SBAS940 December 2018 DAC8742H
In SPI mode, HART data is loaded into a transmit FIFO via the SPI serial interface. In UART mode, the UART BAUD rate matches the HART BAUD rate and therefore the FIFO is bypassed. In both cases the input data is translated into the mark and space, 1200 Hz and 2200 Hz respectively, frequency shift keyed (FSK) analog signals used in HART communication through an internal HART modulator.
The HART modulator implements a look-up table containing 32 6-bit signed values which represent a single phase continuous sinusoidal cycle. A counter is implemented that incrementally loads the table values to a Digital-to-Analog Converter (DAC), at a clock frequency determined by the binary value of the input data, in order to create the mark and space analog output signals used to represent HART data.
The modem operates in half-duplex mode, unless placed in full-duplex mode, where the modulator and demodulator are not active simultaneously. The modem arbitrates over which component is active. To request that the modulator is activated UART devices toggle the RTS pin low, SPI devices toggle the RTS bit in the MODEM CONTROL register. These mechanics are explained in more detail in the respective sections of Device Functional Modes.
In HART mode the MOD_OUT pin requires parallel capacitance of 5-22 nF or 0-100 pF in FOUNDATION Fieldbus and PROFIBUS PA mode for stability.