HART 5 vs HART 7: Technical Differences & Feature Comparison
From a version number perspective, Hart 7 appears to be merely an "upgraded version," yet the two standards exhibit fundamental differences across four dimensions: frame structure, addressing mechanisms, data types, and network topology. While Hart employs "short frames, single variables, and master-slave polling," Hart 7 adopts "long frames, multiple variables, and publish-subscribe architecture." This shift results in approximately an eightfold increase in information density under identical wiring conditions, coupled with a 50% reduction in average response latency. Additionally, Hart 7 imposes stricter requirements on host software: it must support DDL 7 parsing and burst buffering; failure to do so forces a return to Hart 5 compatibility mode, significantly limiting functionality.
Ⅰ.Differences in Communication Protocols and Technologies
The physical layer specifications remain identical for both versions, with differences emerging at the link layer. Hart frames maintain fixed lengths: the user data field is limited to 16 bytes, while command numbers 0–255 are uniformly assigned by the standardization body. Hart 7 frames support extended lengths, allowing a user data field of up to 256 bytes and introducing a "manufacturer-specific command space" (0–31) that enables device manufacturers to implement proprietary algorithms. Regarding address formats, Hart uses either 0–15 short addresses or 38-bit long addresses; Hart 7 further introduces "logical addresses," enabling multiple variables within the same hardware unit to subscribe independently and enhancing multiplexing efficiency. Most critically, Hart 7 features a "programmable burst period" mechanism, allowing slaves to transmit data proactively based on event triggers or periodic schedules, eliminating the need for frequent polling by the master station and reducing cable utilization by approximately 60%. This innovation lays the foundational technology for subsequent time-slot scheduling in Wireless Hart.
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OSI hierarchy |
HART hierarchy |
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The seventh layer |
application layer |
HART command layer |
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the sixth floor |
presentation layer |
Unused |
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the fifth floor |
Session layer |
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the fourth floor |
Transport Layer |
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the third layer |
network layer |
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the second layer |
Data Link Layer |
HART protocol rules |
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the first layer |
physical layer |
Bell 202 FSK |
II.Performance and Compatibility Comparison
In terms of performance, Hart 7 delivers a 16-fold increase in per-frame payload capacity, enabling stable transmission of up to 128 Float32 variables across 4–20 mA ranges with a minimum refresh cycle of 250 ms, whereas Hart under identical conditions can only transmit one variable at approximately 330 ms. Regarding compatibility, Hart 7 slaves defaultingly broadcast a "compatible version number"; if they detect that the master does not support burst mode, they automatically revert to Hart 5 short frames to ensure legacy systems can still access master variables. However, this reverse compatibility does not hold: masters using Hart 6 or earlier cannot parse Hart 7 long frames; attempting to send extended commands will result in a "command not implemented" status code from slaves, requiring gateway or PLC firmware upgrades to enable full functionality. Therefore, in hybrid networks, it is recommended to route Hart 7 devices through dedicated channels or on the new system side to prevent communication timeouts caused by miscommunication of long frames from legacy masters.
III.Function Expansion and Limit Analysis
The functional enhancements of Hart 7 focus on three key areas: multivariable monitoring, event logging, and calibration traceability. Multivariable support enables a differential pressure transmitter to simultaneously output four curves representing differential pressure, static pressure, temperature, and sensor health status, eliminating the need for additional wiring. Event logging can cache up to 256 recent alarms and status transitions, facilitating post-event auditing. Calibration traceability stores calibration dates, responsible personnel, and reference instrument numbers in non-volatile storage, complying with FDA 21 CFR Part 11 requirements for electronic records. However, Hart 7 remains constrained by its 1200 bit/s physical layer rate; attempting to subscribe to all 128 variables at a 250 ms interval would exceed theoretical bandwidth limits, leading to packet loss. Therefore, in practical deployment, variables should be subscribed to via separate channels and frequencies based on priority, with "dead zone" filtering enabled to ensure peak traffic does not exceed 70% of line capacity—a limitation often overlooked by field engineers.
