Selecting Siemens HART Analog Modules SM 331, SM 332, SM 336F

Selecting Siemens HART Analog Modules SM 331, SM 332, SM 336F for S7-300

Overview

Process plants such as sugar mills, refineries, and water-treatment facilities routinely deploy HART (Highway Addressable Remote Transducer) field devices: smart pressure transmitters, Coriolis flow meters, guided-wave radar level probes, temperature transmitters, valve positioners, and pH/conductivity analyzers. To bring these devices into a SIMATIC automation system, the I/O cards must themselves understand the Bell 202 frequency-shift keying (FSK) signal that rides on top of the 4-20 mA loop. For S7-300 and ET 200M stations, Siemens provides three HART-capable analog modules that meet the typical scope of a sugar-plant DCS/PLC upgrade:

These three modules cover the typical sugar-plant signal classes: non-safety process variables (SM 331), modulating control outputs to positioners and VFD references (SM 332), and safety-instrumented functions (SIF) such as burner-management pressure trips and emergency shutdown level interlocks (SM 336F in an S7-300F/ET 200M F-station).

SM 331-7TF01-0AB0 - 8AI HART Analog Input

The SM 331 AI 8x0/4-20 mA HART occupies a single slot in the S7-300 rack and presents eight galvanically isolated current inputs that each carry an overlaid 1200 bps HART signal. Typical use cases include:

• Sugar juice pH and conductivity probes
• Boiler drum level differential-pressure transmitters
• Steam flow (orifice + DP) and feedwater flow
• Crystallizer temperature (RTD via separate SM 331 RTD module; this part number is current-input only)
• Refractometer Brix and color sensor analog outputs

Channel configuration is set per channel group (2 channels per group in 4-channel mode; all 8 in 8-channel mode). Resolution is up to 16 bits depending on integration time selected in STEP 7 HW Config. Each HART-capable channel can operate in one of two modes:

• Point-to-point (PV mode): 4-20 mA loop carries the primary variable; HART is used only for diagnostics and parameterization of the field device. The S7 program reads the mA value as a normal analog input.
• Multidrop (4-20 mA fixed, HART only): Loop current is fixed (typically 4 mA) and the S7 user program reads all four HART variables (PV, SV, TV, QV) from up to 15 addressed devices on the same pair of wires. This mode requires a separate power supply per field device.

Typical per-channel wiring uses the 20-pin front connector (6ES7392-1AJ00-0AA0 screw-type or 6ES7392-1AM00-0AA0 spring-type). The HART signal is demodulated inside the module, so no external HART multiplexer is required - this is one of the principal cost savings of the SM 331-7TF series versus third-party multiplexer solutions.

SM 332-8TF01-0AB0 - 8AO HART Analog Output

The SM 332 AO 8x0/4-20 mA HART provides eight current outputs suitable for driving:

• Pneumatic valve positioners (Fisher DVC, Samson 3725, Siemens SIPART PS2) where the positioner feedback can be read back through the HART layer
• Variable-speed drives configured for 4-20 mA reference with HART status
• Damper actuators and feeder speed references on mill and diffuser lines

The module's HART capability is most often used for read-back of actuator diagnostics (e.g., travel accumulator, seat wear, internal temperature) rather than for command - the AO itself is set by the S7 user program as a 16-bit integer in the output process image. To use the HART read-back, the output module must still close the loop through a HART-aware device.

Important installation notes:

• Outputs are short-circuit proof and galvanically isolated from the backplane bus.
• The 24 V sensor/load power for the field loop is normally supplied from the same 24 VDC distribution that feeds the S7-300 power supply (PS 307). For HART signal integrity, the common of the loop supply should be tied to the S7-300 ground bar at one point only.
• Front connector: same 20-pin 6ES7392-1AJ00-0AA0 / -1AM00-0AA0 as the AI side.

SM 336F-4GE00-0AB0 - 6AI HART, PROFIsafe V2

The SM 336F is the safety-integrity variant of the AI module. Six channels (vs. eight on the standard SM 331) are provided to allow per-channel redundancy and diagnostic headroom required by PROFIsafe V2. The module is recognized by the F-CPU through the PROFIsafe protocol on PROFINET or PROFIBUS, with the safety message CRC and watchdog handled in firmware.

