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ASCII Code in Digital Electronics

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The ASCII Code in Digital Electronics

The term ASCII code (a misnomer, but still used widely) is the short for American Standard Code for Information Interchange (ASCII). The 7-unit ASCII code is only an extension of the Baudot code. Hence all the main features of the Baudot code are applicable in the case of ASCII (to be pronounced as AA-S-KEE) also. The following are the additional features of the ASCII code.

Additional Features of the ASCII code

·         Each code word consists of seven symbols. Hence, we have a total of 27(=128) possible combinations, to represent alphabets, punctuation marks, and numbers.
·         It is divided into two parts. The first part consists of three bits and the second part consists of four bits. This is illustrated in Table 1.16.

Table 1.16 Organization of ASCII code

First part (3 bits)
Second part (4 bits)
xxx
xxxx

In Table 1.16, x’s represent bit 0 or 1. The 3-bit first part is used to identify whether the 4-bits following these 3-bits represent letters, numerals, or punctuation marks. For example the 3-bit combinations of 100 and 101 represent upper-case (capital) letters (A, B, C, etc.). Similarly, bit 3-bit combinations of 110, and 111 represent lower-case (small) letters (a, b, c, etc.). Table 1.17 shows a few examples of ASCII code.

Table 1.17 A few examples of ASCII code

First Part
(3 bits)
Second Part
(4 bits)
Letter/ Punctuation / Number concerned
0 0 0
0 0 0 1
Start of Heading
0 0 0
0 0 1 0
Start of text
0 0 0
0 0 1 1
End of text
0 0 0
0 1 0 0
End of transmission


Since there are 128 possible combinations of 1s and 0s, all letters, numerals and most of the punctuation marks can be represented by using the ASCII code. Since the length of each code word is much larger than that of the Morse code, its efficiency is much less.

Advantages of ASCII code over Morse code

o   In ASCII coding scheme, 27 = 128 code words are possible.  Hence, a large number of symbols, alphabets etc. can be easily represented.
o   There is a definite order in which the alphabets, etc. are assigned to each code word.
o   Parity bits can be added for error-detection and correction.

Disadvantages ASCII code

o   The length of the code is larger; hence, more bandwidth is required for transmission.
o   Number of code words in ASCII is insufficient to represent newly invented and added characters and symbols. 


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