A5191HRT

4.2 Transmit-Signal Shaper

The transmit-signal shaper generates a HART compliant FSK modulated signal at OTXA. Figure 6 and Figure 7 show the transmit-

signal forms of the A5191HRT.

Figure 6: OTXA Waveform (1200Hz)

Figure 7: OTXA Waveform (2200Hz)

4.3 Carrier Detect Circuitry

The carrier detect comparator shown in Figure 8 generates logic low output if the IRXAC voltage is below ICDREF. The comparator

output is fed into a carrier detect block (see Figure 4). The carrier detect block drives the carrier detect output pin OCD high if INRTS is

high and four consecutive pulses out of the comparator have arrived. OCD stays high as long as INRTS is high and the next

comparator pulse is received in less than 2.5ms. Once OCD goes inactive, it takes four consecutive pulses out of the comparator to

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