The falling (negative) edge of the pulse is used to start a monostable multivibrator in the slave device. The multivibrator in the slave reads the data line about 30 us after the falling edge. The slave's internal timer is an inexpensive analog timer. It has analog tolerances that affect its timing accuracy. Therefore, the pulses are calculated to be within margins. Therefore, the "0" pulses have to be 60 us long, and the "1" pulses can't be longer than 15 us.

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