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Abstract
The HART® Field Communications Protocol is widely accepted in the industry as the
standard for digitally enhanced 4-20mA communication with smart field instruments. A wide
range of products from an increasing number of suppliers are available today, and many
more are in development. The enhanced two-way communication capability of instruments
using the HART protocol can significantly improve plant information management, provide
solutions to today's business challenges, and yield substantial cost savings.
Introduction
For many years, the field communication standard for process automation equipment has
been a milliamp (mA) analog current signal. The milliamp current signal varies within a range
of 4-20mA in proportion to the process variable being represented. In typical applications a
signal of 4mA will correspond to the lower limit (0%) of the calibrated range and 20mA will
correspond to the upper limit (100%) of the calibrated range. If the system is calibrated for 0100 PSI, then an analog current signal of 12mA (50% of range) will correspond to a pressure
of 50PSI. Virtually all installed systems use this international standard for communicating
process variable information between process automation equipment.
HART Field Communications Protocol extends
this 4-20mA standard to enhance
communication with smart field instruments.
The HART protocol was designed specifically
for use with intelligent measurement and control
instruments which traditionally communicate
using 4-20mA analog signals. HART preserves
the 4-20mA signal and enables two-way digital
communications to occur without disturbing the
integrity of the 4-20mA signal.
The HART protocol permits the process
variable to continue to be transmitted by the 420mA analog signal and additional information
pertaining to other variable, parameters, device
configuration, calibration, and device
diagnostics to be transmitted digitally at the
same time. Thus, a wealth of additional
information related to plant operation is
available to central control or monitoring
systems through HART communications.
The HART Protocol - An Overview
HART is an acronym for "Highway Addressable Remote Transducer". The HART protocol
makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital
communication signals at a low level on top of the 4-20mA as show in Figures 1 and 2. This
enables two-way field communication to take place and makes it possible for additional
information beyond just the normal process variable to be communicated to/from a smart
field instrument. The HART protocol communicates at 1200 bps without interrupting the 420mA signal and allows a host application (master) to get two or more digital updates per
second from a field device. As the digital FSK signal is phase continuous, there is no
interference with the 4-20mA signal.
HART is a master/slave protocol which means that
a field (slave) device only speaks when spoken to
by a master.
The HART protocol can be used in various
modes for communicating information to/from
smart field instruments and central control or
monitoring systems. HART provides for up to
two masters (primary and secondary) as show
in Figure 3.
This allows secondary masters such as handheld
communicators to be used without interfering with
communications to/from the primary master, i.e. control/monitoring system.
The most commonly employed HART communication mode is master/slave communication of digital
information simultaneous with transmission of the 4-20mA signal as shown in Figure 4.
The HART Command Set is organised into three groups and provides read/write access to
the wealth of additional information available in smart field instruments employing this
technology.
20C15 HART Modem
The 20C15 HART Modem IC is made by LSI Logic.
Description
The 20C15 is a low-power, single-chip, 1200
bps modem specifically designed to satsify
HART Protocol requirements. It contains
nearly all of the circuitry needed to add
HART communication capability to analog 420 mA field instruments.
It is specified for use at supply voltages of 3.3
volt and 5.0 volt and over the industrial
process control temperature range of –40°C to
85°C.
For lowest possible power consumption the
modulator portion of the chip is shut down while the
demodulator is operating and vice-versa.
A crystal or ceramic resonator operating at 460.8 kHz may be used with the chip to generate a 460
kHz
clock. Or a 460 kHz clock may be supplied to one of the 20C15 pins.
An external bandgap voltage reference is required to set up reference voltages and the device operating
current.
The 20C15 includes transmit waveshaping and part of the receive filtering required by HART.
The 20C15 is supplied in a 28-pin PLCC package.
Pinout
Operation
Modulator (Transmit) :
The modulator is operating (and the demodulator is shut down) whenever INRTS is low.
When ITXD is high the modulator output at OTXA will be a trapezoidally shaped wave at nominally
1200 Hz.
When ITXD is low the modulator output is nominally 2200 Hz.
OTXA is usually AC-coupled to an amplifier or buffer stage.
The output voltage levels at OTXA depend on the reference voltage applied at IAREF.
Let VQ, VMIN, and VMAX be as defined in figure 1.
Figure 1
VQ is the quiescent (no modulation) voltage at OTXA, and (VMAX – VMIN) is the pp signal voltage
swing.
VREF is the voltage at pin IAREF.
For VREF = 1.235 volt, VQ is 0.5 volt and the signal swings from 0.25 volt to 0.75 volt. In a HART
Master this is the correct pp voltage to be applied to the network. But the OTXA pin does not have
enough drive capability to drive a HART network directly. A buffer amplifier is usually needed. In a
field instrument, the 0.5 volt pp OTXA output is usually converted to 1 mA pp.
Demodulator (Receiver) :
The demodulator is operating (and the modulator is shut down) whenever INRTS is high.
The received signal is first applied to a bandpass filter. Part of this filter is off-chip.
This is necessary to reduce interference to a level that is within the 20C15 supply rails.
The bandpass filter consists of a 4-pole highpass filter and a single-pole lowpass. When used with the
suggested passive components the overall filter has a gain of 1.6 in the passband. Other pins that are
part of the receive filter are IRXA, ORXAF, and IRXAC.
The filter output is applied to two comparators -one to slice the signal and produce a logic-level
version of the received FSK and a second to act
as a carrier detect.
The reference for the first comparator is the
reference voltage applied at IAREF.
The reference for the second comparator is
applied at ICDREF and is normally set to be 80
mV below the level at IAREF.
The carrier detect comparator is therefore tripped
if the filter output swings low by 80 mV peak or
more. Or, since the filter has a passband gain of
1.6, the carrier detect comparator will trip if the
input swings by 50 mV peak (100 mV pp) or
more. These values are chosen to satisfy the
HART requirement that the carrier detect be OFF
for input signals < 80 mV pp and ON for input
signals > 120 mV pp.
Logic circuits following the carrier detect comparator are used to decide whether carrier is present. If
the input has sufficient amplitude to trip the carrier detect comparator on each of 3 or 4 consecutive
cycles, then carrier is present and OCD goes high. Once carrier is present, if the input signal fails to
trip the comparator for a time of about 3 or 4 cycles, then carrier is no longer present and OCD goes
low.
The logic-level FSK out of the first comparator is applied to a logic block that generates a high level if
a frequency below 1700 Hz is present and a low level if a frequency above 1700 Hz is present.
The output of this logic block is gated with OCD to form the output ORXD. If carrier is not present the
RXD output is always low. If carrier is present, then RXD equals the output of the logic block.
ORXD is normally connected to a UART. The UART sees a low level as an assertion of RXD.
Therefore, when no carrier is present (no reception occuring) ORXD is asserted. In some cases this
will appear to be an extra character at the end of a message. The software that services the UART must
handle this condition appropriately.
Clock :
A 460.8 kHz clock is required. This can be generated by connecting a crystal or ceramic resonator
across pins OXTL and IXTL (see Circuits).
An external clock source can also be used. Connect the external source to OXTL and connect IXTL to
ground.
Parameters
Clock: 460.8 kHz, Tolerance 1%.
Power Supply Current: 400 μA max. (3.3 volt supply), 600 μA max. (5.0 volt supply).
(NOTE: This is the current during receive. It is about 100 μA to 200 μA less during transmit)
Operating Temperature Range: 0°C to 70°C.
Reset Minimum Pulse Time: 2 μS
Transmit Output Drive Capability: Needs minimum of 30 k.
Circuits : Field Instrument, 5.0 Volt Supply: