What is HART protocol?
HART (Highway Addressable Remote Transducer) Protocol is the global standard for sending and receiving digital information across analog wires between smart devices and control or monitoring system. HART is owned by independent non-profit HART communication foundation.
HART is now a global standard because digital information from HART devices can simultaneously communicated with the 4-20mA signal. Without it, there would be no digital communication. Devices that uses HART protocol can provide both 4-20mA and digital signal, it is available any other communication protocol.
Theory of Operation:
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. The FSK standard allows bi-directional communication through 2 wire 4-20mA circuit without interrupting the analog signal.
A HART protocol provides two simultaneous signals one the analog 4-20mA signal and the digital signal. The 4-20mA communicates the primary measured value and digital signal contains information from the device including device status, diagnostics, additional measured or calculated values, etc.
Communication modes:
Master-slave mode:
In the master-slave mode, the teachers initiate communication, request and instruct the slave and then receive the status again. Two teachers can connect in each HART loop, primary and secondary teachers. The main teachers are controllers and secondary teachers can be a handheld terminal or another PC.
The HART protocol mainly uses the master-slave protocol, a master is usually a controller such as DCS, PLC or a PC and the slave devices are transmitters, actuators and controllers that respond to the commands of the primary or secondary master.
Burst mode:
Only some HART devices support Burst mode, they are optional. In burst mode, the master instructs the slave to continuously transmit a standard HART response message (for example, the value of the process variable). The master receives the message at a higher speed until he orders the slave to stop bursting.
HART commands:
Universal commands:
All devices using the HART protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations.
Common practice:
Common practice commands provide functions implemented by many, but not necessarily all, HART communication devices.
Device or Manufacturer specific command:
Command that is specifically done for a proposed or the manufacturer. These commands access setup and calibration information, as well as information about the construction of the device.
Benefits of HART:
- 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.
- The HART protocol extends system capabilities for two-way digital communication with smart field instruments.
- Improved plant operations
- Operational flexibility
- Instrumentation investment protection
- Digital communication
- Continuously validate the integrity of loops and control/automation system strategies.
- Speed the troubleshooting time between the identification and resolution of problems.
- Minimize the impact of deviations by gaining new, early warnings.
