T=0

T=0

T=0T=0ProtocolCharacter-oriented smart card protocol.Click to view → is a character-oriented, half-duplex transmission protocol for contact smart cards, defined in ISO 7816 Part 3. It was the original protocol shipped with the first generation of smart cards and remains widely supported for backward compatibility, particularly in SIM cards and older EMV payment terminals.

How T=0 Works

In T=0, the reader sends the five-byte APDU header (CLA, INS, P1, P2, P3) first. The card responds with a procedure byte that instructs the reader what to do next — send data, receive data, or acknowledge completion. This interleaved exchange means the transport layer has knowledge of the APDUAPDUProtocolCommunication unit between card and reader.Click to view → structure, unlike T=1 which treats the APDU as an opaque payload.

Key characteristics of the T=0 protocol:

• Character-level error detection: Each byte is transmitted with a parity bit; if the receiver detects a parity error, it signals NACK by pulling the I/O line low, and the sender retransmits.
• No block framing: There are no packet headers, length fields, or CRC checksums at the block level.
• Half-duplex only: Data flows in one direction at a time, governed by the procedure byte handshake.
• Single logical channel: T=0 does not natively support multiplexed channels.

T=0 vs T=1

Where T=0 Is Still Used

Despite being the older protocol, T=0 persists in several domains. Most SIM cards default to T=0 because telecom standards (ETSI TS 102 221) specify it as mandatory. Many legacy payment terminals and government PIV readers also rely on T=0. New deployments generally prefer T=1T=1ProtocolBlock-oriented smart card protocol.Click to view → or contactless protocols, but readers must still implement T=0 to handle the installed card base.

Protocol Selection

A card advertises its supported protocols in the ATR interface bytes. If both T=0 and T=1 are offered, the reader may issue a PPS request to select the preferred protocol. If no PPSPPSProtocolCard-reader parameter negotiation.Click to view → is sent, the first protocol indicated in the ATRATRProtocolInitial response from card after power-on.Click to view → is used by default.