Functional features per the official designation string:

• AI 6x0/4-20 mA HART
• PROFIsafe V2 (up to SIL 3 / Category 4 / PL e per IEC 61508 / ISO 13849-1, application-dependent)
• Safety-related operation with diagnostic interrupt
• HART communication maintained alongside the safety layer
• Firmware updateable in service (via SIMATIC PDM or STEP 7 HW Config) - useful for end-customers running long-life plants who must patch against HART device-library changes
• Redundancy-capable when deployed in ET 200M with two SM 336F modules feeding the same PROFIsafe address

Typical safety loops in a sugar plant that use this module:

• Boiler drum low-low level trip (SIF)
• Bagasse-fired boiler furnace pressure high-high trip
• Crystallizer overspeed / overset safety interlock
• Molasses tank high-high level with hardwired ESD
• Sugar-mill hydraulic roll-gap safety trip (SIL 2/3 depending on MR calculation)

The fail-safe behavior (power-down to safe state, de-energize to trip) is configured per channel in the F-CPU's safety program; HART variables remain accessible in the standard program for predictive maintenance.

SIMATIC PDM Integration

The HART variables, device parameters, and EDD-based diagnostic menus are not interpreted by STEP 7 itself. They are exposed through SIMATIC PDM (Process Device Manager). The classic order number discussed in project scoping is:

PDM V6.0 is an older product line. The original release targets Windows 2000 / Windows XP, which are no longer supported by Microsoft. For any new sugar-plant project, engineers should confirm with Siemens Industry Online Support the current PDM version (the PDM 9.x line supports Windows 10/11 and Server 2016/2019/2022) and the matching device library. The 128-TAG limit in the V6.0 bundle is small for a complete mill - factor in either multiple PDM server stations or an upgrade to the larger TAG-count bundle.

PDM accesses the HART modules through:

1. Direct HART modem on the module's serial service port - older approach, requires the module to be physically accessible.
2. PROFIBUS DP-V1 / PROFINET with HART tunnel - the preferred modern path. The HART variables are tunneled through the I/O backplane to the PDM server using the "HART over PROFINET" or "PROFIBUS PA" route. No additional multiplexer is needed.

S7-300 System Prerequisites

To deploy the modules listed, the S7-300 station must meet the following baseline:

• CPU: Any S7-300 CPU with sufficient work memory for the configured channels. For F-modules, an F-CPU variant (CPU 315F-2 DP, CPU 317F-2 PN/DP, CPU 319F-3 PN/DP, or equivalent ET 200S/ET 200pro F-head module). Verify the F-capability in the CPU's datasheet.
• Power supply: PS 307 (5 A, 10 A) sized for backplane current plus the 24 V field-side loop load. Each HART module draws approximately 100-150 mA from the backplane.
• STEP 7 version: STEP 7 V5.4 SP5 or later for the V6.0 PDM bundle. Newer PDM releases require TIA Portal or STEP 7 V5.6 depending on the product.
• Front connectors: 6ES7392-1AJ00-0AA0 (screw) or 6ES7392-1AM00-0AA0 (spring). Order 1 per AI/AO module.
• Labeling strips: 6ES7392-2XX00 series for channel identification.
• HART handheld communicator (optional): For loop-check before connecting to the module, a 475/375 Field Communicator lets the engineer confirm the device's PV range and HART tag without the PLC online.

Wiring and Field-Side Considerations

HART communication is sensitive to loop impedance, cable capacitance, and grounding. The following rules apply on every channel of the SM 331-7TF01, SM 332-8TF01, and SM 336F-4GE00:

