Secret Numbers Behind Text: Reading and Writing ASCII Codes

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Technology and Computer Science · Computer Science
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Secret Numbers Behind Text: Reading and
Writing ASCII Codes
Technology and Computer Science · Computer Science · Grades 8-10 · 50 minutes
Time needed
50 minutes (one class period)
Prep
Copy the 2-page worksheet (one per student). Write the place-value row from Before Class on
the board.
Technology
None.
Student
independence
High. Students decode and encode with the reference tables, then compare with a partner.
Copies to make
One 2-page worksheet per student
Materials
Pencils, Worksheet, Whiteboard or chalkboard
Goal for today: Students will use an ASCII reference table to encode and decode short words, read an 8-bit binary
pattern as a letter, and explain why computers need agreed-upon codes for text.
Before the bell
☐ Copy the worksheet, one per student (2 pages).
☐ On the board write this row of place values: 128 64 32 16 8 4 2 1. Under it write 0 1 0 0 0 0 0 1 and the words
“64 + 1 = 65 = A.”
☐ Keep the answer key on your desk. Every decoded message is a short everyday word or phrase.
☐ Let students know no devices are needed; all codes are on the worksheet.
Say this to start
Good morning! A computer cannot store letters. It stores numbers. So people agreed on a code that gives every letter,
digit and symbol its own number. One famous code is called ASCII. Today you will use an ASCII table to turn words into
numbers and numbers back into words, like a code breaker. All the tools you need are printed on the worksheet.

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Page 3
Run of show
Minutes
What you do
What students do
0–5
Take attendance. Hand out the worksheet and ask
students to look at the code tables.
Read the two reference tables at the top.
5–12
Use the board example: “The letter A has the code 65, B is
66, C is 67. So the word CAB is 67, 65, 66.” Ask a student
to find the code for the letter D (68).
Follow along and find a code in the table.
12–22
Tell students to do questions 1–3 (writing words as codes)
and 4–6 (decoding messages). Walk the room.
Look up each letter or number in the tables.
22–32
Tell students to complete the lowercase letters (questions
7–9). Point out the pattern in the rule box: lowercase =
capital + 32.
Use the +32 rule and the tables.
32–42
Tell students to do the binary section (questions 10–13).
Point at the place-value row on the board for help.
Add up the place values that have a 1 to
get each code.
42–47
Read answers to questions 1–13 aloud from the key.
Students check their work.
Mark and correct answers.
47–50
Collect all worksheets.
Hand in the worksheet.
If things go wrong
• If students are lost with binary, point to the board row and say: “Wherever there is a 1, add that place value.
Where there is a 0, add nothing.”
• If students ask why the digit 5 has code 53, say that the character “5” is a symbol in text, and the computer
gives it its own code, just like a letter.
• If a decoded message looks like nonsense, tell the student to double-check each number against the table.
Every message is real words.
• If students want to make secret messages to pass around, tell them to wait until the worksheet is done, then
trade with a partner.
Early finishers
• Write your favorite word as ASCII codes and have a neighbor decode it.
• Add 1 to every code in the word CODE and decode the new word (it gives DPEF, which is not a real word).
• Explain in two sentences why people and computers all around the world need to agree on the same code.
Support and challenge
• Support: Let students skip the binary questions (10–13) and use the reference tables for all encoding and
decoding.
• Challenge: Ask students to find the binary pattern for the letter C (67 = 01000011) and the letter a (97 =
01100001) using the place-value row.

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Technology and Computer Science · Computer Science
SP-CS-014 · Free to print and use in your classroom · substituteplans.com
Page 4
Notes for the teacher (you fill this in)
Jot a few lines before you leave. A full Daily Sub Report is in the free Sub Survival Kit at SubstitutePlans.com.
How far the class got / what to pick up next:
Students who were especially helpful:
Anything the teacher should know: