TD3 - Structural, encodes Y4 and TThe ISO 7816-3 specification is not public. So I can't copy/paste part of the text. I will use Wikipedia instead.
Refer to TD1 - Structural, encodes Y2 and T since the definition of TD3 is identical to TD1.
TD3 (as the other TDi bytes) is structural and indicates:
- How to interpret the other ATR bytes
- What communication protocol the card wants to use
For 96.96% of the ATRs no TD3 is present. So no other TA4, TB4, TC4 or TD4 is present and no new protocol is defined so the protocol(s) defined by TD1 and TD2 will be used.
For 2.94% of the ATRs TD3 = 0x1F. The high nibble is 0001b so TA4 is present and T=15 protocol is defined. One such ATR is 3B 94 18 81 B1 80 7D 1F 03 19 C8 00 50 DC.
For 0.10% of the ATRs TD3 = 0x3F. The high nibble is 0001b so TA4 and TB4 are present and T=15 protocol is defined. One such ATR is 3B DE 18 FF 81 F1 FE 43 00 3F 07 83 44 45 53 46 69 72 65 38 20 53 41 4D 2D 58 17.