ASCII

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Introduction

If computers operate in binary, then how are we able to store letters and words? To do this, we assign numbers to characters. This is known as character encoding.

Example of ASCII encoding

Looking at the internals of a simple text document

To understand how character encoding works, let’s create a simple example. First, assign the numbers 1-26 to the English alphabet:

1  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
a  b  c  d  e  f  g  h  i  j  k  l  m  n  o  p  q  r  s  t  u  v  w  x  y  z

To write a simple encoded message, we substitute the numbers for the letters. For example, 8 5 12 12 15. By using numbers, we have constructed the word h e l l o.

But to completely capture the English alphabet – including upper and lower-case letters, numbers, and punctuation – we needed more than 26 characters. As a result, the American Standard Code for Information Interchange (ASCII) was created as one of the first character encoding standards for computers.

What You Will Learn

The following topics will be covered in this tutorial:

  • A brief history of ASCII
  • How to translate decimal, binary, and hexadecimal numbers to ASCII

Suggested Reading

There are a few concepts that you might want to be familiar with before starting to read this guide:

  • Binary - Knowing how a computer stores numbers is useful to translating those numbers to characters.
  • Hexadecimal - Hexadecimal is often used to express binary numbers in groups of 4 bits.
  • Installing Arduino IDE - Arduino is a good way to try printing ASCII characters.

History

The American Standards Association (ASA), now the American National Standards Institute (ANSI), began work on ASCII on October 6, 1960. The encoding scheme had origins in the 5-bit telegraph codes invented by Émile Baudot. The committee eventually decided on a 7-bit code for ASCII.

7 bits allow for 128 characters. While only American English characters and symbols were chosen for this encoding set, 7 bits meant minimized costs associated with transmitting this data (as opposed to say, 8 bits).

The first 32 characters of ASCII were reserved control characters. These characters were used to relay special instructions to other devices, like printers. For example, a user could advance a line, delete a character, and on some devices, ring a bell (such as on the Teletype Model 33 ASR).

ASA published the first version of ASCII in 1963 and revised it in 1967. The last major update to the standard occurred in 1986. ASCII first saw commercial use in the American Telephone & Telegraph (AT&T) TeletypeWriter Exchange (TWX) network.

Teletype Model 33 ASR

Teleprinters, like this Teletype Model 33 ASR, were used to send typed messages to one or more other teleprinters across various communication channels (Image courtesy of Arnold Reinhold of Wikimedia Commons)

On March 11, 1968 President Lyndon B. Johnson mandated that all US federal government computers must support ASCII, thus cementing ASCII’s place in American computing history.

Other encoding schemes existed at the time, such as the International Telegraph Alphabet No. 2 (ITA2), but ASCII quickly became the standard for American English encoding. ASCII was the most common encoding found on the Internet until it was surpassed by UTF-8 in 2007.

ASCII Table

To identify a character’s ASCII value, it is common to look it up on an ASCII table. The ASCII table pairs each character to its assigned value between 0 and 127.

Control Characters

Control characters make up the first 32 characters of the ASCII table. These characters are not intended to be printed, instead they are used to send command instructions to another device, such as a printer. Note that we have included the octal representation of the ASCII characters in the off chance that you might be working with a particularly old system (such as the 12-bit PDP-8).

Dec Bin Oct Hex Char Description
0 0000 0000 000 00 NUL null
1 0000 0001 001 01 SOH start of heading
2 0000 0010 002 02 STX start of text
3 0000 0011 003 03 ETX end of text
4 0000 0100 004 04 EOT end of transmission
5 0000 0101 005 05 ENQ enquiry
6 0000 0110 006 06 ACK acknowledge
7 0000 0111 007 07 BEL bell
8 0000 1000 010 08 BS backspace
9 0000 1001 011 09 TAB horizontal tab
10 0000 1010 012 0A LF line feed, new line
11 0000 1011 013 0B VT vertical tab
12 0000 1100 014 0C FF form feed, new page
13 0000 1101 015 0D CR carriage return
14 0000 1110 016 0E SO shift out
15 0000 1111 017 0F SI shift in
16 0001 0000 020 10 DLE data link escape
17 0001 0001 021 11 DC1 device control 1
18 0001 0010 022 12 DC2 device control 2
19 0001 0011 023 13 DC3 device control 3
20 0001 0100 024 14 DC4 device control 4
21 0001 0101 025 15 NAK negative acknowledge
22 0001 0110 026 16 SYN synchronous idle
23 0001 0111 027 17 ETB end of transmission block
24 0001 1000 030 18 CAN cancel
25 0001 1001 031 19 EM end of medium
26 0001 1010 032 1A SUB substitute
27 0001 1011 033 1B ESC escape
28 0001 1100 034 1C FS file separator
29 0001 1101 035 1D GS group separator
30 0001 1110 036 1E RS record separator
31 0001 1111 037 1F US unit separator
127 0111 1111 177 7F DEL delete

Printable Characters

There are 95 printable characters in the ASCII encoding scheme. Note that the “space” character denotes a printable space (“ ”).

