TB1 - Global, deprecatedThe 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_byte_TB1 (with some edition to remove extra details):
TB1, if present, is global. The usage of TB1 is deprecated since the 2006 edition of the standard, which prescribes that cards should not include TB1 in the ATR, and readers shall ignore TB1 if present. EMV still requires that the card includes TB1 = ‘00’, and that remains common practice; doing so explicitly indicates that the card does not use the dedicated contact C6 for the purpose of supplying a programming voltage (VPP) to the card; the cards might however use C6 for Standard or Proprietary Use (SPU), such as communicating with a NFC front end by the Single Wire Protocol (SWP). On the reader side, EMV requires making a warm ATR for cards with TB1 other than ‘00’ in the cold ATR, and handling any TB1 in a warm ATR as if it was ‘00’.
TB1 was previously indicating (coarsely) the programming voltage VPP and maximum programming current required by some cards on the dedicated contact C6 during programming of their EPROM memory. Modern Smart Cards internally generate the programming voltage for their EEPROM or Flash memory, and thus do not use VPP. In the 1997 and earlier editions of the standard:
- The low 5 bits of TB1 (5th MSbit to 1st LSbit) encode PI1; if TB2 is absent, PI1 = 0 indicates that the C6 contact (assigned to VPP) is not connected in the card; PI1 in range [5..25] encodes the value of VPP in Volt (the reader shall apply that voltage only on specific demand by the card, with a tolerance of 2.5%, up to the maximum programming current; and otherwise leave the C6 contact used for VPP within 5% of the VCC voltage, up to 20 mA); if TB2 is present, it supersedes the indication given by TB1 in the PI1 field, regarding VPP connection or voltage.
- The high bit of TB1 (8th bits) is reserved, shall be 0, and can be ignored by the reader.
- The 6th and 5th bits of TB1 encode the maximum programming current (assuming neither TB1 nor TB2 indicate that VPP is not connected in the card).
(#) This was 100 mA in ISO/IEC 7816-3:1989.
7th and 6th bits 00 01 10 11 Maximum programming current 25 mA 50 mA RFU(#) RFU
Only 68 ATRs (3.2%) have a TB1 different from 0x00.
The TB1 value found are (in order of appearance):
- 0x25 "Programming Param P: 5 Volts, I: 50 milliamperes"
- 0x2F "Programming Param P: 15 Volts, I: 50 milliamperes"
- 0x35 "Programming Param P: 21 Volts, I: 50 milliamperes"
- 0x20 "VPP is not electrically connected"
- 0x3F "Programming Param P: 31 Volts, I: 50 milliamperes"
- 0xFF "Programming Param P: 31 Volts, I: RFU"
Most of the smart cards with TB1 = 0x25 are PayTV cards. maybe TB1 is using a non standard value to confuse the smart card reader.