space. (Although we use 2764, an 8k
EPROM actually since now-a-days only
8k EPROM ICs are easily available and
easily programmable while 2k capacity
EPROMs are almost obsolete and difficult
to program—we need 25V programming
pulse etc.) The address range for EPROM-
1 is 0000-07FF. Similarly, pins 11 and 12
are joined together to provide address
range from 0800-0FFF. This is the chip
select signal for the second ERPOM, which
stores the character dot patterns for the
four languages. This too is a 2764, and the
chip select signal ranges only 2k, but the
total 8k range is for storing four language
dot patterns, each in one 2k range. Thus
the selection of the range/language is done
by signals from the 7475-output port bits
D0’ and D1’, which are wired to A11 and
A12 address lines of the 2764 character
generator EPROM, which can be selected
using the function keys as explained be-
low.
The input port 80H, using 74365, is
for reading the language selection made.
The language is selected by pressing keys
F1 through F4 on the PC’s keyboard. This
causes bits D0 and D1 to be output on
the 7475 output ports to indicate the se-
lection by two of the LEDs wired at its
output. Two other bits, D4 and D5 of this
input port, are connected to the data and
clock pins of the IBM PC keyboard con-
nector.
The RAM chip 6264 (8k memory) is
used in the circuit. However, only 1k
(1000-13FF) of its address space is
utilised. So, its ‘high’ address pins A11
and A12 are permanently made ‘high’.
A chip-select 1 (CS1) is obtained from
pin 6 of the 74156 IC, which covers 1400-
17FF address range. This goes to select
the video RAM 62256. Though a 62256 of
32k memory is used, only 16k is actually
utilised. Its pin 1 is made permanent
‘high’. This chip select uses the address
lines A0 to A5 having an address range
of just 64 bytes, just the low order memory
of the video RAM.
A chip-select 2 (CS2) signal is used to
select a 74LS373 latch used with the video
RAM circuit. This is used to supply the
high order addresses (A6 through A13) to
the video memory.
An additional chip-select 3 (CS3) sig-
nal is used for accessing the 6845 CRT
controller to program its mode of opera-
tion, so as to get a raster of 312 lines and
50 Hz frame frequency.
In the earlier design by the authors,
an ASCII keyboard had been used. This
ASCII keyboard used a dedicated key-
board controller IC, and the keys were
wired in the fashion of the typewriter
keys, making use of switches fixed on to
a plain PCB and wiring the contacts to
the IC as per its data sheet. There are
ICs for making such an ASCII keyboard.
The AY3-5376 is one such IC. The ASCII
code for the key pressed is output as a 7-
bit code by this IC.
In this new design, the authors have
used an IBM PC (AT) keyboard. The au-
thors have given such a PC keyboard for
their Home Computer Project (Refer EFY
Electronics Projects, Vol. 11). This was a
keyboard of the older type, the XT key-
board, but now the freely available (for
Rs 300) AT keyboard has been employed
for the current design.
The keyboard is labeled with English,
Hindi, and Tamil characters, as per the
standard typewriter format. The format
for Hindi and Tamil characters are shown
in Fig. 3.
The 8085 generates the control sig-
nals IO/M, WR, RD (active low signals).
These are used in conjunction with
74LS02 and 74LS00 gates shown in
Fig. 2(a) to obtain separate read and write
control signals for memory or input-out-
put, i.e. MR, MW, IOR, IOW for use in
the circuit.
PARTS LIST
Semiconductors:
IC1 - 8085 8-bit microprocessor
IC2, IC21 - 74LS373 octal transparent
latch
IC3, IC4 - 2764 8k byte EPROM
IC5 - 6264 8k byte RAM
IC6 - 74LS245 octal transceiver
IC7 - 74LS156 dual 2-line to 4-line
decoder
IC8 - 74LS365 8-line to 1-line
multiplexer
IC9 - 74LS75 4-bit latch
IC10, IC13 - 74LS02 quad NOR gate
IC11 - 74LS04 hex inverter
IC12, IC14 - 74LS00 quad NAND gate
IC15 - 74LS132 quad NAND
Schmitt trigger
IC16 - 6845 CRT controller
IC17, IC18 - 74LS157 quad 2-line to 1-line
data selector
IC19 - 74LS244 octal bus buffer/
driver
IC20 - 62256, 32k byte static RAM
IC22 - 74LS165 parallel-in shift
register
IC23 - 74LS190 synchronous decade
counter
T1 - BC148B npn transistor
D1 - 1N4148 switching diode
LED1-LED4 - Red LEDs
Resistors (all ¼-watt, ±5% carbon, unless
stated otherwise):
R1-R5 - 1-kilo-ohm
R11-R16,
R6, R17 - 4.7-kilo-ohm
R7-R10,
R22-R23 - 220-ohm
R18, R19 - 10-kilo-ohm
R20 - 220-kilo-ohm
R21 - 680-ohm
VR1 - 470-ohm preset
Capacitors:
C1, C3 - 1 µF, 16V electrolytic
C2 - 22 pF ceramic disk
Miscellaneous:
X
TAL 1 - 4 MHz crystal
PCKBD - Keyboard interface connector
To be continued next month
126