HART (Highway Addressable Remote Transducer) was developed to add digital communication capabilities to existing 4-20mA analog loops. It uses frequency-shift-keying to modulate two frequencies onto the analog signal without affecting the average current. The HART protocol allows simultaneous analog and digital multi-drop communication for monitoring and configuration of smart field devices. A C8051F340 microcontroller and HART modem chip like the DS8500 can be used to implement the physical and data link layers of the HART protocol for communication with field transmitters.

1. HART Protocol Physical and
Data link layer Implementation
Guided by Dr. C.B.Bhatt

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

3. 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

4.  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

5. HART Format
 The HART protocol has two formats for digital
transmission of data:
 Poll/response mode
 Burst (or broadcast) mode

6.  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.

7. 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)

8. 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.

9. FREQUENCY SHIFT KEYING (FSK)

10. HART PROTOCOL DATA LINK LAYER
HART data link frame format

11.  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

12. HART NETWORK
 Point to Point
 Multidrop

13. 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.

14. POINT TO POINT

15. BLOCK DIAGRAM OF PROJECT

16. 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.

17. 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.

18. Applications
 4–20mA Loop-Powered Transmitters for Temperature,
Pressure, Flow, and Level Measurement
 HART Multiplexers
 HART Modem Interface Connectivity

19. 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

20. Pin Configuration

21. Block Diagram

22. 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.

23. Modulator waveform

24. Demodulator waveform

25. Typical Application Circuit

26. C8051F340 Processor

27. 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

28. 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)

29. 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

30. Thank You