ISO 7816 Part 3: Electronic Signals and Transmission Protocols
ISO/IEC 7816-3 is the part that turns eight metal pads into a working communication channel. It defines the electrical conditions the reader must provide, the sequence that brings the card to life, and the two transmission protocols — T=0 and T=1 — that carry bytes across the single I/O line.
The card is a peripheral with no power of its own
Everything in Part 3 follows from one fact: the card has no battery and no oscillator. The reader supplies VCC and CLK, and holds RST low until the supply is stable. The card does nothing until the reader releases RST. This is why “the card is unresponsive” is nearly always a reader-side or contact-side problem rather than a card fault.
Activation and the Answer to Reset
The activation sequence is deliberately ordered: apply ground, apply VCC, apply the clock, set I/O to reception mode, then release RST. The card responds with the Answer to Reset (ATR), a byte string that is the card’s self-description and the first thing worth capturing when debugging any integration.
The ATR is structured rather than opaque:
- TS — the initial character, which establishes bit convention.
3Bindicates direct convention,3Finverse convention. A reader that gets this wrong reads every subsequent byte as nonsense. - T0 — the format byte, indicating which interface bytes follow and how many historical bytes there are.
- TA/TB/TC/TD — interface bytes carrying timing, guard time, and protocol selection. TD indicates whether further interface bytes follow and which protocol they describe.
- Historical bytes — up to 15 bytes describing the card, discussed further in the historical bytes page in this section.
- TCK — a check byte, present when any protocol other than T=0 is offered.
Where the card offers more than one protocol or a faster speed than the default, the reader may negotiate using PPS (Protocol and Parameter Selection) before the first command.
T=0 and T=1: two different mental models
T=0 is byte-oriented and asymmetric. The reader sends a five-byte header, the card replies with procedure bytes that control the flow byte by byte, and data moves under the card’s direction. There is no notion of a command that both sends and receives data in one exchange, which is why T=0 needs GET RESPONSE to retrieve output after a command that also carried input.
T=1 is block-oriented and symmetric. Each transfer is a framed block with a node address, a protocol control byte, a length, an information field and an error-detection code. Blocks come in three kinds — information blocks carrying application data, receive-ready blocks acknowledging or requesting retransmission, and supervisory blocks handling housekeeping such as information-field size negotiation and resynchronisation. Because framing and error recovery live in the protocol itself, T=1 carries a full command and its response naturally.
The practical consequence: an application that works over T=1 can fail over T=0 purely because of the GET RESPONSE and length-correction handling, and vice versa. If you support both, test both.
Practical implementation notes
- Log the raw ATR. It identifies the card, the convention, the protocols offered and the negotiable speed. Most integration questions are answered by it.
- Respect voltage classes. Cards declare which supply classes they accept. Driving a card outside its class is a hardware compatibility failure, not a protocol one.
- Timing parameters are not decorative. Guard time and waiting time extensions exist because cryptographic operations take real time; a reader stack that ignores them will time out on perfectly compliant cards.
- PC/SC hides much of this. On a desktop stack the driver handles activation and protocol selection for you, which is convenient until something fails and you need to reason about the layer underneath.
Relationship to the rest of the series
This page is part of Ambimat’s ISO 7816 smart card standard developer reference.
Part 3 delivers bytes reliably. What those bytes mean — the APDU structure, the file system, access conditions — is ISO/IEC 7816-4. How APDUs specifically map onto T=0 and T=1 framing is covered in the transport pages in this section.
Authoritative source
For the normative specification, refer to ISO/IEC 7816-3:2006, Identification cards — Integrated circuit cards — Part 3: Cards with contacts — Electrical interface and transmission protocols, published by ISO. Note that an Amendment 1:2025 adding voltage classes exists and a further revision is in progress, so confirm the current edition before citing it in a specification. This page is Ambimat’s own explanatory summary and does not reproduce or replace the standard.