ISO/IEC 15693 forms part of a series of International Standards that specify a contactless smart card. The card can be carried by members of the public in a purse or wallet and when presented nearby a terminal device give access to places, goods or services. In addition, the card can be attached to objects like bags and valuable items which can then be tracked whilst in the vicinity of a reading device. ISO/IEC 15693-2:2006 defines the power and communications interface between the vicinity card and the reading device. Other parts of ISO/IEC 15693 define the physical dimensions of the card and the commands interpreted by the card and reader.
Power is coupled to the vicinity card by an a.c. field produced in the reader, also known as a coupler; the powering field has a frequency of 13,56 MHz and is one of the industrial, scientific and medical (ISM) frequencies available for worldwide use. When sufficient power is received by the card, it is able to respond to commands sent from the coupler. The coupler sends commands to the card by modulating the powering field and by using a modulation system known as pulse position modulation, whereby the position of a single pulse relative to a known reference point codes the value of a nibble or byte of data. This allows the card to draw the maximum energy from the field almost continuously. Vicinity cards, which have no power source, can be energized at ranges of up to 1 m from a coupler that can only transmit power within the limits permitted by international radio frequency (RF) regulations.
A vicinity card only responds when it receives a valid command that selects a single card from a possible collection of cards within range of the coupler. This process of collision detection and selection, also known as anti-collision, is made possible by detecting the unique identification number encoded into every card. Anti-collision and the commands used are defined in ISO/IEC 15693-3. The card responds to the coupler by drawing more or less power from the field and generates one or two sub-carriers of around 450 kHz. These are switched on and off to provide Manchester-encoded data that are then detected by the coupler.
Thus both power and bi-directional communications form the air interface between the vicinity card and the coupler. It is the flexibility of the interface to select one or two sub-carriers when communicating from card to coupler, whilst also using slow or fast data rates from the coupler to the card, that allows systems to be tuned to suit different operational requirements ranging from use with high RF noise at short range to low RF noise at long range.