External hardware devices revision notes

1. Input and output devices

The specification requires consideration of the main characteristics, purposes, suitability and principles of operation of four external devices:

• Barcode reader: an input device. Its purpose is to provide barcode data to a computer system. When revising it, be able to explain what information it supplies, the characteristics that make it suitable for its intended use, and the principle by which it reads the barcode.
• Digital camera: an input device used to provide digital image data to a computer system. A good answer should connect its purpose and characteristics to the application in which it is used, rather than simply naming it.
• Laser printer: an output device that produces printed output. Explain why its characteristics make it suitable or unsuitable for a particular application and describe its operating principle at the level expected by the specification.
• RFID: an external device or technology considered as an input method. Explain the information it provides, its purpose, its suitability for an application and its principle of operation.

For any device question, use the chain: device -> input or output -> purpose -> relevant characteristic -> suitability -> operating principle. A bare list of devices is not enough.

2. Why secondary storage is needed

Secondary storage is required within a computer system so that data can be stored using a secondary-storage device rather than relying only on the computer's immediate working storage. In an exam answer, explain the need first, then compare devices using capacity and speed of access before reaching a judgement about suitability for the stated application.

The devices to know are:

• Hard disk
• Optical disk
• Solid-state disk (SSD)

3. SSD operation

An SSD consists of NAND flash memory and a controller. The controller manages pages and blocks, including the complexities of writing. The storage is based on floating-gate transistors that trap and store charge.

A block is made up of many pages. A page cannot simply be overwritten: it has to be erased before it can be written to. However, the technology requires the whole block to be erased. This creates a distinction between the size of the data being changed and the larger unit that must be erased. The controller manages these pages, blocks and writing complexities.

An SSD has lower latency and faster transfer speeds than a magnetic disk drive. This makes speed an important part of a suitability judgement, but an answer should still compare both capacity and speed of access for the particular application.

Common errors

• Calling every listed device an input device: a laser printer is an output device.
• Describing an SSD only as fast without explaining NAND flash memory, floating-gate transistors, pages, blocks and the erase restriction.
• Saying that an SSD overwrites a page directly. The page must be erased before being written to, and the whole block must be erased by the technology.
• Comparing devices without making a final suitability judgement.
• Giving a device name without explaining its purpose, characteristics or principle of operation.