HART is right now the most used protocol for communication from field to DCS in process plants all over the world. It has proved itself in field communications and has various advantages over the conventional 4-20mA signal protocol.
The HART protocol is acronym for Highway Addressable Remote Transducer. Although it is a new concept it uses the conventional modeof 4-20mA for its communication from field to PLC or DCS.
include transmitters, actuators, and controllers that respond to commands from the primary or secondary master.
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.
Leaving all the technicalities behind, comparing with conventional 4-20mA the HART provides a whole bunch of diagnostic and configuration features to the control Engineer to setup and monitor the process effectively. One of the example that i found online is:
suppose we are monitoring a small process based on a temperature transmitter having thermocouple sensor. Then HART trnamsitter can tell me at any point of time about the
sensor: Has it failed or cable break from the sensor into the transmitter,
transmitter: is the value given by sensor in range or is it out of service,
cabling: is the cable from the transmitter to the DCS proper.
more coming soon...
The HART protocol is acronym for Highway Addressable Remote Transducer. Although it is a new concept it uses the conventional modeof 4-20mA for its communication from field to PLC or DCS.
This protocol was developed by rosemount Inc in early nineties. The HART communication protocol is based on the Bell 202 telephone communication standard and operates using the frequency shift keying (FSK) principle. Since I dont know much about FSK, we can understand it from here.
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. The average value of the FSK signal is always zero so the 4–20 mA analog signal is not affected. The digital communication signal has a response time of approximately few data updates per second without interrupting the analog signal. A minimum loop impedance of 230 W is required for communication.
HART communication can happen in two ways. 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 like a Handheld Communicator or another PC. Slave devicesinclude transmitters, actuators, and controllers that respond to commands from the primary or secondary master.
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.
Leaving all the technicalities behind, comparing with conventional 4-20mA the HART provides a whole bunch of diagnostic and configuration features to the control Engineer to setup and monitor the process effectively. One of the example that i found online is:
suppose we are monitoring a small process based on a temperature transmitter having thermocouple sensor. Then HART trnamsitter can tell me at any point of time about the
sensor: Has it failed or cable break from the sensor into the transmitter,
transmitter: is the value given by sensor in range or is it out of service,
cabling: is the cable from the transmitter to the DCS proper.
more coming soon...
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