Peripherals and Input Devices

What counts as a peripheral

A peripheral is a device that connects to a computer from the outside and extends what it can do, without being part of its core — the processor, memory, motherboard and internal storage described in What's Inside a Computer. The word comes from "periphery": the edge. A keyboard, a webcam and a printer are peripherals; the central processing unit (CPU) is not.

Peripherals are usually grouped by the direction of the data:

The grouping is not always clean — a touchscreen is a display with a touch sensor on top — but it is useful when you troubleshoot: an input problem makes you look at what the device sends, an output problem at what it receives. Monitors and printers have their own pages: Displays and Printers.

Keyboards, mice and other input devices

Keyboards

A keyboard does not send letters. It sends a code for each key that is pressed or released (a scan code), and the operating system (OS) turns that code into a character using the keyboard layout set in its language settings. The layout is named after the first six letters on the top row:

• QWERTY: the United States, the United Kingdom and many other countries (the United States and United Kingdom versions differ in several symbol keys, such as @ and ").
• AZERTY: France and Belgium.
• QWERTZ: Germany, Austria and much of central Europe.

Because the character comes from the OS setting and not from the keyboard, a keyboard with QWERTY keycaps on a computer set to AZERTY types an "a" when you press Q. The physical keyboard is fine; the layout setting is wrong.

Under the keycaps, keyboards are built in two main ways:

• Membrane keyboards have a rubber dome under each key that presses two thin plastic layers with printed contacts together. They are cheap, quiet and light, and most office and laptop keyboards are built this way (laptop keys add a small scissor mechanism for stability).
• Mechanical keyboards have a separate switch with a spring under every key. They cost more, are louder, and last longer; individual switches can often be replaced. Gamers and people who type all day often prefer them.

Mice and touchpads

A modern mouse is optical: a small camera takes thousands of pictures of the surface every second, and a chip calculates the movement from how the pictures shift. Glass and glossy surfaces give the camera nothing to track, which is why a mouse can "jump" on a glass desk. Sensitivity is given in dots per inch (DPI, sometimes counts per inch, CPI): how many steps the mouse reports for an inch of movement.

A touchpad (trackpad) is the laptop equivalent: a capacitive surface that senses the position of one or more fingers and supports gestures such as two-finger scrolling. A frequent ticket is "my touchpad stopped working" after the user pressed the function (Fn) key combination that disables it, or after an external mouse was connected on a system set to turn the touchpad off in that case. Check the touchpad settings before you suspect hardware. Other pointing devices include trackballs (the ball is rolled by hand and the device stays still) and the stylus for touchscreens and graphics tablets.

Scanners and barcode readers

A scanner turns a paper document, photo or code into a digital image by moving a row of light sensors across it. The main types:

• Flatbed: the original lies face down on a glass plate and the sensor moves underneath. Good for photos, books and fragile documents.
• Sheet-fed: the scanner pulls the pages past a fixed sensor, usually from an automatic document feeder (ADF). Fast for stacks of loose pages; many multifunction printers combine a flatbed with an ADF. A "duplex" ADF scans both sides in one pass.
• Barcode scanners: handheld or built into a counter at a point of sale. Linear (laser) scanners read one-dimensional barcodes; image-based scanners also read two-dimensional codes such as Quick Response (QR) codes, which need a camera to capture the whole square at once.

Scan resolution is given in dots per inch (dpi). About 300 dpi is enough for text documents and for optical character recognition (OCR), the software that turns an image of text back into editable text. Higher resolutions are for photos and fine detail, and they cost disk space quickly: doubling the resolution quadruples the number of pixels, because it doubles both the width and the height.

WIDTH_IN, HEIGHT_IN = 8.5, 11          # a US Letter page, in inches

for dpi in (150, 300, 600):
    width_px = round(WIDTH_IN * dpi)
    height_px = round(HEIGHT_IN * dpi)
    pixels = width_px * height_px
    color_mb = pixels * 3 / 1_000_000   # 24-bit color: 3 bytes per pixel
    gray_mb = pixels * 1 / 1_000_000    # 8-bit grayscale: 1 byte per pixel
    print(f"{dpi:>3} dpi: {width_px} x {height_px} px, "
          f"color {color_mb:5.1f} MB, grayscale {gray_mb:4.1f} MB")

150 dpi: 1275 x 1650 px, color   6.3 MB, grayscale  2.1 MB
300 dpi: 2550 x 3300 px, color  25.2 MB, grayscale  8.4 MB
600 dpi: 5100 x 6600 px, color 101.0 MB, grayscale 33.7 MB

An A4 page, the standard size in most of the world including Europe and the UK, is 210 by 297 mm (about 8.27 by 11.69 in), slightly larger than US Letter, so its numbers are a few percent higher. These are uncompressed sizes. Saved as a compressed image or as a Portable Document Format (PDF) file, the file is much smaller, but the ratio holds: a color scan at 600 dpi is sixteen times the data of the same page at 150 dpi. When a user complains that scans are too large to email, the scan profile's resolution and color mode are the first settings to check.

