DB14
Scientech Technologies Pvt. Ltd.
5
Theory
The availability of a large variety of codes for the same discrete elements of
information results in the use of different codes by different digital system. It is
sometimes necessary to use the output of one system as the input to another. A
conversion circuit must be inserted between the two systems if each uses different
code for the same information. Thus, a code converter is a circuit that makes the two
systems compatible even though each uses a different code.
BCD is an abbreviation for binary-coded decimal. Binary codes for decimal digits
require a minimum of four bits. The BCD code expresses each digit in a decimal
number by its nibble equivalent. Weights are assigned to binary bits according to their
positions. The weights in the BCD codes are 8, 4, 2, 1. For example, the bit
assignment 0110 can be interpreted by the weights to represent the decimal digit 6
because 0
×
8 + 1
×
4 + 1
×
2 + 0
×
1 = 6
A seven segment decoder /driver is an IC decoder that can be used to drive a seven
segment indicator. There are two types of decoder/drivers, corresponding to common
anode and common cathode indicators. Each decoder driver has 4 BCD inputs and 7
output pins (a to g segment).
The 74LS47 is a seven segment decoder for common anode display. There are two
important types of 7-segment LED display. In a common cathode display, the
cathodes of all the LEDs are joined together and the individual segments are
illuminated by HIGH voltages. In a common anode display, the anodes of all the
LEDs are joined together and the individual segments are illuminated by connecting
to a Low voltage. The figure 1, figure 2 shows the two displays.
Common Cathode
Figure 1
Common Anode
Figure 2