TB2 - 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_TB2
TB2, if present, is global. The usage of TB2 is deprecated since the 2006 edition of the standard, which prescribes that cards should not include TB2 in the ATR, and readers shall ignore TB2 if present.
In the 1997 edition of the standard, TB2 (8th to 1st bit) encode PI2, which when in range 50..250 (other values being RFU) encode VPP in increments of 0.1 V, and subsumes the coarser indication given by PI1 of TB1. Refer to that section for why modern Smart Cards have no use of VPP, and thus of TB2.
Historical note: Provision for TB2 did not exist in ISO/IEC 7816-3:1989, and was introduced because VPP = 12.5 V became a popular value in EEPROM technology, replacing 25 V and 21 V.
Only 2 cards use a TB2 value.
- 3B FF 00 FF 81 31 00 45 65 63 00 00 00 00 00 00 00 00 00 00 00 00 00 00
- TB2= 0x45 Programming param PI2 (PI1 should be ignored): 69 (dV)
- This card also has TB1 = 0xFF Programming Param P: 31 Volts, I: RFU
- 3F FF 3F 3F 3F 3F 00 3F 3F FF 3F 3F 3F 3F 3F FF 3F FF 95 3F FF 95 3F FF
- TB2 = 0x3F Programming param PI2 (PI1 should be ignored): 63 (dV)
- This card also has TB1 = 0x3F Programming Param P: 31 Volts, I: 50 milliAmperes