Cards, Tape and Discs: How the Bit Is Read

About this interactive

The first three Storage Media lessons show three ways to hold a bit, and how a machine reads each one back. Punch cards: a column on the lesson's card has ten rows, 0 to 9, and the punched row is the digit, so the card is decimal, not binary. A reader finds the holes either with a probe that completes a circuit through the hole, or with a light and a photo sensor that sees light only where there is a hole. Magnetic media: tape, floppy disks and hard drives are divided into tiny sections, and each section is magnetized in one direction or the other. The read/write head's coil sets that direction to write a bit and senses it to read one. Optical media: a focused laser bounces off a reflective layer into a sensor. Disrupt the reflection and the sensor sees the difference. The lesson calls removing the reflective surface the subtractive method, and adding something that blocks it the additive method. Read each situation, choose what happens, then read why.

What you'll learn

Predict how a punch card reader detects a punched hole by electrical contact or by light, and read a column as one decimal digit
Predict how a read/write head writes and reads bits as the direction of magnetized sections on tape or a disk
Predict how a laser, a reflective layer and a sensor read optical media, and tell the subtractive method from the additive one

Aligned to

CompTIA Tech+

2.5 Compare and contrast storage types.

CompTIA A+ Core 1

3.3 Given a scenario, select and install storage devices.

Key terms

Electrical Contact: A punch card reading method in which a probe and conductor complete an electrical circuit through a punched hole, sending a signal to the computer to register the encoded data.
Optical Sensing: A punch card reading method that uses a light source and photo sensor to detect holes in a card; when light passes through a hole, the sensor registers it as a signal for the computer to read.
Magnetic Polarity: The directional orientation of a magnetic field (north or south) used to represent binary values of 0 and 1 on magnetic storage media.
Read/Write Head: A component in magnetic storage devices that uses a magnetic coil to both write data by setting magnetic polarity and read data by sensing the direction of existing magnetic fields.
Laser: A highly focused beam of light used in optical media drives to read and write data by reflecting off or interacting with a disc surface.
Optical Media: Storage media, such as CDs and DVDs, that uses laser light reflected off a disc surface to store and read binary data.

Topics

Punch Cards Magnetic Storage Optical Storage Storage Media Interactive Predict

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