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.
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
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.
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.
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
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.
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
