A character that is transmitted over the I/O line is embedded in a character frame. Before the transmission of a character, the I/O line shall be in state H. Depending upon the convention used, the logical '1' in a character is either represented by state H on the I/O line, direct convention, or state L on the I/O line, inverse convention. A character consists of 10 consecutive bits (see Figure 7.2):

• 1 start bit in state L;
• 8 bits, which comprise the data byte;
• 1 even parity checking bit.

The parity bit is set, in a way, that there is an even number of bits set to '1' including the parity bit in the character frame. The time origin is fixed as the mean between the last observation of state H and the first observation of state L. The receiver shall confirm the existence of a start bit before 0,7 etu (receiver time). Then the subsequent bits shall be received at intervals of (n + 0,5 ± 0,2) etu (n being the rank of the bit). The start bit is bit 1. Within a character, the time from the leading edge of the start bit to the trailing edge of the nth bit is (n ± 0,2) etu. The interval between the leading edges of the start bits of two consecutive characters comprises the character duration (10 ± 0,2) etu, plus a guardtime. Under error free transmission, during the guardtime both the UICC and the terminal shall be in reception mode (I/O line in state H), unless specified otherwise.

The data shall always be passed over the I/O line with the most significant byte first. The order of bits within a byte (that is, whether the least significant or most significant bit is transferred first) is specified in character TS returned in the answer to reset (see ISO/IEC 7816-3 [11]).

If the low impedance driver on the I/O line has been selected, as the result of a successful PPS exchange, the following protocol on the I/O line applies. The transmission state is defined as the period starting from the start bit of the first character to the end of the guardtime of the last character to transmit. During the transmission state the transmitter shall drive the I/O line to the desired level using the low impedance driver, with the exception of the error indication period, e.g. character guardtime of T = 0. After reception of the last character in a command or response sequence when the communication direction is changed, the entity that is in turn to transmit, terminal or UICC, shall drive the I/O line to the high level using the low impedance driver during the interface inactivity period During clock stop the terminal shall drive the I/O line to high state. The interface inactivity period ends when the transmission of a new command or its response starts.

The T = 0 is a half-duplex asynchronous character based transmission protocol. All commands using the protocol T = 0 are initiated from the terminal by sending a five byte header, which informs the UICC what to do. The terminal will always act as master and the UICC as a slave. The direction of the transmission is assumed to be known to both the UICC and the terminal.

The minimum interval between the leading edge of the start bits of two consecutive characters shall be at least 12 etu. The maximum interval between the start leading edge of any character sent by the UICC and the start leading edge of the previous character sent either by the UICC or the terminal is the WWT. The value of the WWT shall not exceed 960 × WI × Fi/f. WI is an integer received in the specific interface byte TC2. If no TC2 is available, default value of WI is 10. The clock rate conversion factor, Fi, and the baud rate conversion factor Di, may be indicated in TA1. If TA1 is absent the default values 372 and 1 respectively are used. If the WWT is exceeded, the terminal shall initiate a deactivation within 960 etu.

A command is always initiated by the terminal which sends an instruction to the UICC in the form of a five byte-header called the command header. The command header comprises five consecutive bytes, CLA, INS, P1, P2 and P3. These bytes together with any data to be sent with the command form the Command-Transport Protocol Data Unit (C-TPDU) for T = 0. The mapping of the C-APDU onto the C-TPDU is described in clause 7.3. The terminal transmits the header to the UICC and waits for a procedure byte or a status byte.

The error detection and correction procedure is mandatory for T = 0 protocol except for the terminal during the ATR-procedure. An error, from the receiver's point of view, is defined by an incorrect parity. The error is indicated on the I/O line, which is set to state L at (10,5 ± 0,2) etu after the leading edge of the start bit for the character. The I/O line shall be in state L for a maximum of 2 etu and a minimum of 1 etu. The transmitter shall check the I/O line for parity error indication at (11 ± 0,2) etu starting from the leading edge of the start bit, in the character being transmitted. If the UICC or terminal as receiver detects a parity error in a character just received, it shall set the I/O line to state L at (10,5 ± 0,2) etu after the leading edge of the start bit for the character for a maximum of 2 etu to indicate the error to the sender (see Figure 7.2). If the transmitter detects an error indication at (11 ± 0,2) etu starting from the leading edge of the start bit, in the character being transmitted, the character shall be sent again after a minimum delay of 2 etu.