Answer To Reset (ATR) is a byte sequence from a smart card to the card reader after reset. The ATR instruction includes several parameters for communication between smart card and smart card terminal. It allows the read-write unit of a smart card terminal to determine the card parameters and the transport protocol used.
ATR protocols are distinguished between synchronous and asynchronous smart card communication. Synchronously operating smart cards know three different ATR protocols: the ISO 7816-3 with 16 bits and 7816-4 with 32 bits protocols specified according to ISO 7816, and the Telekom protocol according to B1. The 32-bit protocol according to ISO 7816-4 is compatible with the 16-bit protocol ISO 7816-3. The different bits of the protocols are used for the 16-/32-bit distinction, the regulation of the protocol parameters, the identification of the card manufacturer, the pointer for the memory area, the country code and the card application.
For the asynchronously operating chip cards, the clock frequency for the interface is obtained from the applied clock signal by frequency division. The interface format corresponds as far as possible to V.24 with one start bit, eight data bits, one parity bit and two stop bits. The entire ATR protocol comprises 31 bytes. The older, character-oriented T=0 protocol and the block-oriented T=1 protocol are used as protocol types.