HART 5.2 vs 7.2 Compatibility on Siemens MAG 6000 Flowmeters

Overview

The Siemens MAG 6000 electromagnetic flowmeter ships with the FDK-085U0226 HART add-on module, which implements HART protocol revision 5.2. Many modern process transmitters and valve positioners now ship at HART revision 7.2 or higher, raising a recurring integration question: can a HART 5.2 slave coexist with a HART 7.2 slave on the same 4-20 mA loop, and if so, which HART 7 features remain inaccessible to the legacy module?

The short answer is yes, the two revisions interoperate on the same current loop at the standard 1.2 kb/s signaling rate. The longer answer, detailed in this reference, defines which commands, variables, and burst modes degrade gracefully, which are silently dropped, and how multi-vendor hosts (SIMATIC S7-1500/ET 200MP, ControlLogix, Honeywell MasterLogic PLC) interpret the heterogeneous traffic.

HART Protocol Revision Background

HART (Highway Addressable Remote Transducer) is a master/slave, half-duplex, frequency-shift-keyed protocol superimposed on a 4-20 mA analog signal. The physical layer is defined by the Bell 202 modem specification at 1200 bit/s with a 1200 Hz / 2200 Hz mark-space pair. This physical layer has not changed since the protocol's introduction in the late 1980s, which is the foundation for cross-version coexistence.

Version evolution has been additive rather than disruptive:

• HART 5 – Stable Universal and Common Practice command set; standardized burst mode; defined Device Description Language (DDL) baseline.
• HART 6 – Added enhanced burst mode, device variable classifications, and expanded command 33 (Device Variable).
• HART 7 – Introduced WirelessHART (IEC 62591), Time-Stamped Variables, Block Data Transfer, Catch Device Variable, and long tag names (32 characters).

The FieldComm Group (successor to the HART Communication Foundation) maintains a public comparison matrix of revisions 5, 6, and 7. The matrix confirms that all revisions use the identical 1.2 kb/s physical layer and identical Universal Commands 0, 1, 2, 3, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21.

Backward Compatibility Mechanics

Backward compatibility in HART is implemented at three layers:

1. Physical layer: Every revision uses Bell 202 FSK at 1.2 kb/s centered on the 4-20 mA loop. The 1200 Hz mark and 2200 Hz space tones are unchanged, so any HART host modem can decode any HART slave.
2. Universal command layer: Universal Commands 0 through 21 are mandatory and revision-stable. A HART 5.2 slave answers Universal Commands in the same format as a HART 7.2 slave, including the PV in engineering units (Command 3) and the loop current (Command 2).
3. Common Practice command layer: Commands 22 through 130 are optional. A HART 5.2 slave returns a "not implemented" response code (byte 1, bit 7 of the response status byte = 1) for any HART 7-specific command it does not recognize. The host interprets the response code and continues without error.

The MAG 6000 with FDK-085U0226 therefore responds to any HART host running Universal Commands regardless of host revision. The host can poll the PV, SV, TV, QV, tag, descriptor, date, and message fields without revision-specific negotiation. The constraint appears only when the host attempts a Common Practice command unique to revision 6 or 7.

Siemens MAG 6000 HART Module (FDK-085U0226) Specifics

The FDK-085U0226 is a plug-in communication module for the MAG 5000 / MAG 6000 / MASS 6000 transmitter family. It installs in the transmitter's communication slot and presents a HART slave to the 4-20 mA loop. The accompanying operating manual is document order number FDK-521H1197, publication code SFIDK.PS.023.F1.02. The manual confirms HART revision 5.2 as the implemented protocol level.

Functional capabilities at HART 5.2:

The MAG 6000's PV represents the volumetric flow rate in the configured engineering unit (m³/h, l/s, US gpm, etc.). SV, TV, and QV map to totalizer counts, electronics temperature, and empty-pipe detection flag, respectively. All four dynamic variables are accessible to a HART 7.2 host via Command 9 (read device variables with status).

Feature Matrix: HART 5.2 Versus HART 7.2

Multi-Master and Multi-Slave Network Topology

A HART loop supports up to two masters and up to 15 slaves per multidrop network segment. The two master "slots" are reserved for a primary host (typically a DCS or PLC analog input channel) and a secondary handheld configurator (typically a 475/375 Field Communicator). Masters arbitrate by carrier detect; the master that detects the loop idle first transmits.

In a mixed-revision network, the topology shown below is common in retrofit installations:

+-----------------+      4-20 mA + HART 1.2 kb/s      +------------------+
| HART 7.2 Host  |-----------------+------------------| FDK-085U0226     |
| (SIMATIC S7-   |                 |                  | on MAG 6000      |
|  1500 AI 8x    |                 |                  | (HART 5.2 slave) |
|  HART HF)      |                 |                  +------------------+
+-----------------+                 |
                                    +------------------+
                                    | HART 7.2 Valve   |
                                    | Positioner       |
                                    +------------------+
                                              |
                                    +------------------+
                                    | 475 Field        |
                                    | Communicator     |
                                    | (Secondary       |
                                    | Master)          |
                                    +------------------+

The HART 7.2 host polls the MAG 6000 with Universal Commands first, confirms PV availability (Command 3), then issues only the Common Practice commands the MAG 6000 firmware recognizes. Any unrecognized command returns response code byte 1 bit 7 set, which the host logs and skips. The host's DD (Device Description) library either downgrades to the MAG 6000's revision 5.2 DD or presents a "limited feature" indicator in the host HMI.

