HT1200M Hart Modem Typical General Description

The HT1200M is a single-chip, CMOS modem for use in Highway Addressable Remote Transducer (HART) field instruments and masters. The modem and a few external passive components provide all of the functions needed to satisfy HART physical layer requirements including modulation, demodulation, receive filtering, carrier detect and transmit-signal shaping. The HT1200M is pin-compatible with the SYM20C15. See the Pin Description and Functional Description sections for details on pin compatibility with the SYM20C15.

The HT1200M uses Phase Continuous Frequency Shift Keying (FSK) at 1200 bits per second. To conserve power the receive circuits are disabled during transmit operations and vice versa. This provides the half-duplex operation used in HART communications.

Features

• Can be used in designs presently using the SYM20C15 or equivalent type chip

• Single-chip, half-duplex 1200 bits per second FSK modem

• Bell 202 shift frequencies of 1200 Hz and 2200 Hz

• 3.3V-5.0V power supply

• Transmit-signal wave shaping

• Receive band-pass filter

• Low power: optimal for intrinsically safe applications CMOS compatible

• Internal oscillator requires 460.8kHz crystal or ceramic resonator

• Meets HART physical layer requirements

• Industrial temperature range of -40 °C to +85 °C

• Available in a 28-pin PLCC and 32-pin LQFP packages

Typical Application

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.

Characteristics