Department of Computing and Information Science
Digital Systems
(CIS*3120)
Code Converters
The conversion from one binary code to another is common in digital systems. In this
exercise, you will design and construct two combinational-circuit converters.
Grey Code to Binary
Your first task is to design a combinational circuit with four inputs and four outputs that
converts a four-bit Grey code number (see Table 1) into the equivalent four-bit binary
number. Implement the circuit with exclusive-OR gates. Connect the circuit to four
binary switches and to four binary probes and verify that the circuit is working correctly.
Make sure to show your truth table, Simplified Boolean functions, and correctly
implemented circuit to the Teaching Assistant.
Binary Grey Code
0000
0000
0001
0001
0010
0011
0011
0010
0100
0110
0101
0111
0110
0101
0111
0100
1000
1100
1001
1101
1010
1111
1011
1110
1100
1010
1101
1011
1110
1001
1111
1000
Table 1: 4-bit Grey
code
1
Seven-Segment Display
A seven-segment display is used for displaying any one of the decimal digits 0 through 9.
Usually, the decimal digit is available in BCD. A BCD-to-seven-segment decoder accepts
a decimal digit in BCD and generates the corresponding seven-segment code. For
example, for an input of 616 the output 01101102 (the ASCII code for the character ‘6’)
would be produced. Figure 1 shows the connections necessary between the decoder and
the display.
0 1 2 3
4 5 6 7
8 9AB
CDEF
7
Your
Circuit
A
ASCIIDisplay
Hex Keyboard
0
Figure 1: Block diagram of circuit.
Your task is to construct the circuit shown in Fig. 1. As each hexadecimal digit
corresponds to four binary digits, your circuit will have four inputs. ASCII is a 7-bit code,
therefore your circuit will require 7 outputs. The input to your circuit should be provided
using a hex-keyboard, which can be found in the primio.clf library. Use an ASCIIdisplay, which can be found in the same library, to display the ASCII code (character)
output by your circuit. (Note: When using the ASCII-display, make sure to ground the
input to pin number 7 (which is actually the eighth input counting from one). You can do
this using a switch permanently set to zero. Although ASCII is a 7-bit code, an extra
(eighth) bit is sometimes used to represent an additional 128 characters (not part of the
English language) whenever the eighth bit is 1. )
Procedure:
Follow the five steps below.
1. Use the ASCII table at the bottom of this page to create a truth table for the problem.
2. Employ a K-map to find a simplified Boolean expression for each output of the table.
(You will need to show these maps to the TA when being marked.)
2
3. Using the simplified expressions, implement your circuit in LogicWorks’ design
window.
4. Use LogicWorks’ DevEditor to create a symbol for your circuit. (See lab #1 if you
don’t remember how to do this.)
5. Connect the hex-keyboard and ASCII-display to the input and output of your circuit
and verify that it is working correctly.
Make sure to show your truth table, Simplified Boolean functions, and correctly
implemented circuit to the Teaching Assistant.
Table2: American Standard Code for Information Interchange (ASCII)
Bits
4321
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
000
NUL
S0H
STX
ETX
EOT
ENQ
ACK
BEL
BS
HT
LF
VT
FF
CR
SO
SI
001
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN
EM
SUB
ESC
FS
GS
RS
US
010
SPACE
!
“
#
$
%
&
‘
(
)
*
+
‘
.
/
Bit Positions 765
011
100
0
@
1
A
2
B
3
C
4
D
5
E
6
F
7
G
8
H
9
I
:
J
;
K
<
L
=
M
>
N
?
O
101
P
Q
R
S
T
U
V
W
X
Y
Z
[
\
]
^
_
110
`
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
111
p
q
r
s
t
u
v
w
x
y
z
{
|
}
~
DEL
3