Inductive data transfer
Different types of modulation are used for data transmission in the two directions.
Data transmission from the card to the terminal
For data transmission from the card to the terminal, a 307.2 kHz subcarrier is first generated using load modulation (see Figure 3.81), with a load variation of at least 10 %. Data modulation is achieved by switching the phase of the subcarrier by 180 degrees, producing two phase states that can be interpreted as logic 1 and logic 0. The initial state after the magnetic field has been established is defined to be logic 1. This initial state (interval t3 in Figure 3.84) remains stable for at least 2 ms. Following this, every subcarrier phase shift represents a reversal of the logic state, yielding non-return to zero (NRZ) coding. The data transmission rate, at least for the ATR, is 9600 bits per second.

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Figure 3.81 Operating principle of phase modulation for data transmission with a contactless smart
card. The upper diagram shows the alternating magnetic field, and the associated phase states are shown in the lower diagram. The carrier frequency is 4.9152 MHz, and the subcarrier frequency is 307.2 kHz

Data transmission from the terminal to the card
To transfer data from the terminal to the card, the four alternating magnetic fields F1 through F4, which pass through coupling surfaces H1 through H4, are phase modulated using phase-shift keying (PSK). This causes the phase of all four fields to simultaneously shift by 90 degrees. In this way, two phase states A and A’are defined. Depending on the orientation of the card relative to the terminal, this yields two different constellations of phase states, as shown in Figures 3.82 and 3.83. Since the card must work in all four possible orientations with respect to the terminal, the initial state (intervals t2 and t3 in Figure 3.84) is interpreted as a logic 1, regardless of which of the indicated alternatives is actually present. Following this, every phase change represents a reversal of the logic state, which again produces an NRZ encoding.

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Figure 3.82 The first phase modulation variant for data transmission with a contactless chip card. The four arrows represent phase vectors

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Figure 3.83 The second phase modulation variant for data transmission with a contactless chip card. The four arrows represent phase vectors

Capacitive data transfer
For capacitive data transmission from the card to the terminal, one pair of coupling surfaces is used, depending on the orientation of the card relative to the terminal – either E1 and E2 or E3 and E4 as shown in Figure 3.80. The other pair of coupling surfaces can be used for data transmission in the opposite direction. Since the card sends the ATR via one particular pair of coupling surfaces, the terminal can recognize the relative orientation of the card. The maximum potential difference between a pair of coupling surfaces is limited to 10V, but it must at least exceed the minimum differential voltage of the receiver (± 300 mV). Differential NRZ encoding is used for data transmission. The transmitter generates the encoding by reversing the voltage between surfaces E1 and E2 or E3 and E4. The state representing a logic 1 is again established in interval t3 (see Figure 3.84). Following this, every polarity reversal represents a change in the logic state.

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Figure 3.84 Timing diagram for data transmission with a contactless smart card according to ISO/IEC 10536 3. Here t0 ≥ 8 ms, t1 ≤ 0.2 ms, t2 = 8 ms, t3 = 2 ms and t4 ≤ 30 ms

Initial state and answer to reset (ATR)
In order for the terminal to unambiguously determine the type of data transmission and the orientation of the card at the beginning of a data exchange, certain time intervals must be defined for initiating energy and data transfers. Figure 3.84 shows the constraints and values for the reset recovery time t0, power-up time t1, initialization time t2, stable logic state time t3 and answer to reset time t4.
Minimum reset recovery time: t0
If a reset is to be produced by switching the energy-transfer field off and back on, the time between switching the field off and then on again, during which no energy is transferred, must be equal to or greater than 8 ms.
Maximum power-up time: t1
The time required for the energy-transfer field produced by the terminal to be established must be less than or equal to 0.2 ms.
Initialization time: t2
The initialization time, which is the time allowed for the card to attain a stable operating state, is 8 ms.
Stable logic state time: t3
Prior to the Answer to Reset, the logic state is held at the logic 1 level for 2 ms. During this interval, the card and the terminal are set to logic 1 for inductive data transmission.
Maximum response time for ATR: t4
The card must start sending the ATR before 30 ms have elapsed. The card can use the ATR to indicate that the conditions for subsequent operation must be changed with regard
to energy level, data transmission rate or the frequency of the fields. The ‘maneuvering room’ provided here can be utilized according to the requirements of the application.
For instance, a significantly higher data transmission rate can be selected for a time-critical application.