Input & Output Devices

🎯 Syllabus & Goals 3 min

Cambridge 3.2 · Input and output devices Paper 1 · Computer Systems

By the end of this lesson you can:

1. Define input and output devices and classify the devices at a supermarket checkout.
2. Describe how a barcode is read, and how it drives automatic stock control.
3. Describe how a QR code is read, and compare QR codes with barcodes.

Textbook: Chapter 3, §3.2.1 (pp. 88–92) — barcode and QR code scanners.

Recap / Warm-Up 5 min

Lessons 1–3 looked inside the processor. Data has to get in before the CPU can process it. Results have to get out before anyone can use them. That is the job of input and output devices.

Quick starter

A barcode is a pattern of dark and light stripes. Which Unit 1 idea lets a computer store that pattern?

🧠 Key Concept 14 min

1 · Input and output devices

Section 3.2 of the syllabus names every device you can be asked about. This map shows where each one is taught.

2 · Barcodes — binary printed as lines

A barcode is a row of dark and light parallel lines of different widths. Each digit 0–9 has its own pattern. Every digit takes up the same width: seven thin modules, forming two dark bars and two light spaces.

• Guard bars mark the start, the centre and the end. The centre guard splits the left digits from the right digits.
• Left-hand digits always start with a light module and have an odd number of dark modules.
• Right-hand digits always start with a dark module and have an even number of dark modules.
• Because the two halves look different, the scanner can tell which way round it is reading. A barcode can be scanned in either direction.

3 · What happens when a barcode is scanned

1. A red laser or red LED shines light across the barcode.
2. Light areas reflect a lot of light; dark bars reflect little or none.
3. Sensors (photoelectric cells) detect the reflected light.
4. As the light sweeps across, the pattern of light and dark is turned into digital data (light = 0, dark = 1).

4 · The checkout: input and output devices working together

5 · Automatic stock control

The barcode number is the key field of the product's record in the stock database. It uniquely identifies that product. Each scan triggers this sequence:

6 · QR (quick response) codes

A QR code is a two-dimensional barcode: a square matrix of dark and light squares called pixels (or modules). Because it stores data both across and down, it holds far more.

• Up to 4296 characters (or 7089 digits), compared with about 30 digits for a barcode.
• It can hold a web address, a phone number or a whole boarding pass.
• The three large squares in the corners are for alignment. A smaller square near the fourth corner checks the size and angle of the camera shot.

How a phone reads a QR code:

1. The user points the phone's camera at the code.
2. A QR app (or the camera software) processes the image and turns the squares into data.
3. The phone's browser reads that data and decodes any web address in it.
4. The user is taken to the website — or to the phone app if a number was stored, or the boarding pass is saved.

Worked Example 12 min

(a) Writing a barcode section as binary: 5 4 3 | 0 5 2

1. Start guard: 101.guard bars frame the code so the scanner knows where it starts.
2. Left 5 → 0110001, left 4 → 0100011, left 3 → 0111101.read each straight off the left-hand table.
3. Check: dark counts are 3, 3 and 5 — all odd, and all start with 0 (light).this is the self-check that proves you used the left-hand set.
4. Centre guard: 01010.it separates the two halves.
5. Right 0 → invert 0001101 = 1110010; right 5 → invert 0110001 = 1001110; right 2 → invert 0010011 = 1101100.right-hand patterns are the left ones with every bit flipped.
6. Check: dark counts 4, 4 and 4 — all even, all start with 1 (dark). End guard 101.even and dark-first means right-hand — correct.

(b) Automatic stock control, scan by scan

1. Each scan: the barcode (key field) finds the record; the price goes to the checkout.the lookup is what every other step depends on.
2. The stock level is reduced by 1 and written back: 23 → 22 → 21 → 20 → 19.one scan = one item sold.
3. After scan 3, stock is 20 ≤ 20, so an order is placed and a flag is set."less than or equal to" — 20 itself triggers it.
4. Scan 4 finds the flag already set, so no second order is made.the flag stops an order on every later scan.

How the marks are earned: barcode looked up in database / key field (1); stock reduced by 1 and written back (1); compared with re-order level (1); order placed and flag set to stop repeat orders (1).

(c) How much can a small QR code hold?

1. A square code of 40 × 40 pixels, each 1 bit.each pixel is dark or light: 1 or 0.
2. Bits: 40 × 40 = 1600 bits.it is a 2D grid, so multiply rows by columns.
3. Bytes: 1600 ÷ 8 = 200 bytes.8 bits = 1 byte (Unit 1).

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

Input devices bring data in; output devices present results. A barcode is binary printed as bars: light reflects (0), dark absorbs (1). Its number is the key field for automatic stock control. A QR code stores data in a 2D grid, so it holds far more and is read by a phone camera.

Input device

Hardware that converts external data into a form the computer can process.

Output device

Hardware that shows the results of processing in a form people can understand.

Barcode

A series of dark and light lines of varying thickness that represents data; read with a laser or LED light source.

Key field

The field that uniquely identifies a record in a file — here, the barcode number.

QR code

A matrix of dark and light squares representing data; read by a smartphone camera and a QR app.

Frame QR code

A QR code that includes a canvas area for advertising graphics.

Attagging

Hiding malicious code or a fake web link inside a QR code.

Homework 1 min

Task (≤ 15 min): An airline sends boarding passes to phones as QR codes. Explain two reasons why a QR code is used rather than a traditional barcode, and describe one risk to the passenger. [5]