Although wired HART uses only a single pair of cables, it contains two signal transmission channels: the main measurement and control information of the process is transmitted by the
4~20mA analog channel in the HART protocol; other more measurements, process parameters, device configuration, calibration, diagnostic information, etc. are transmitted through the
digital channel in the HART protocol.
Since most users use HART devices that only transmit 4~20mA analog signals to the upper control system to achieve basic process control, the only benefit of the HART protocol with digital
channels for them is to use handheld communicators for engineering installation/commissioning, troubleshooting and calibration of HART field devices. Since handheld communicators can
be
accessed at any point in the 4~20mA loop, such as in a control room with a good environment and personal safety, instead of having to go to the site, this is very popular with field maintenance
personnel.
When users are interested in the digital information of HART field devices (such as device status and diagnostic alarms, process variables and measurement units, loop current and percentage
range
, manufacturer and device tags, etc.), they need to release this digital information through some means and transmit this digital information to the upper control system or asset management
system.
Foreign products include KJ300-X1-BA1 input module and KJ300-X1-BE1 output module for DeltaV of Emerson Company in the United States, AAI14-H input module and AAI841-H input and
output
module for DCS of Yokogawa Company in Japan, and domestic products include FW351H 8-channel HART current signal input card and FW372H 8-channel HART current signal output card for
ECS100
of Zhejiang Zhongkong Company. Another way is to use a multiplexer to collect the digital signal of the HART device and convert it into the required signal output. For example, the Pepperl+Fuchs
HiDMux2700H multiplexer can realize the conversion of HART signals of 32 devices into RS-485 signals.
There is also a special case. Some multivariable HART transmitters (such as flow transmitters) can transmit one information (such as instantaneous flow) by the analog signal part, and up to 4
variables
(such as differential pressure, temperature, and pressure) by the HART digital signal part. However, when this transmitter only transmits analog information, the rest of the information cannot
be sent
to the control system. If the remaining three pieces of information are important, a multivariable output converter can be used, which can convert the remaining three variables of the HART
digital
signal part into independent three-way 4~20mA signal output.
By the 20s of the 21st century, HART has become the most widely used communication protocol for field instruments in the past 40 years, and most of the more than 40 million field instruments
installed worldwide use wired HART technology. Unfortunately, 90% of the data owned by HART field instruments are not transmitted to the control system, so the most common digital technology
in the HART device transmission information is only used during the commissioning and calibration of field equipment, which leads to the failure to fully utilize the HART function. This situation is
currently changing, especially in recent years, many large petrochemical projects in my country have begun to gradually use asset management systems (Asset Management System, AMS). This is to
make full use of the HART digital communication capabilities of existing field equipment to provide users with intelligent digital information such as diagnosis and alarm.