A detail that surprises many technicians: most Universal Serial Bus (USB) barcode scanners present themselves to the computer as a keyboard. The scanned code arrives as if someone had typed it very quickly, usually followed by Enter. That is why they need no driver — and why a scanner "types" into whatever window has the focus.

Signature pads and game controllers

A signature pad is a small touch-sensitive surface, usually with a stylus and often with a screen, on which a customer signs. Banks, shops, parcel services and hospitals use them to capture a handwritten signature digitally. They normally need the vendor's driver and software, because the application must receive the signature data in its own format.

Game controllers (gamepads, joysticks, steering wheels) connect over USB or Bluetooth. Most present themselves as a standard human interface device (HID; see below), so basic buttons and sticks work at once; advanced features such as force feedback may need extra software.

Webcams, microphones and headsets

A webcam is a small video camera for video calls and recording, built into the display bezel of a laptop or connected over USB. Almost all USB webcams follow the USB Video Class (UVC) standard, so the operating system's built-in class driver handles them without extra software. Many have a microphone built in, and business models often have a physical privacy shutter.

Microphones, speakers and headsets connect in two ways:

• Analog, through the 3.5 mm jacks: traditionally green for line out (speakers or headphones) and pink for the microphone. Laptops and phones often have one combined jack for a headset with a four-contact plug.
• Digital, over USB or Bluetooth: the device contains its own sound hardware and appears to the computer as a separate audio device.

KVM switches

A KVM switch (keyboard, video, mouse) lets one keyboard, one monitor and one mouse control two or more computers. Each computer connects to the switch with a video cable and a USB cable; a button, a keyboard shortcut or a hotkey switches the set to another computer. Uses:

• A desk with a work laptop and a personal computer, or a developer with a test machine next to the main one.
• A server rack, where a rack-mounted KVM with a fold-out screen and keyboard serves many servers.
• A KVM over IP (Internet Protocol) switch sends the keyboard, video and mouse over the network, so an administrator can operate a server's console — including its firmware setup screens — from elsewhere.

When buying one, match the video connectors (High-Definition Multimedia Interface, HDMI, or DisplayPort) and the resolution and refresh rate of the monitor; cheap switches may not pass through high refresh rates or audio.

Connecting peripherals

USB and device classes

Most wired peripherals use USB. It is hot-swappable — devices can be connected and removed while the computer runs — and it supplies power, so a keyboard or webcam needs no power adapter. The versions, speeds and connectors are covered in Ports, Connectors and Cables.

What makes USB peripherals so easy is the idea of device classes. The USB standards define common classes, and every modern OS ships a driver for each of them. A device that follows a class works without a vendor driver:

• Human interface device (HID): keyboards, mice, game controllers, most barcode scanners.
• Video (UVC): webcams.
• Audio: USB headsets, microphones, speakers.
• Mass storage: flash drives, external disks.
• Printer: basic driverless printing, using the Internet Printing Protocol (IPP) over USB.

Bluetooth

Bluetooth is a short-range wireless standard for connecting devices directly to each other, without an access point or router: a small personal area network (PAN). It uses the 2.4 GHz band, like many Wi-Fi networks, and is managed by the Bluetooth Special Interest Group (SIG). A few points a technician should know:

• Versions: Bluetooth 4.0 added Bluetooth Low Energy (BLE), which lets devices such as fitness trackers and sensors run for months on a small battery. Bluetooth 5 improved the range and speed of Low Energy connections. At the time of writing, devices with 5.x are the norm and version 6 (6.0 in 2024, followed by 6.1 and 6.2) has been published. Versions are backward compatible, but a connection only uses the features both sides support.
• Range: the specification defines power classes; the rough figures usually quoted are about 1 m for Class 3, about 10 m for Class 2 (typical for headsets and mice) and up to about 100 m for Class 1. Walls, bodies and interference reduce this considerably in practice.
• Profiles define what a connection can do: audio streaming, hands-free calling, input devices, file transfer. A headset that plays music but does not work as a microphone in calls is often connected with the wrong profile.

Before two Bluetooth devices can talk, they must be paired: they exchange keys once and remember each other afterward. The steps are the same on every platform:

1. Turn Bluetooth on at the computer or phone.
2. Put the peripheral in pairing (discoverable) mode — often by holding a button until a light flashes.
3. Select the device in the list of discovered devices.
4. Confirm the pairing: compare a numeric code on both screens, type a code, or — for devices without a display — accept a fixed personal identification number (PIN) or nothing at all.
5. Test that the device works.

Wireless dongles

Many wireless keyboards and mice do not use Bluetooth but a proprietary 2.4 GHz radio with a small USB receiver, the dongle. The receiver is paired with its devices at the factory, so the set works the moment it is plugged in — also in the firmware setup screen before the OS has loaded, where Bluetooth is usually not yet available. The downsides are a USB port taken by the receiver, and a lost receiver usually means the vendor's pairing software is needed to pair a replacement. If a wireless mouse stutters, move the receiver to a front port or an extension cable closer to the mouse: a receiver behind a metal case, next to a USB 3 port or disk, can suffer interference.

