04-05-2013, 01:07 PM
HART APPLICATION GUIDE
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MASTER-SLAVE MODE
HART is a master-slave communication protocol, which means that during
normal operation, each slave (field device) communication is initiated by a
master communication device. Two masters can connect to each HART
loop. The primary master is generally a distributed control system (DCS),
programmable logic controller (PLC), or a personal computer (PC). The
secondary master can be a handheld terminal or another PC. Slave devices
include transmitters, actuators, and controllers that respond to commands
from the primary or secondary master.
BURST MODE Some HART devices support the optional burst communication mode.
Burst mode enables faster communication (3–4 data updates per second). In
burst mode, the master instructs the slave device to continuously broadcast
a standard HART reply message (e.g., the value of the process variable).
The master receives the message at the higher rate until it instructs the slave
to stop bursting.
Frequency Shift Keying
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
(Figure 1). Because the average value of the FSK signal is always zero, the
4–20 mA analog signal is not affected. The digital communication signal
has a response time of approximately 2–3 data updates per second without
interrupting the analog signal. A minimum loop impedance of 230 W is
required for communication.
DEVICE DESCRIPTION
Some HART host applications use device descriptions (DD) to obtain
information about the variables and functions contained in a HART field
device. The DD includes all of the information needed by a host application
to fully communicate with the field device. HART Device Description
Language (DDL) is used to write the DD, that combines all of the
information needed by the host application into a single structured file. The
DD identifies which common practice commands are supported as well as
the format and structure of all device-specific commands.
A DD for a HART field device is roughly equivalent to a printer driver for a
computer. DDs eliminate the need for host suppliers to develop and support
custom interfaces and drivers. A DD provides a picture of all parameters
and functions of a device in a standardized language. HART suppliers have
the option of supplying a DD for their HART field product. If they choose
to supply one, the DD will provide information for a DD-enabled host
application to read and write data according to each device’s procedures.
DD source files for HART devices resemble files written in the C
programming language. DD files are submitted to the HCF for registration
in the HCF DD Library. Quality checks are performed on each DD
submitted to ensure specification compliance, to verify that there are no
conflicts with DDs already registered, and to verify operation with standard
HART hosts. The HCF DD Library is the central location for management
and distribution of all HART DDs to facilitate use in host applications such
as PCs and handheld terminals.
Benefits of HART Communication
The HART protocol is a powerful communication technology used to
exploit the full potential of digital field devices. Preserving the traditional
4–20 mA signal, the HART protocol extends system capabilities for
two-way digital communication with smart field instruments.
The HART protocol offers the best solution for smart field device
communications and has the widest base of support of any field device
protocol worldwide. More instruments are available with the HART
protocol than any other digital communications technology. Almost any
process application can be addressed by one of the products offered by
HART instrument suppliers.
Unlike other digital communication technologies, the HART protocol
provides a unique communication solution that is backward compatible
with the installed base of instrumentation in use today. This backward
compatibility ensures that investments in existing cabling and current
control strategies will remain secure well into the future.
COST SAVINGS IN MAINTENANCE
The diagnostic capabilities of HART-communicating field devices can
eliminate substantial costs by reducing downtime. The HART protocol
communicates diagnostic information to the control room, which
minimizes the time required to identify the source of any problem and take
corrective action. Trips into the field or hazardous areas are eliminated or
reduced.
When a replacement device is put into service, HART communication
allows the correct operational parameters and settings to be quickly and
accurately uploaded into the device from a central database. Efficient and
rapid uploading reduces the time that the device is out of service. Some
software applications provide a historical record of configuration and
operational status for each instrument. This information can be used for
predictive, preventive, and proactive maintenance.