HART Protocol Physical and Data link layer Implementation

Guided by Dr. C.B.Bhatt

HART - Principle HART (Highway Addressable Remote Transducer) was developed by Fisher-Rosemount to retrofit 4-to-20mA current loop transducers with digital data communication.

HART - Principle HART modulates the 4-20mA current with a low-level

frequency-shift-keyed (FSK) sine-wave signal, without affecting the average analogue signal.

HART uses low frequencies (1200Hz and 2200 Hz) to deal with poor cabling, its rate is 1200 Bd - but sufficient.

At a basic level, most smart instruments provide core functions such as:

• Control of range/zero/span adjustments • Diagnostics to verify functionality • Memory to store configuration and status information

Average current change during communication = 0 The HART protocol can be used in three ways: • In conjunction with the 4–20 mA current signal in point-

to-point mode • In conjunction with other field devices in multidrop

mode • In point-to-point mode with only one field device

broadcasting in burst mode

HART Format The HART protocol has two formats for digital

transmission of data: Poll/response mode Burst (or broadcast) mode

In the poll/response mode the master polls each of the smart devices on the highway and requests the relevant information.

In burst mode the field device continuously transmits process data without the need for the host to send request messages.

Although this mode is fairly fast (up to 3.7 times/second), it cannot be used in multidrop networks.

HART PROTOCOL PHYSICAL LAYER The physical layer of the HART protocol is based

on two methods of communication.

Analog 4–20 mA Digital frequency shift keying (FSK)

FREQUENCY SHIFT KEYING (FSK) The HART Communication Protocol is based on

the Bell 202 telephone communication standard and operates using the frequency shift keying (FSK) principle. The digital signal is made up of two frequencies — 1,200 Hz and 2,200 Hz representing bits 1 and 0, respectively. Sine waves of these two frequencies are superimposed on the direct current (dc) analog signal cables to provide simultaneous analog and digital communications (Fig 1). Because the average value of the FSK signal is always zero, the 4‐20mA analog signal is not affected.

FREQUENCY SHIFT KEYING (FSK)

HART PROTOCOL DATA LINK LAYER

HART data link frame format

Two-dimensional error checking, including vertical and longitudinal parity checks, is

implemented in each frame. Each character or frame of information has the following

parameters: 1 start bit 8 data bits 1 odd parity bit 1 stop bit

HART NETWORK Point to Point

Multidrop

POINT‐TO‐POINT In point‐to‐point mode, the 4‐20mA signal is

used to communicate one process variable, while additional process variables, configuration parameters, and other device data are transferred digitally using the HART Protocol. The 4‐20mA analog signal is not affected by the HART signal and can be used for control. The HART communication digital signal gives access to secondary variables and other data that can be used for operations, commissioning, maintenance and diagnostic purposes.

POINT TO POINT

BLOCK DIAGRAM OF PROJECT

HART MODEM General Description The DS8500 is a single-chip modem with Highway

Addressable Remote Transducer (HART) capabilities and satisfies the HART physical layer requirements.

The device integrates the modulation and demodulation of the 1200Hz/2200Hz FSK signal, has very low power consumption, and needs only a few external components due to the integrated digital signal processing.

The input signal is sampled by an analog-to-digital converter (ADC), followed by a digital filter/demodulator.

HART MODEM This architecture ensures reliable signal detection in noisy

environments. The output digital-to-analog converter (DAC) generates a sine wave and provides a clean signal with phase-continuous switching between 1200Hz and 2200Hz. Low power is achieved by disabling the receive circuits during transmit and vice versa.

The DS8500 is ideal for low-power process control transmitters.

Applications 4–20mA Loop-Powered Transmitters for Temperature,

Pressure, Flow, and Level Measurement HART Multiplexers HART Modem Interface Connectivity

Features Single-Chip, Half-Duplex, 1200bps FSK Modulation and

Demodulation Digital Signal Processing Provides Reliable Input Signal

Detection in Noisy Conditions Sinusoidal Output Signal with Lowest Harmonic Distortion Few External Components Enable a Space-Saving Solution Standard Component 3.6864MHz Crystal Complies to HART Physical Layer Requirements 2.7V to 3.6V Operating Voltage 285μA (max) Current Consumption Space-Saving, 5mm x 5mm x 0.8mm, 20-Pin TQFN

Package

Pin Configuration

Block Diagram

Functional Description The DS8500 modem chip consists of a demodulator, carrier

detect, digital filter, ADC for input signal conversion, a modulator and DAC for output signal generation, and receive and transmit state machine blocks to perform the HART communication.

The Block Diagram illustrates the interface between various blocks of circuitry.

The input HART signal’s noise interference is attenuated by a one-pole high pass filter that is external to the chip; the attenuated signal is digitized by the ADC and filtered by the receive state machine.

The transmit state machine modulates the input to the HART-compliant signal with the help of the modulator and the DAC.

Modulator waveform

Demodulator waveform

Typical Application Circuit

C8051F340 Processor

Features of C8051F340

High-speed pipelined 8051-compatible microcontroller core (up to 48 MIPS)

In-system, full-speed, non-intrusive debug interface (on-chip) Universal Serial Bus (USB) Function Controller with eight flexible

endpoint pipes, integrated transceiver, and 1 kB FIFO RAM Supply Voltage Regulator True 10-bit 200 ksps differential / single-ended ADC with analog

multiplexer On-chip Voltage Reference and Temperature Sensor On-chip Voltage Comparators (2) Precision internal calibrated 12 MHz internal oscillator and 4x clock

multiplier Internal low-frequency oscillator for additional power savings

Features of C8051F340 Up to 64 kB of on-chip Flash memory Up to 4352 Bytes of on-chip RAM (256 + 4 kB) External Memory Interface (EMIF) available on 48-pin

versions. SMBus/I2C, up to 2 UARTs, and Enhanced SPI serial

interfaces implemented in hardware Four general-purpose 16-bit timers Programmable Counter/Timer Array (PCA) with five

capture/compare modules and Watchdog Timer function On-chip Power-On Reset, VDD Monitor, and Missing Clock

Detector Up to 40 Port I/O (5 V tolerant)

Summary of HART benefits Simultaneous analog and digital communications Allows other analog devices on the highway Allows multiple masters to control same smart instrument Multiple smart devices on the same highway Long distance communications over telephone lines Two alternative transmission modes Flexible messaging structure for new features Up to 256 process variables in any smart field device

Thank You