Device drivers

A device driver is software that lets the operating system use one specific kind of hardware. Applications do not talk to hardware directly. A word processor asks the OS to "print this page"; the OS passes the request to the printer's driver, which knows the commands that particular printer understands. The driver is the translator between the general requests of the OS and the specific language of the device.

Where drivers come from

• Built into the OS: class drivers for HID, video, audio and storage devices, and a large library of drivers for common hardware.
• Through the OS update service: on Windows, Windows Update can deliver drivers automatically when a device is connected.
• From the manufacturer's support website: the safest source for a driver that is missing, outdated or needs the vendor's full software. Pick the driver by the exact model number and the OS version and architecture (for example 64-bit Windows 11 on x64 or on ARM).

Avoid "driver updater" tools and download sites that are not the manufacturer's: they are a well-known route for malware. A driver runs with the highest privileges on the system, so a malicious one can do anything.

Signed drivers and plug and play

That is also why operating systems check driver signing: a digital signature proves who published the driver and that it was not changed afterward. 64-bit versions of Windows refuse to load kernel-mode drivers that are not properly signed. If a driver install stops with a signature warning, treat that as a red flag, not as an obstacle to click away.

Plug and play (PnP) is the process that makes a new device work without manual setup: the OS notices the device, reads its identifiers (on USB, a vendor ID and product ID), finds a matching driver, loads it and assigns the resources the device needs. When no matching driver is found, the device still appears, but as an unknown device.

Configuring and troubleshooting peripherals

Work from the outside in, starting with what is quickest to check:

1. Physical: is the cable seated, is the device on, are the batteries charged? Try another port — preferably directly on the computer, not through a hub or dock — and another cable.
2. Power: a bus-powered hub may not supply enough current for several devices; an external drive or scanner that disconnects at random may need a powered hub or its own adapter.
3. Wireless: is it paired, is the receiver plugged in, is Bluetooth or airplane mode switched the right way?
4. Driver: does the OS see the device, and is its driver working?
5. Settings: default device, keyboard layout, privacy permissions, application settings.
6. Isolate: try the device on another computer, or another device on this computer, to learn whether the problem follows the device or stays with the computer.

The general method behind these steps is on Hardware Troubleshooting.

Device Manager on Windows

Device Manager (devmgmt.msc) lists every device Windows knows about, grouped by type. The icons tell you the state at a glance:

• A yellow triangle with an exclamation mark: the device has a problem. Its Properties show an error code, such as Code 28 (the drivers for this device are not installed) or Code 43 (Windows stopped the device because it reported problems).
• A down arrow: the device is disabled.
• An entry under Other devices or named Unknown device: Windows found hardware but no driver for it.

From the device's Properties you can update the driver, uninstall it (and let Windows reinstall it after a restart or a hardware scan), or roll back to the previous driver — the quickest fix when a device stopped working right after a driver update. The same information is available in PowerShell. This command lists only the devices that report a problem (illustrative output):

Get-PnpDevice -PresentOnly | Where-Object Status -ne 'OK'

Status     Class           FriendlyName                 InstanceId
------     -----           ------------                 ----------
Error      Camera          USB2.0 HD Webcam             USB\VID_…&PID_…&MI_00\…
Error                      Unknown device               USB\VID_…&PID_…\…

lsusb on Linux

On Linux, lsusb lists the USB devices, and lsusb -t shows them as a tree, with the device class, the driver bound to each interface and the negotiated speed. A device with no driver shows Driver=[none]. Illustrative output for a keyboard and a webcam:

lsusb -t

/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 480M
    |__ Port 2: Dev 2, If 0, Class=Human Interface Device, Driver=usbhid, 12M
    |__ Port 2: Dev 2, If 1, Class=Human Interface Device, Driver=usbhid, 12M
    |__ Port 3: Dev 3, If 0, Class=Video, Driver=uvcvideo, 480M
    |__ Port 3: Dev 3, If 1, Class=Video, Driver=uvcvideo, 480M
    |__ Port 3: Dev 3, If 2, Class=Audio, Driver=snd-usb-audio, 480M

Read it from left to right: the keyboard (device 2) has two interfaces, both handled by the generic HID driver; the webcam (device 3) has video interfaces handled by the UVC driver and an audio interface for its microphone. The last column is the speed: 12M is USB full speed (12 Mbit/s), plenty for a keyboard; 480M is USB 2.0 high speed.

Accessibility devices

Peripherals are also how many people with a disability use a computer. Examples a technician may be asked to set up:

• Alternative keyboards (large keys, one-handed layouts) and alternative pointing devices such as trackballs, head pointers and eye-tracking cameras.
• Switch access: one or more large buttons that, with scanning software, let someone operate a computer with a single movement.
• Refreshable braille displays, which raise pins to form braille characters, used together with a screen reader.
• Headsets and microphones for speech recognition, and adaptive game controllers.

Most of these connect over USB or Bluetooth like any other peripheral, and many work together with accessibility features built into the OS. Treat them as critical equipment: for their user, a broken switch is not an inconvenience but a computer that cannot be used at all.