How does the magnetic strip of credit cards work?

The credit card magnetic stripe or magstripe as they are sometimes called carries loads of information. Each stripe is made up of iron-based magnetic particles that are in a plastic film with each one being a very small bar magnetic of about 20 millionths of an inch. These magnetic stripes are able to have information placed on them because each magnet can be magnetized in the direction of the north or south pole. Each magstripe has three tracks that are one-tenth of an inch wide and each track contains unique data.

Before describing how a magnetic card reader works, look at how data is stored on a magnetic card and the format of data storage collected.

• Data Storage: There are up to three tracks on credit card magnetic stripes or magstripes. These are numbered tracks 1, 2, and 3. Track 3 is rarely used, and may not even exist on credit cards because the magnetic stripe may not be wide enough to accommodate it.According to existing standards, a magnetic card stores information in three separate tracks. All three tracks possess different bit densities and encoded character sets. The average bit density of the first track is 210 bits per inch (bpi). A 64-bit character data set stores information in track 1. The characters are made up of six data bits and an odd parity bit. The encoding format grants the least significant bit to come first and the parity bit, last. So, track one can hold around 79 characters.

“Two-Frequency, Coherent Phase Recording” is used for encoding magnetic cards. It is also called F/2F sampling-encoding. By using combined data and clock bits, self-clocking is achieved.

• How data is stored? For all 3 tracks, the data storage format is as follows: Leading zero bits are encoded to specify the presence of an encoded magnetic card and provide synchronization pulses to the read head electronics and, ultimately, to the controller. The Start Sentinel character then indicates the initiation of actual data. The coded data follows the Start Sentinel character, which the End Sentinel character follows. The End Sentinel character terminates the card’s data portion, which an LRC byte (used for error detection) follows. Zero bits fill the remaining card.

Now that it is understood how data is stored on a magnetic card, it is a bit easier to build on that foundation to gain an understanding of how the reader actually works.

The magnetic card reader uses a specific component to read data from a magnetic card, which is referred to as the read head. The magnetic card reader is a microcontroller-based device. The read heads contain signal amplifiers and line drivers. All modern magnetic read heads contain integrated F/2F bit recovery circuitry and interface with the host controller.

The magnetic card reader is a microcontroller-based device and has been programmed for a specific application. That program simply reads the card in a forward direction in a simple data format or it can be complex enough to read the card in any direction with a corresponding encoding format.