Integration With SIMATIC S7-1500 / ET 200MP AI 8xHART HF

The SIMATIC AI 8xHART HF analog input module (part of the ET 200MP distributed I/O family) processes HART communication in multiplex mode, channel-by-channel in succession. The module supports up to eight HART variables per channel and is documented in the Siemens Industry Online Support portal under entry ID 109821074.

Configuration steps for adding a MAG 6000 to a SIMATIC HART channel:

1. In TIA Portal, add the AI 8xHART HF module to the ET 200MP station and assign it a PROFINET device name.
2. Open the module properties, select the channel connected to the MAG 6000 loop, and enable HART for that channel.
3. Set the channel's HART "short address" to 0 for point-to-point, or assign a unique 1-15 address if multidropping multiple MAG 6000 transmitters on a single loop.
4. Map the desired variables (PV = flow, SV = totalizer, TV = temperature) to the HART channel's cyclic data area. Each variable occupies 5 bytes (4-byte IEEE-754 float + 1-byte status).
5. Download the project. The module polls each HART channel in turn; mixed-revision slaves (HART 5.2 and HART 7.2 on different channels of the same module) are managed independently.

Integration With Allen-Bradley ControlLogix HART I/O

The 1756 family includes analog input modules with HART pass-through, notably the 1756-IF8H and 1756-IF8IH. The product profile 1756-PP016 confirms that these modules expose the full HART command set to the Logix Designer application, including Universal and Common Practice commands. The MAG 6000 PV, SV, TV, and QV appear as REAL tags in the controller's tag namespace.

To enable HART on a 1756-IF8H channel:

1. Add the 1756-IF8H module to the ControlLogix chassis in Logix Designer.
2. Open Module Properties > Configuration tab, select the channel, and set HART Enable = Yes.
3. Choose "Point-to-Point" for a single MAG 6000 per channel, or "Multidrop" with the MAG 6000 polling address set to 1-15.
4. Enable the HART status tag HART:.Sts to capture device revision, command response codes, and configuration changed flag.
5. Use an AOI (Add-On Instruction) or the pre-built HART Command Send/Receive ladder instructions to issue Common Practice commands if needed.

Integration With Honeywell MasterLogic 2MLF-DC4H

The Honeywell 2MLF-DC4H HART analog output module mounts in the MasterLogic ML200 rack and outputs a 4-20 mA signal while simultaneously passing HART traffic upstream to the controller. The user guide is published at Honeywell EDAM. The module implements HART master protocol revision 7 and accepts HART 5.x slaves transparently in the same manner as the SIMATIC and ControlLogix hosts above.

Configuration and Commissioning Procedure

When integrating a MAG 6000 with FDK-085U0226 into a HART 7.2 host system, follow this sequence:

1. Verify hardware revision. Read the HART revision with a 475/375 Field Communicator: Online > Device Information > Revision. Confirm "HART Revision 5.2" and "Device Revision 1" (or as labeled on the FDK-085U0226 label).
2. Set the polling address. For point-to-point, leave address = 0. For multidrop with up to 15 MAG 6000s on one loop, assign addresses 1-15. Each transmitter must have its own loop power supply capable of sustaining 4 mA at the multidrop idle current.
3. Configure PV damping and engineering units. Use Command 138 (or the local MAG 6000 keypad) to set PV damping between 0.1 and 99.9 seconds.
4. Download the correct DD to the host. Import the FDK-085U0226 DD (revision 5.2) into the host's DD library. The host will present HART 5.2 menu items; HART 7-specific menu items (wireless configuration, block transfer, long tag) will display as "not available."
5. Map variables to the controller. Map PV to the primary process value register; map SV to totalizer; map TV to electronics temperature.
6. Verify loop integrity. Measure loop resistance (250-600 Ω typical), confirm 24 VDC compliance voltage, and check for shield grounding at a single point only.
7. Validate communication. From the host, issue Command 0 (read unique identifier) and confirm Manufacturer ID = 0x56 (Siemens) and Expanded Device Type matches the MAG 6000 family code.

Verification Checks

After commissioning, perform these verification steps to confirm proper interoperation:

Troubleshooting Matrix

Safety and Functional Considerations

For new installations requiring HART 7 features (wireless, block transfer, long tag), specify a current-generation Siemens flowmeter such as the SITRANS F M MAG 6000 with the appropriate HART 7 add-on module, or migrate to the SITRANS F M MAG 8000 / MAG 8000 CT with native HART 7 support. The MAG 6000 with FDK-085U0226 remains fully supported for maintenance and spare-parts purposes.

Summary of Interoperability Limits

The MAG 6000 with FDK-085U0226 interoperates with HART 7.2 hosts on the same 4-20 mA loop at 1.2 kb/s without modification. The shared Universal Command set (Commands 0-21) returns identical data to a HART 5.2 host, a HART 6 host, or a HART 7.2 host. HART 7-only Common Practice commands (long tag, block transfer, wireless join, time-stamped variables, catch device variable) return "not implemented" response codes and do not interrupt host polling. WirelessHART functionality is physically unavailable because the FDK-085U0226 carries no radio transceiver. For mixed-version installations where HART 7 features are required on the flow measurement point, replace the legacy module with a current-generation HART 7 transmitter; otherwise, the existing MAG 6000 installation continues to deliver PV, SV, TV, and QV over the standard 4-20 mA + HART signal pair indefinitely.