Dec Bin Oct Hex Char
32 0010 0000 040 20 space
33 0010 0001 041 21 !
34 0010 0010 042 22 "
35 0010 0011 043 23 #
36 0010 0100 044 24 $
37 0010 0101 045 25 %
38 0010 0110 046 26 &
39 0010 0111 047 27 '
40 0010 1000 050 28 (
41 0010 1001 051 29 )
42 0010 1010 052 2A *
43 0010 1011 053 2B +
44 0010 1100 054 2C ,
45 0010 1101 055 2D -
46 0010 1110 056 2E .
47 0010 1111 057 2F /
48 0011 0000 060 30 0
49 0011 0001 061 31 1
50 0011 0010 062 32 2
51 0011 0011 063 33 3
52 0011 0100 064 34 4
53 0011 0101 065 35 5
54 0011 0110 066 36 6
55 0011 0111 067 37 7
56 0011 1000 070 38 8
57 0011 1001 071 39 9
58 0011 1010 072 3A :
59 0011 1011 073 3B ;
60 0011 1100 074 3C <
61 0011 1101 075 3D =
62 0011 1110 076 3E >
63 0011 1111 077 3F ?
Dec Bin Oct Hex Char
64 0100 0000 100 40 @
65 0100 0001 101 41 A
66 0100 0010 102 42 B
67 0100 0011 103 43 C
68 0100 0100 104 44 D
69 0100 0101 105 45 E
70 0100 0110 106 46 F
71 0100 0111 107 47 G
72 0100 1000 110 48 H
73 0100 1001 111 49 I
74 0100 1010 112 4A J
75 0100 1011 113 4B K
76 0100 1100 114 4C L
77 0100 1101 115 4D M
78 0100 1110 116 4E N
79 0100 1111 117 4F O
80 0101 0000 120 50 P
81 0101 0001 121 51 Q
82 0101 0010 122 52 R
83 0101 0011 123 53 S
84 0101 0100 124 54 T
85 0101 0101 125 55 U
86 0101 0110 126 56 V
87 0101 0111 127 57 W
88 0101 1000 130 58 X
89 0101 1001 131 59 Y
90 0101 1010 132 5A Z
91 0101 1011 133 5B [
92 0101 1100 134 5C \
93 0101 1101 135 5D ]
94 0101 1110 136 5E ^
95 0101 1111 137 5F _
Dec Bin Oct Hex Char
96 0110 0000 140 60 `
97 0110 0001 141 61 a
98 0110 0010 142 62 b
99 0110 0011 143 63 c
100 0110 0100 144 64 d
101 0110 0101 145 65 e
102 0110 0110 146 66 f
103 0110 0111 147 67 g
104 0110 1000 150 68 h
105 0110 1001 151 69 i
106 0110 1010 152 6A j
107 0110 1011 153 6B k
108 0110 1100 154 6C l
109 0110 1101 155 6D m
110 0110 1110 156 6E n
111 0110 1111 157 6F o
112 0111 0000 160 70 p
113 0111 0001 161 71 q
114 0111 0010 162 72 r
115 0111 0011 163 73 s
116 0111 0100 164 74 t
117 0111 0101 165 75 u
118 0111 0110 166 76 v
119 0111 0111 167 77 w
120 0111 1000 170 78 x
121 0111 1001 171 79 y
122 0111 1010 172 7A z
123 0111 1011 173 7B {
124 0111 1100 174 7C |
125 0111 1101 175 7D }
126 0111 1110 176 7E ~

Try It

If you would like to try printing something using ASCII encoding, you can try it out using Arduino. See this tutorial for getting started with Arduino.

Open the Arduino IDE and paste in the following code:

language:c
void setup() 
{
  Serial.begin(9600);
}

void loop() 
{
  Serial.write(0x48); // H
  Serial.write(0x65); // e
  Serial.write(0x6C); // l
  Serial.write(0x6C); // l
  Serial.write(0x6F); // o
  Serial.write(0x21); // !
  Serial.write(0x0A); // \n

  delay(1000);
}

Run it on your Arduino, and open a Serial console. You should see the “Hello!” appear over and over:

Arduino saying Hello!

Notice that we had to use Serial.write() instead of Serial.print(). The write() command sends a raw byte across the serial line. print(), on the other hand, will try and interpret the number and send the ASCII-encoded version of that number. For example, Serial.print(0x48) would print 72 in the console.

Also, notice that we used the ASCII character 0x0A, which is the “line feed” control character. This causes the printer (or console in this case) to advance to the next line. It is similar to pressing the ‘enter’ key.

Resources and Going Further

There are many character encoding sets available. The most popular encoding for the World Wide Web is UTF-8. As of June 2016, UTF-8 is used in 87% of all web pages.

UTF-8 is backwards compatible with ASCII, which means the first 128 characters are the same as ASCII. UTF-8 can use 2, 3, and 4 bytes to encode characters from most modern written languages, including Latin, Greek, Cyrillic, Arabic, Chinese, Korean, and Japanese characters.

Knowledge of basic ASCII encoding can be useful when working in serial terminals. See the Serial Terminal Basics to learn how to use some of the many serial terminal programs available.

If you are interested in downloading the ASCII table in image format, click the button below. With an image, you can print it out and hang it on your wall, put it on a coffee mug, or have it printed on a mouse pad.

ASCII Table Images