An input device sends data and instructions into a computer, for example a keyboard, mouse, microphone, scanner or sensor. An output device takes the results of processing out of the computer in a form people (or other machines) can use, for example a monitor, printer or speakers. Some devices, such as a touch screen, do both. When we compare input and output devices we look at port connectivity (how they connect), speed and the quality of output.

This is the third lesson on the parts of a computer, after Memory Types and Features and Storage Devices. If you did CSEC, Choosing the Right Input and Output Devices is a short refresher.

Port connectivity

A port is a socket on the computer where a device is connected. The port decides what the cable can carry (data, power, video, sound or a network) and how fast. Many devices now connect wirelessly instead, using Bluetooth or Wi-Fi. When a device is connected, the operating system uses a small program called a device driver to talk to it.

Things to think about when choosing a connection: does the computer have that port; is the port fast enough for the device (a webcam or external SSD needs more speed than a mouse); does the cable also need to carry power or sound; and would wireless be more convenient (no cable), or is wired better (faster, more reliable, no batteries to charge)?

Speed

Speed means different things for different devices:

• Input speed: how quickly data can be entered. A keyboard is limited by how fast a person types. A barcode reader, or the MICR reader that reads the magnetic ink numbers on bank cheques, enters data much faster and with fewer errors, because no one types it.
• Printer speed is measured in pages per minute (ppm). Laser printers are usually faster than inkjet printers for large amounts of text.
• Monitor speed is the refresh rate in hertz (Hz): how many times a second the picture is redrawn. 60 Hz is normal; gaming monitors use 144 Hz or more for smoother movement.
• Connection speed: the port or wireless link must be fast enough for the data (see the table above).

Quality of output

The quality of output is how good the result looks or sounds.

• Screens: the resolution is the number of pixels across and down, for example 1920 × 1080 (Full HD) or 3840 × 2160 (4K). More pixels give a sharper picture. The colour depth is the number of bits used for each pixel’s colour: 24 bits gives 2²⁴, about 16.7 million colours.
• Printers: quality is measured in dots per inch (dpi). A laser printer gives sharp text (typically 600 dpi or more) and is cheap per page for large jobs. An inkjet printer gives good colour photos and is cheap to buy, but the ink is expensive per page. A dot-matrix (impact) printer is noisy and gives low quality, but it can print on multi-part forms, where a copy is made by pressing through the sheets.
• Sound: better speakers and headphones give clearer sound with less distortion.

Higher quality costs something: more pixels and more colours mean bigger files, more memory and more data to send through the port.

Specialised devices

The syllabus names four specialised devices. Each one solves a particular problem.

Braille keyboard

A braille keyboard lets a blind or visually impaired person type by touch. Some are ordinary keyboards with raised braille dots on the keys. Others have six main keys, one for each dot of a braille cell, as on a Perkins Brailler: you press the keys for the dots together to make one letter. Braille users often also use a refreshable braille display (an output device whose pins rise and fall to show text as braille) and a screen reader, software that reads the screen aloud.

Image: Perkins brailler keyboard layout by Emflazie, CC0 (public domain), via Wikimedia Commons.

Sensors

A sensor is an input device that measures a physical quantity, such as temperature, light, moisture, pressure, humidity or movement. Most sensors give an analogue signal, so an analogue-to-digital converter (ADC) turns it into a binary number the computer can process. Sensors are used in monitoring and control systems, for example greenhouses, security alarms, traffic lights and car parking sensors. They take readings automatically and continuously, day and night, and are more accurate than a person.

In a control system, the computer compares each reading with a set value called the threshold. If the reading crosses it, the computer sends a signal to an actuator (a motor, fan, pump, lamp or alarm) to switch on or off.

Touch screen

A touch screen is both an output device (the display) and an input device (it detects where it is touched). A resistive touch screen detects pressure, so it works with a stylus or gloves. A capacitive touch screen detects the small electrical charge of a finger, is more sensitive and supports multi-touch, such as pinching to zoom. Touch screens are used on phones, tablets, ATMs, airport kiosks and shop tills. They are easy to learn and need no keyboard or mouse, but they get fingerprints, are slow for typing a lot of text, and small buttons are hard to hit. That is why the buttons on this page are at least 48 pixels tall.

