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HART Modem ASIC | HT1200M | Page
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10
VSSA: Analog Ground
Analog Ground
Function Description
The HT1200M is a functional equivalent of the SYM20C15 HART Modem. It
contains a transmit data modulator and signal shaper, carrier detect circuitry, analog
receiver and demodulator circuitry and an oscillator, as shown in Figure 4.
The internal HART modulates the transmit-signal and demodulates the receive signal.
The transmit-signal shaper enables the HT1200M to transmit a HART compliant
signal. The carrier is detected by comparing the receiver filter output with the
difference between two external voltage references. The analog receive circuitry band-
pass filters the received signal for input to the modem and the carrier detect circuitry.
The oscillator provides the modem with a stable time base using either a simple
external resonator or an external clock source.
Figure 4 HT1200M Block Diagram
HT1200M LOGIC
The modem consists of a modulator and demodulator. The modem uses shift
frequencies of nominally 1200 Hz (for a 1)and 2200 Hz (for a 0). The bit rate is1200
bits/second.
Modulator
The modulator accepts digital data in NRZ form at the ITXD input and generates the
FSK modulated signal at the OTXA output. INRTS must be a logic low for the
modulator to be active.
Demodulator
The demodulator accepts an FSK signal at the IRXA input and reproduces the original
modulating signal at the ORXD output. The nominal bit rate is 1200 bits per second.
Figure 5 illustrates the demodulation process.
