Type-A communications interface
With Type-A cards, data transmission takes place in both directions at a bit rate of fC/128 (≈106 kbit/s).
Data transmission from the terminal to the card
Digital amplitude modulation (100% ASK) with modified Miller coding is used for data transmission from the PCD to the card, with the length of the blanking interval (gap) being
limited to 3 μs. This relatively short blanking interval makes it easier to provide a steady supply of energy to the card. The exact specification of the length of the blanking interval and its rise and decay characteristics are shown in Figure 3.87. The card recognizes the end of the pause during interval t4, which means after the magnetic field has reached 5 % of HINITIAL and before it exceeds 60 % of HINITIAL. Overshoots must be limited to HINITIAL ± 10 %.

An example of the coding of a bit sequence using modified Miller coding is shown in Figure 3.88. The following coding rules apply here:
–logic 1: blanking interval after half the bit interval
–logic 0: no blanking, with the following exceptions:
  -if there are two or more logic 0 states in succession, there is a blanking interval at the start of the bit interval
  -if the first bit of a protocol frame is a 0, it is represented by a blanking interval at the start of the bit interval
–start of a message: blanking interval at the start of a bit interval
–end of a message: logic 0 followed by one bit with no blanking interval
–no data: no blanking interval for the duration of at least two bits.
Data transmission from the card to the terminal
The bit rate for data transmission from the card to the terminal is also fC/128 (≈106 kbit/s). Load modulation with a subcarrier is used, which means that the subcarrier is generated by switching a load inside the card. The frequency of the subcarrier is specified to be fS = fC/16 (≈847 kHz). The subcarrier is modulated by switching the subcarrier on and off (on–off keying, or OOK) using Manchester coding. An example of the coding of a bit sequence is shown in Figure 3.89.
The coding is defined as follows:
–logic 1: the carrier is modulated by the subcarrier during the first half of the bit interval
–logic 0: the carrier is modulated by the subcarrier during the second half of the bit interval
–start of a message: the carrier is modulated by the subcarrier during the first half of the bit interval
–end of a message: the carrier is not modulated for one-bit interval
–no data: no subcarrier modulation.

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Figure 3.87 Specification of the blanking interval (gap) according to ISO/IEC 14 433-2. The maximum duration of the gap is limited to 3 μs in order to interrupt the energy supply to the card as briefly as possible. Here 2.0 μs ≤ t1 ≤ 3.0 μs; 0.5 μs ≤ t2 ≤ t1 if t1 > 2.5 μs, or 0.7 μs ≤ t2 ≤ t1 if t1 ≤ 2.5 μs; and 0 μs ≤ t4 ≤ 0.4 μs

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Figure 3.88 Coding of a bit sequence transmitted from the terminal to the card for a Type-A communications interface with 100%ASK and modified Miller coding at 106 kbit/s. The figure shows the voltage at the terminal aerial

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Figure 3.89 Load modulation for data transmission from the card to the terminal using a subcarrier at a frequency of fC/16(≈847 kHz) and Manchester coding with a bit rate of 106 kbit/s and OOK. The figure shows the voltage on the card coil

Type-B communications interface
Data transmission from the terminal to the card
With Type-B cards, ASK modulation with a modulation index of 10 % (–2 %, +4 %) is used for data transmission from the PCD to the card. In contrast to the Type-A method, in which continuity of the energy supply is assured by very short blanking intervals, with the Type-B method it is assured by the small modulation index, which is defined such that at least 86 % of the carrier field is always available. The bit rate is again fC/128 (≈106 kbit/s). The exact form of Type-B modulation is shown in Figure 3.90.

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Figure 3.90 Type-B carrier modulation. A continuous supply of energy is made possible by the small modulation index (10 %). Here tf, tr ≤ 2 μs, y = 0.1 · (a – b) and hf, hr ≤ 0.1 · (a – b)

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Figure 3.91 Coding of a bit sequence from the terminal to the card for a Type-B communications interface with 10% ASK, NRZ coding and a bit rate of 106 kbit/s. The figure shows the voltage on the terminal aerial

Simple non-return to zero (NRZ) bit coding is used, with the following coding rules:
–logic 1: high carrier amplitude
–logic 0: low carrier amplitude.
Data transmission from the card to the terminal
With the Type-B method, load modulation with a subcarrier is also used for data transmission from the card to the terminal. The frequency of the subcarrier is again fC/16 (≈847 kHz). In contrast to Type A, the subcarrier is modulated by shifting the phase by 180 degrees (binary phase-shift keying, or BPSK), again using a bit rate of fC/128 (≈106 kbit/s) and NRZ coding. In order to have an unambiguous initial state, the following sequence must be observed at the start of each protocol frame:
–No subcarrier is generated during a guard time interval TRO > 64/ fS following reception of data from the terminal.
–After the guard time, the card generated the subcarrier with no phase shifting for a synchronization time interval TR1 > 80/ fS. The phase during this interval is defined to be the reference phase φ0.
The initial phase φ0 is defined to be logic 1, so the first phase shift means a change from a logic 1 to a logic 0. The following rules apply to the remainder of the data transmission:
–logic 1: φ0
–logic 0: φ0 + 180◦