Data Sheet
A5191HRT AMIS HART™ Modem
4.0 Functional Description
The A5191HRT 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 modem modulates the transmit-signal and demodulates the receive signal. The transmit-signal shaper enables the A5191HRT 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: A5191HRT Block Diagram
4.1 A5191HRT Logic
The modem consists of a modulator and demodulator. The modem uses shift frequencies of nominally 1200Hz (for a 1) and 220Hz (for a 0). The bit rate
is 1200 bits/second.
4.1.1 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.
4.1.2 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.
Figure 5: Demodulator Signal Timing
AMI Semiconductor - Rev. 2.0, Mar. 05
4
www.amis.com
AMI [ AMI SEMICONDUCTOR ]
