ATR in Asynchronous Transmission
The standard defining the ATR in asynchronous transmission is ISO/IEC 7816-3. Subsets of the full ATR specification are used for some Smart Card applications, e.g. EMV.
The ATR proceeds in five steps: initial character TS; format byte T0; interface bytes TAi, TBi, TCi, TDi (optionals, variable number); historical bytes Ti (optionals, up to 15), and the check byte TCK (optional). There are a total of 2 to 33 characters including TS.
The initial character TS is always physically present, but is not part of the Answer-to-Reset in ISO/IEC 7816-3:2006, defined as: the value of the byte string (at most 32 bytes) encoded in the sequence of characters following the initial character TS. ISO/IEC 7816-4:2005 states that TS is a character or synchronization pattern, not a byte]. However practice (in PC/SC, EMV, ETSI, and Calypso at least) is still to consider that TS is part of the ATR, as it was in ISO/IEC 7816-3:1997 and former. In particular, the ATR returned by PC/SC card readers and software stacks includes TS as the first byte, with the value explicitly given in every edition of ISO/IEC 7816-3, see below.
Read more about this topic: Answer To Reset