ATR statistics: TD1 - Structural, encodes Y2 and T

Article from the series "ATR statistics"

TD1 - Structural, encodes Y2 and T

The ISO 7816-3 specification is not public. So I can't copy/paste part of the text. I will use Wikipedia instead.

From Wikipedia https://en.wikipedia.org/wiki/Answer_to_reset#Interface_bytes_TDi:
Interfaces bytes TDi for i≥1, if present, are structural.
TDi encodes in its 4 high-order bits the presence of at most 4 other interface bytes: TAi+1 (resp. TBi+1, TCi+1, TDi+1) follow, in that order, if the 5th (resp. 6th, 7th, 8th) bit of TDi is 1.
TDi encodes in its 4 low-order bits (4th MSbit to 1st LSbit) an integer T, in range [0..15]. T = 15 is invalid in TD1, and in other TDi qualifies the following TAi+1 TBi+1, TCi+1, TDi+1 (if present) as global interface bytes. Other values of T indicates a protocol that the card is willing to use, and that TAi+1 TBi+1, TCi+1, TDi+1 (if present) are specific interface bytes applying only to that protocol. T = 0 is a character-oriented protocol. T = 1 is a block-oriented protocol. T in the range [3..14] is RFU.
Historical note: provision for dynamically qualifying interface bytes as global using T = 15 did not exist in ISO/IEC 7816-3:1989.

TD1 # %
899 43.39 %
0x80 459 22.15 %
0x81 372 17.95 %
0x40 95 4.58 %
0x00 69 3.33 %
0x91 46 2.22 %
0xC0 45 2.17 %
0xC1 27 1.30 %
0x10 22 1.06 %
0x50 16 0.77 %
0x01 9 0.43 %
0x0E 8 0.39 %
0x11 2 0.10 %
0x1F 1 0.05 %
0x31 1 0.05 %
0x3F 1 0.05 %



TD1 (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 43% of the ATRs no TD1 is present. So no other TA2, TB2, TC2 or TD2 is present and no protocol is defined so the default T=0 will be used.

For 22% of ATRs TD1 = 0x80 so bit 8 is set to indicate that a TD2 is present and T=0 is used. One such ATR is 3B 80 80 01 01.

For 17% of ATRs TD1 = 0x81 so, as in the previous case, TD2 is present but T=1 is used. One such ATR is 3B 82 81 31 76 43 C0 02 C5

For 5% of ATRs TD1 = 0x40 so TC2 is present and T=0 is used. One such ATR is 3B 85 40 20 68 01 01 00 00

I will not document all the other cases. I let this exercise to the reader.

One special case is TD1 = 0x?E to indicate the T=14 protocol.

From ISO 7816-3:
The type T refers to a transmission protocol and/or qualifies interface bytes.
  • T=0 refers to the half-duplex transmission of characters specified in clause 10.
  • T=1 refers to the half-duplex transmission of blocks specified in clause 11.
  • T=2 and T=3 are reserved for future full-duplex operations.
  • T=4 is reserved for an enhanced half-duplex transmission of characters.
  • T=5 to T=13 are reserved for future use by ISO/IEC JTC 1/SC 17.
  • T=14 refers to transmission protocols not standardized by ISO/IEC JTC 1/SC 17.
  • T=15 does not refer to a transmission protocol, but only qualifies global interface bytes.

As you can see in the list above "T=14 refers to transmission protocols not standardized by ISO/IEC JTC 1/SC 17." In my list T=14 is used only by pay TV cards like 3B 9F 21 0E 49 52 44 45 54 4F 20 41 43 53 03 83 95 00 80 55.

TD1 = 0x?F is also another special case to indicate T=15, which is not a protocol, and will change the interpretation of the following ATR bytes.