4.7.4.1 - Input and output devices

Barcode readers

A barcode reader obtains an encoded identifier without requiring someone to type it.

For a conventional one-dimensional barcode, dark bars and light spaces form a pattern. In a laser reader, a light beam scans across the code and a detector receives reflected light: light spaces reflect more than dark bars. The changing signal represents the pattern, which the reader's decoding circuitry converts into characters. Imaging readers instead capture an image and decode the pattern from it. The method differs, but both convert the printed code into data.

A shop normally uses the decoded product identifier to look up the item in a database. The price and description need not be printed inside the barcode. Changing a price can therefore update the database without changing every label. Codes can also identify parcels, library books or patient samples. A barcode is inexpensive to print, and automated capture reduces typing errors and speeds up repeated entry.

The reader must be able to see a sufficiently clear code. Dirt, damaged printing, glare, obstruction or unsuitable positioning can prevent a read. Conventional readers need line of sight and a suitable working distance; they cannot read a label hidden inside an opaque box. A successful read identifies the code, but does not itself prove that the correct label was attached to the item.

Digital cameras

A digital camera converts light from a scene into a stored digital image.

A lens focuses light onto an image sensor, commonly a CMOS or CCD sensor. Its array of light-sensitive elements produces electrical signals related to the light received. Sampling and analogue-to-digital conversion turn these signals into numerical values. The camera processes the values into pixels, including colour information, and encodes the image for storage. A memory card or other secondary storage retains the image; the sensor is not the long-term file store.

Resolution describes the number of pixels recorded, often stated as width × height or a total pixel count. More pixels can record finer spatial detail when the lens, focus, lighting and sensor permit it, but also produce more data at the same bit depth and compression. Pixel count alone does not guarantee better photographs: noise, blur, poor lighting or a limited lens can dominate. Colour depth concerns the number of bits used for colour values and is a different characteristic from spatial resolution.

Digital cameras are suitable when the computer needs visual evidence, such as documenting an object, capturing a scene or inspecting a manufactured part. Images can be checked, stored, processed and transmitted without developing photographic film. A camera captures the appearance of a scene; identifying a specific object or reading a code from the image requires subsequent processing. For simply capturing a known product code, a barcode reader may be the more direct and reliable tool.

Laser printers

A laser printer turns an electronic page description into a permanent pattern of toner on paper.

The printer/controller receives page data, stores it in a buffer and forms a raster image describing where marks belong. A photosensitive drum is given an electrostatic charge. A laser is directed across the rotating drum, changing charge in a pattern corresponding to the page image. Fine toner powder is attracted to the appropriate charged areas, forming the image on the drum. The toner image is transferred to paper using electrostatic attraction. Heated pressure rollers in the fuser bond the toner to the paper; the drum is cleaned and prepared for reuse.

The laser creates the charge pattern on the drum. It does not burn the letters directly into the paper. Toner is a powder, rather than the liquid ink sprayed by an inkjet printer. Describing the controlled charge pattern avoids assuming that every printer uses the same charge polarity.

Laser printers are suitable for repeated text-heavy office output because they can produce many sharp pages quickly. Resolution, often expressed in dots per inch, affects how finely marks can be placed; pages per minute describes throughput, not image resolution. A networked office printer can buffer jobs from several users. The suitability decision also includes initial cost, consumables, energy, maintenance, colour requirements and the volume of printing. A fast monochrome office printer cannot satisfy a requirement for colour photographs merely by being fast.

RFID and device suitability

RFID uses radio communication to read data from an electronic tag rather than an optically visible printed pattern.

An RFID tag contains an integrated circuit storing data and an antenna. An RFID reader emits a radio signal and receives a tag's response, then decodes its identifier or other stored data. A passive tag obtains energy from the reader's field; it does not need its own battery. It communicates a response using the supplied radio energy. An active tag has its own power source. Power source, frequency and design affect range, cost and intended use, so there is no single range for all RFID systems.

RFID is suitable for stock identification, access cards and tracking tagged assets. It does not require optical line of sight, so tags may be read through suitable packaging within the system's working range. Suitable readers can identify multiple tags using protocols that manage competing replies; they do not just treat overlapping responses as one code. Metal, liquids, interference, orientation and shielding can still affect a read. Lack of optical line of sight does not mean every tag is readable through every material at any distance.

Compare the requirement with the operation. A cheap printed label deliberately presented at a checkout favours a barcode. Identifying several tagged boxes without exposing their labels may favour RFID, provided cost and the materials/range are suitable. Capturing a scene needs a camera. Producing a permanent paper record needs a printer. A reader's decoded ID often selects a database record; the tag need not store the whole stock database.

Contactless capture also creates access-control and privacy questions. A system should limit who can read or use the identifiers and consider whether a tag should remain readable after sale. These concerns affect suitability; they do not change RFID's basic input mechanism.