Introduction to Hart 5 and Hart 7 protocols

1、Definition and Core Principles

  The Hart protocol can be summarized as an "analog+digital" dual channel specification: the 4-20 mA channel carries the main variable, and the FSK channel carries instructions and auxiliary data. The physical layer adopts the Bell 202 standard with a baud rate of 1200 bit/s. The master station sends request frames and the slave station sends response frames back. The frame format includes preamble, delimiter, address, command, data, and checksum, ensuring reliable communication of about 1 km in the factory area with severe electromagnetic interference. Due to the low impedance and high noise resistance advantages of the current loop itself, Hart can directly use intrinsic safety barriers in explosion-proof situations without additional isolation, greatly simplifying wiring. The protocol stack is only defined at the data link layer and application layer. If the upper system needs high-speed batch data, it still needs to be translated through a gateway or PLC. This "lightweight" positioning makes it almost cost-effective to implement in old DCS renovation projects.

2、Main features and application scenarios

The biggest feature of Hart is "backward compatibility", any instrument that supports Hart can directly replace traditional two-wire meters without changing cables and power supplies. In the early days, its instruction set only included eight basic commands such as read main variable, read current, and write range, but it was sufficient for remote calibration and fault identification, and was widely used in continuous process industries such as oil and gas fields, refining, power, and papermaking. Due to limited data throughput, Hart typically serves "point-to-point" maintenance scenarios: technicians carry handheld terminals to access the circuit nearby, complete inspections, calibration, and recording, and then leave the site, causing communication loads to be interrupted. This mode has the highest cost-effectiveness among thousand point level devices, which is also the reason why the stock market still maintains a proportion of about 60% to this day.

二、Introduction to Hart 7 Protocol Fundamentals

Hart 7 is the seventh edition specification released by the Hart Communication Foundation in 2007, with the internal code name "Hart 7.0". Its core goal is to "provide industrial Ethernet level information richness to field instruments while maintaining 4-20 mA compatibility". To achieve this goal, Hart 7 redefined the data link layer and application layer above the physical layer, introducing mechanisms such as burst mode, time synchronization, event notification, and massive data block transmission, allowing a single instrument to broadcast multiple process variables, diagnostic status, and timestamps to the outside world, providing native support for subsequent advanced analysis, predictive maintenance, and digital twins. This version has also been included in the IEC 61158-2-2010 layout, becoming an internationally recognized open standard.

1、Definition and Development Background

The proposal of Hart 7 stems from the rigid demand of the process industry for "higher resolution, lower latency, and more variables". The traditional Hart polling cycle is about 500 ms, which is only suitable for reading single point data; Modern factories need to simultaneously monitor multidimensional information such as temperature, pressure, flow rate, density, valve position, and actuator health, and require millisecond level refresh. Hart 7 uses "burst+time-division multiplexing" to increase the effective payload from 16 bytes per frame to 256 bytes. A single message can carry up to 128 variables while maintaining a physical layer rate of 1200 bit/s, balancing the bandwidth limit of old cables with the appetite of upper layer applications. The foundation has also launched DDL (Device Description Language) 7.0, which enables host software to automatically recognize new variables without the need to upgrade drivers, reducing the adaptation threshold for instrument manufacturers.

 2.Key improvements and advantages

The core improvements of Hart 7 can be summarized as "four new": new burst mode, new variable type, new time synchronization, and new security model. The new burst mode supports active push from the slave station, and the master station can subscribe to up to 8 logical channels, achieving "one request, multiple data" reverse broadcasting; The new variable type adds floating-point arrays, strings, and structures, which can return the complete valve signature curve at once; The new time synchronization adopts UTC relative seconds with a resolution of 1 ms, which facilitates horizontal comparison among multiple instruments; The new security model adds access permission bytes at the application layer, supports write protection, calibration locks, and OEM signatures, and reduces the risk of misoperation. Thanks to this, Hart 7 has rapidly become popular in high-end instruments such as intelligent remote transmission, multivariable vortex street, and acoustic gas ultrasound, becoming the standard communication core for "smart field".