Input and Output Devices

An input device brings data into the system; an output device presents results to the outside world. Most devices are clearly one or the other, and questions test the few that are not.

A touchscreen is both — it displays and it senses. So is a modem, sending and receiving. Storage devices are neither in the usual sense: data goes both ways, but to and from the machine's own memory rather than to a person.

A sensor measures a physical quantity continuously and passes the reading to a system. It is the input device that makes automated systems possible, and the syllabus expects specific types matched to specific applications.

The key point about every sensor: it produces an analogue reading, a continuously varying quantity. A computer works in digital values, so an analogue-to-digital converter stands between the two. A sensor cannot be wired straight into a processor.

The syllabus names several and asks you to choose between them, so the distinguishing characteristic of each is what to know.

For printers: an inkjet sprays droplets, giving high-quality colour at low purchase cost, but is slow and expensive per page — right for photographs and low volumes. A laser uses toner and a rotating drum, printing quickly and cheaply per page in large volumes — right for an office. A 3D printer builds an object in layers, used for prototypes and custom parts.

Other outputs include screens, speakers, projectors, and actuators — motors, valves, heaters — which are the output devices of a control system, doing something physical rather than presenting information.

Input devices are usually studied one at a time, but a sensor rarely works alone. It sits at the start of a chain, and the chain is what the exam asks about.

The reading is analogue, so it passes through an analogue-to-digital converter before a processor can use it. The processor compares it with a stored value and decides. If action is needed, the signal goes to an actuator — and the action changes the very quantity the sensor is measuring.

Step from Sense to Act. The sensor is only the first of four stages, and the loop closes because the actuator changes what the sensor is measuring.

Some input devices are designed to capture data without anyone typing it, which is faster and far more accurate. The syllabus names several and asks where each is appropriate.

A barcode reader identifies a product from a printed pattern. QR codes hold much more data and can be read from any angle by a phone. RFID reads a tag by radio without needing line of sight, so a whole pallet can be scanned at once. Magnetic stripe and chip readers handle cards, and OMR — optical mark recognition — reads pencil marks in fixed positions, which is how multiple-choice answer sheets are marked.