Concept keyboard

A concept keyboard is a flat board divided into areas. Each area stands for a whole item or idea, not a single letter, and is shown by a picture or word on an overlay. The areas can be programmed for each use. Concept keyboards are used on fast-food tills (one press for "large fries"), and by young children and people with learning difficulties. Data entry is fast with few errors, and the sealed surface is easy to wipe clean and resists spills. The drawback is that only a fixed set of choices can be entered.

Summary

Input and output in C

In C, scanf reads input (normally from the keyboard) and printf sends output (normally to the screen). Your program does not need to know which keyboard or monitor is attached: the operating system and its device drivers deal with the actual devices.

#include <stdio.h>

int main()
{
    int age;

    printf("Enter your age: ");
    scanf("%d", &age);
    printf("Next year: %d\n", age + 1);

    return 0;
}

Whatever whole number the user types is stored in the variable age in RAM, and the program prints one more than it. More on this in Variables, format specifiers and printf and Adding 2 numbers: outputting the result with printf().

Practice questions

Homework

1. Look at the ports on a computer or laptop at home or at school. List each port, say what it is (use the table above), and name one device you could connect to it.
2. On a Windows computer, open Device Manager and list the input and output devices installed (keyboards, mice, monitors, audio). The CAPE syllabus suggests this activity.
3. Choose one specialised device from this lesson. Write a short paragraph on who uses it, what problem it solves, and one advantage and one disadvantage.

Exam tips

• Use the syllabus headings. For this objective the syllabus compares input and output devices by port connectivity, speed and quality of output, and names four specialised devices: the braille keyboard, sensors, the touch screen and the concept keyboard. When you compare devices in an answer, organise it under those headings.
• 2024 Unit 1 Paper 02, Question 2(b). Sensors turn up inside a logic question here: several sensors in a security system have to share one line, and you are asked how a multiplexer makes that possible. It tests whether you can link input devices to the building blocks from Module 1. Explain which inputs the sensors connect to, what the select lines do, and how each sensor gets the line in turn. Try it, then compare with the solution on this site: 2024 U1 Q2. See also Multiplexers.
• 2024 Unit 1 Paper 02, Question 1(b)(ii). This part compares RAM with a flash drive on volatility and portability. It tests whether you can explain portability in terms of the device itself, and the port it uses is part of that answer. The solution on this site points out that a flash drive plugs into a USB port. Try the part first, then compare.
• Specimen Unit 1 Paper 02 (2021), Question 2(b)(ii). This part is about the kinds of instruction a CPU has in its instruction set. One kind moves data, including to and from input and output devices, which is where this lesson meets the processor. For each kind you name, give a short description and an example. Try it when you study the CPU, then check the mark scheme.
• Specimen Unit 2 Paper 01 (2021), Item 45. A multiple-choice item on what happens to a running process while it waits for input from a device, and when that input arrives. It belongs to Unit 2 (operating systems: process states, device drivers, polling, buffering and spooling), so it is only a preview here. When you get there, draw the process state diagram before you choose an answer. Open the specimen paper and try it then.
• More worked answers are on the CS Solutions page.

General advice (from the lesson content, not from a particular paper): when asked to choose a device for a scenario, name the device, then justify it with the features in the question, such as who the user is, the port or connection, the speed needed and the quality of output.

Previous: Storage Devices (and before that, Memory Types and Features). Next: the processor (CPU), and how it moves data between memory and the input and output devices (coming soon in this course; for now see 2009 U1 Q2 Part C). Sensor readings become binary numbers, so data representation follows: see Decimal to binary. For how users interact with a computer, see User Interfaces: Command Line, Menus, Graphical and Touch.

Syllabus link: CAPE Computer Science (CXC A19/U2/21), Unit 1, Module 1: Computer Architecture. Specific Objective 1: state the purpose of the main components of a computer system (input/output devices: port connectivity, speed, quality of output; specialised devices: braille keyboard, sensors, touch screen, concept keyboard). U1 M1 SO1.

Attributions to media used in this post:

Braille keyboard diagram: Emflazie, CC0 1.0 (public domain), via Wikimedia Commons (File:Perkins brailler keyboard layout.png).

Updates

7 Nov 2026: Added the shared course badge summary and lesson mastery quiz, with feedback, retries and browser-saved progress.

28 Oct 2026

- Draft created.

- Exam references reworded and quiz questions rewritten.

© 2026 Vedesh Kungebeharry. All rights reserved.