Hardware Devices: I/O and Secondary Storage

A barcode reader reads a pattern of parallel black and white bars that encodes a product identifier.

How it works:

1. A laser or LED emits light across the barcode
2. A photodetector measures the reflected light: black bars absorb light (low reflection); white spaces reflect (high reflection)
3. The pattern of reflection times is decoded into the widths of alternating bars and spaces
4. These widths map to binary digits: narrow = 0, wide = 1 (or similar encoding depending on the barcode standard)

Key point: the barcode encodes data as the relative widths of bars, not as the presence/absence of a single mark.

A digital camera captures a scene as a grid of pixel colour values.

How it works:

1. Light passes through the lens onto a CCD (Charge-Coupled Device) or CMOS sensor
2. The sensor is an array of millions of light-sensitive cells (photosites)
3. Each photosite converts the intensity of incoming light into an electrical charge proportional to brightness
4. An ADC converts each charge to a digital value (the pixel's colour/brightness)
5. The digital values form the bitmap image

CCD vs CMOS: both convert light to charge, but differ in how the charge is read out. CMOS sensors are lower power and are now standard in smartphones.

A laser printer produces high-quality text and images using an electrostatic process.

How it works:

1. The laser scans across a charged photosensitive drum, removing charge from areas that should be printed
2. Toner (fine powder) is applied to the drum — it sticks only to the discharged (printed) areas
3. The drum rolls over the paper, transferring toner to it
4. A fuser heats the paper, melting the toner and bonding it permanently

Key point: the laser never touches the paper. It charges/discharges the drum, and the drum transfers toner. Heat is used to fuse, not to print.

RFID (Radio Frequency Identification) allows data to be read from a small tag without physical contact.

Components:

• RFID tag: stores a small amount of data; contains an antenna; passive tags have no battery
• RFID reader: emits radio waves that power the passive tag and request data

How it works:

1. Reader emits radio waves
2. The antenna in the passive tag harvests energy from the radio waves
3. The tag uses this energy to transmit its stored data back to the reader
4. Reader decodes the response

Uses: stock tracking in warehouses, contactless payments, library book management, access control cards, passports.

Advantage over barcodes: RFID tags can be read without line-of-sight, through packaging, and from greater distances.

An HDD stores data magnetically on spinning platters.

Structure:

• Multiple magnetic platters mounted on a central spindle — both surfaces used
• Read/write heads on movable arms float nanometres above the platter surface
• Actuator arm moves heads across the platter (seek motion)
• Platters spin at 5400–7200 RPM (revolutions per minute)

How data is stored: each tiny area of the platter can be magnetised in one of two directions (N or S pole), representing 0 or 1.

Access time: determined by seek time (moving heads) + rotational latency (waiting for data to spin under the head) + transfer time.

Characteristics: high capacity (TB range), relatively low cost per GB, but slower and more fragile than SSDs (moving parts).

Optical disks (CD, DVD, Blu-ray) store data as a spiral track of pits and lands on a reflective disc.

• Pit: a depression in the surface; laser reflects less (or phase changes in re-writable disks)
• Land: flat area; laser reflects more

A laser reads the transitions between pits and lands; the timing of transitions encodes binary data.

Type	Capacity	Use
CD	~700 MB	Audio, small software
DVD	4.7 GB (single layer)	Video, software
Blu-ray	25 GB (single layer)	HD video, large data

Advantages: portable, durable against magnetic fields, cheap to manufacture. Disadvantages: fragile to scratches, much slower access than HDD/SSD.

An SSD stores data in NAND flash memory — arrays of floating-gate transistors that trap charge to represent 0s and 1s.

Structure:

• Data is stored in pages (typically 4–16 KB each)
• Pages are grouped into blocks (typically 128–256 pages per block)
• A controller manages reads, writes, wear levelling, and garbage collection

Critical constraint: a block must be erased entirely before any page within it can be rewritten. This is why writes to an SSD are more complex than reads — the controller must erase a block (copy live data elsewhere, erase, rewrite) before updating a single page.

Characteristics:

• No moving parts — silent, shock-resistant
• Much faster random access than HDDs (no seek time)
• Lower latency — data accessible in microseconds vs milliseconds for HDDs
• More expensive per GB than HDDs
• Finite write endurance (cells wear out after many erase cycles)

	HDD	Optical	SSD
Capacity	TB range	GB range	GB–TB range
Speed	Slow (mechanical)	Very slow	Fast
Access time	~5–15 ms	~100–150 ms	~0.1 ms
Cost per GB	Low	Very low	Higher
Durability	Poor (fragile, moving parts)	Poor (scratches)	Good (no moving parts)
Suitable for	Bulk storage, backups	Distribution, archiving	OS, applications, portables

1. Stating a laser "burns" data onto the drum in a laser printer

The laser does not burn. It selectively discharges (removes charge from) the photosensitive drum. Toner then adheres to discharged areas. The fuser uses heat to bond toner to paper — not the laser.

2. Confusing pits/lands with 0/1 directly

It is the transition between pits and lands (and the timing of transitions) that encodes data — not pits as 0 and lands as 1. The actual encoding depends on the standard (CD uses EFM encoding).

3. Claiming SSDs can rewrite individual pages directly

SSDs cannot overwrite a page without first erasing the entire block it belongs to. Pages can be read and written fresh (to an already-erased block), but updating existing data requires an erase cycle at block level.

4. Saying RFID requires line-of-sight

RFID does not require line-of-sight — the radio signal passes through packaging, clothing, and many other materials. This is what differentiates RFID from barcodes (which require a clear optical path).