Proximity card (or prox card) is a generic name for contactless integrated circuit devices used for security access or payment systems. The standard can refer to the older 125 kHz devices or the newer 13.56 MHz contactless RFID cards, most commonly known as contactless smartcards.
Modern proximity cards are covered by the ISO/IEC 14443 (proximity card) standard. Proximity cards typically have a range of 0 to 3 inches (0 to 8 cm) for reading, with a lower range for writing. The user will usually be able to leave the proximity card inside his wallet or purse. The price of the cards is low, usually US$2–$5 (retail), allowing them to be used in applications such as identification cards, keycards, payment cards and public transit fare cards.
There is also a related ISO/IEC 15693 (vicinity card) standard for longer range operation. Proximity cards as well as vicinity cards are powered by resonant inductive coupling via an LC circuit including an IC, capacitor, and coil are connected in parallel. The card reader produces an electromagnetic field that excites the coil and resonant current charges the capacitor, which in turn energizes and powers the IC.
In reading process, the IC transmits the card number via the coil to the card reader. The card readers communicate in various protocols, for example the Wiegand protocol that consists of a data 0 and a data 1 circuit (or binary or simple on/off (digital) type circuit). The earliest cards were 26 bit. As demand has increased bit size has increased to continue to provide unique numbers. Often, the first several bits can be made identical; these are called facility or site code. The idea is that company A has a facility code of xn and a card set of 0001 through 1000 and company B has a facility code of yn and a card set also of 0001 through 1000. Numbering system is internationally harmonized and allocated by Netherlands based NEN as international registration authority according to ISO 6523 and ISO/IEC 15459 standard.