1.1.3 - Choosing input, output and storage

Capturing the right data

An input device supplies data or control signals to a computer. Start a choice by identifying what must be captured: typed text, a selection, sound, an image, an identifier or a physical measurement. The device must also fit the people and environment using it.

A keyboard suits long written responses because it supports precise text entry. A touchscreen suits a public ticket machine with a small set of large choices: the user can select the displayed destination directly. A mouse gives precise pointer control but needs a suitable surface. A microphone captures sound; a camera captures images or video. None automatically determines the meaning of what it captures: interpreting speech or recognising an object also requires software.

For automatic data collection, a temperature sensor supplies measurements without a person repeatedly typing readings. A barcode reader captures the encoded item identifier quickly at a checkout. The identifier is then looked up in a database to obtain a price; the barcode need not contain the price itself. A card reader can capture an identifier from a membership card. These devices reduce repeated manual entry, but damaged labels, poor lighting or failed sensors can still produce missing or wrong data. Validation and a fallback entry method may be needed.

Consider a greenhouse that records temperature every minute, including overnight. A temperature sensor is appropriate because the required input is a physical measurement and the collection must operate unattended. A keyboard could enter measurements, but it would require someone to read a thermometer and type each value. A camera would capture a scene, not directly provide the required numeric temperature. The reasoning links the device's data to the actual requirement.

Presenting information and causing action

An output device presents processed results or turns a computer's decision into a physical action. Choose the form of output needed, rather than assuming every result should appear on a screen.

A display can show changing information immediately, such as a train's current platform. A printer produces a physical copy that can be carried or read without the computer. Speakers provide sound, including an alarm when a user is not looking at a display. Headphones suit private audio but would be unsuitable as the only public evacuation alarm. An actuator, such as a motor or valve, changes the physical environment: a greenhouse controller can open a vent when a temperature exceeds a threshold. A visual message alone would leave a person to open it.

Suitability also depends on accessibility and conditions. A ticket machine might combine a display with spoken instructions for users who cannot see small text. A noisy workshop may need a visible warning as well as sound. A touchscreen combines input and output roles: it displays choices and senses a touch. Classify the particular operation, not the whole device as permanently belonging to just one category.

For a remote pumping station, the controller compares a water-level measurement with a limit. Sending a control signal to a valve actuator can stop the inflow. Showing the level on a display helps an operator monitor the station, but it does not itself stop water. The same processed result can therefore require more than one output with different purposes.

Magnetic disks and tape

Secondary storage retains programs and data when power is removed. Its capacity is the amount it can hold; its access time is the delay before required data can be reached; its transfer rate concerns how quickly data moves once transfer is underway. These are different properties.

Magnetic storage represents data using magnetised regions of a surface. In a hard disk drive, spinning platters and read/write heads give access to locations on tracks. The head must reach the appropriate track and wait for the required region to rotate into position. This mechanical movement adds delay and makes shock while operating a concern. Hard disks can provide large capacity at a relatively low cost per unit of storage, making them useful for substantial collections of files where those trade-offs are acceptable.

Magnetic tape instead stores data along a long strip. Reaching a particular late record may require winding past earlier regions: it is primarily sequential access. A tape system can transfer a large backup stream efficiently once positioned, so “sequential” does not mean that every transfer is slow. It is poorly suited to continually retrieving unrelated small files, but well suited to large scheduled backups or archives that are rarely retrieved. Tape media can also be removed and stored away from the original machine.

A video archive that frequently opens different recordings might use hard disks for its active collection. A second copy of the full collection written overnight could use tape because the backup is a large sequential operation. A removable copy is useful, but merely owning tape does not guarantee a good backup: copies must be current, protected and tested for restoration.

Optical and flash storage

Optical storage uses a laser and differences in how areas of a disc reflect light to read encoded data. On a manufactured read-only disc, pits and lands produce detectable changes; recordable and rewritable media use other physical changes to create the readable pattern. Avoid treating a visible pit as necessarily one binary digit: the drive decodes an encoding of the changing signal.

CD, DVD and Blu-ray are optical formats with different capacities. A supplied film on a read-only disc can be distributed without its contents being casually overwritten. Discs are removable and relatively inexpensive individually, but require a compatible drive, can be scratched, and generally provide less convenient high-capacity, frequent updating than disk or flash solutions. A rewritable disc is not permanent read-only storage.

Flash storage stores data through electrical charge states in memory cells. The charge state remains without power. Reading does not require a spinning platter or moving head. Solid-state drives, USB flash drives and memory cards use flash, although their interfaces and controllers differ.

The absence of mechanical movement gives flash short access delays, low physical bulk and resistance to knocks, which suit portable devices. Writes and erases cause wear: flash has a finite endurance, and controllers may distribute writes across cells. It is therefore wrong to describe flash as indestructible or able to be rewritten forever. Cost per unit of storage and endurance must be considered alongside speed.

A helmet camera needs small removable storage that survives movement and records a continuous stream. A suitable flash memory card meets these needs, provided its capacity and write rate meet the recording requirements. An optical disc is bulkier and needs a moving drive; a hard disk has moving components vulnerable to knocks. This is a contextual argument, not a claim that one technology wins for every workload.

Justifying a complete device choice

A sound recommendation states the device or technology, identifies a relevant property and explains its consequence for the requirement. “It is faster” needs a meaning: short random-access delays may matter to an application opening many small files, while sustained writing matters to a recording device.

Suppose a school stores a large, frequently accessed media library on a fixed server. Capacity and cost per unit make a hard disk array a plausible choice. If rapid access to many small files is the dominant constraint and the budget allows it, solid-state storage may be justified instead. Neither choice can be marked solely from the word “school”; the workload and constraints determine the reasoning.

Now suppose the school sends each pupil one unchanging collection to use offline, and every pupil has a compatible DVD drive. A read-only optical disc may be suitable for distribution, although its capacity must hold the collection. Remove the stated drive availability and that recommendation becomes weaker. Stated assumptions can change a correct choice.

Use a matched comparison: compare cost per capacity with cost per capacity, access behaviour with access behaviour, and durability with durability. Then explain which feature is decisive in the given situation. It is acceptable to reach a different choice when the supplied constraints permit it and the reasoning is technically sound.