1. Loop resistance budget. The total loop resistance (transmitter + wiring + receiver input) must be between 230 and 600 ohm for reliable HART. The S7 module itself presents roughly 50-250 ohm depending on configuration; budget the rest to the field device and cable.
2. Shielding. Use twisted, overall-shielded pairs (e.g., Belden 8761 / 8762 or equivalent). Ground the shield at the cabinet entry only; do not ground at the field device to avoid ground loops.
3. Separation from VFD cables. Analog/HART cables should maintain at least 200 mm separation from VFD power cables, or cross at 90 degrees. VFD-induced common-mode noise is a leading cause of intermittent HART communication errors.
4. Cable length. Typical HART twisted-pair runs work to 1500 m (multidrop less, depending on capacitance). Verify against the device datasheet.
5. Surge protection. For long outdoor runs (raw juice lines, molasses tank farms), install surge arrestors (e.g., Phoenix Contact PT-IQ series) on each analog/HART pair to protect the module's input stage.
6. Multidrop addressing. If the loop is configured for multidrop, each device must have a unique HART short address (1-15) and the loop current must be set to fixed 4 mA in the device configuration. The S7 user program reads PV via the HART library rather than from the analog value.

HART Variable Mapping into S7

Each HART-capable SM 331 channel exposes the four standard HART variables (PV, SV, TV, QV) in addition to the loop-current value. In STEP 7 V5.x with PDM, the variables are mapped into the input process image as follows (per channel, verify exact offsets in HW Config > Module Properties > HART):

The exact offsets depend on the configured number of channels and the HART operating mode (PV-only vs. full four-variable). Always read the module's HW Config online diagnostic and use the PDM-exported symbol table when wiring the user program. Reading the wrong offset is a frequent commissioning defect.

Selection Matrix for Sugar-Plant Applications

Field Commissioning Procedure

1. Loop check before module power-up. With the SM 331 or SM 336F module removed, power the transmitter from a temporary 24 V supply with a 250 ohm resistor and a multimeter in mA mode. Verify 4 mA at LRV and 20 mA at URV. Use a 475/375 communicator to confirm HART tag, range, and engineering units.
2. Install module, connect front connector. Torque front-connector screws to 0.6-0.8 Nm. Confirm shield is bonded at the cabinet gland only.
3. HW Config in STEP 7. Insert the module from the catalog (SM 331 > AI8 x 0/4-20 mA HART, or AI6 x 0/4-20 mA HART F). Assign channel diagnostics, integration time (typically 20 ms for noise rejection), and HART operating mode.
4. Download and go online. In online mode, observe the module's diagnostic buffer. A green RUN with no SF/FB errors indicates the module is healthy and HART communication is established.
5. Open PDM and assign device. PDM > Device > Assign. PDM will discover the device via the module's HART tunnel. Load the matching EDD from the device library. Confirm PV, SV, TV, QV mapping.
6. Verify in user program. Force a known PV in the field device and observe the corresponding REAL variable in the VAT table. Cross-check against the multimeter mA reading.
7. Document tag database. Export the PDM project for the plant asset management system. This is the deliverable used by maintenance for future EDD updates.

Troubleshooting Matrix

Accessories and Companion Parts Checklist

When scoping the bill of materials for a sugar-plant HART upgrade, the following accessories are typically co-ordered:

• Front connector, 20-pin screw: 6ES7392-1AJ00-0AA0
• Front connector, 20-pin spring: 6ES7392-1AM00-0AA0
• SIMATIC PDM V6.0 with 128 TAGs: 6ES7 658-3KX06-0YA5 (legacy) or current equivalent per Siemens IA/DT
• PDM service pack / device library update (subscription)
• HART handheld communicator (Emerson 475/375, or current equivalent) for loop commissioning
• 250 ohm precision resistor per HART channel (mounted in marshalling cabinet)
• Surge arrestor per outdoor channel
• Shielded twisted-pair field cable, instrumentation grade
• Cabinet gland and EMC backshells for each cable entry

Notes on Migration to S7-1500 / ET 200SP

Although this article targets S7-300, sugar plants expanding to S7-1500 should note that the equivalent HART modules are in the ET 200SP family (e.g., AI 4xI 2-/4-wire HART, order numbers in the 6ES7534 / 6ES7134 range). The HART variable mapping and PDM integration model remain identical; the rack and backplane change only. Existing SM 331-7TF01, SM 332-8TF01, and SM 336F-4GE00 modules can be re-used unchanged in an ET 200M station on PROFINET, preserving investment while modernizing the controller.

Safety and Standards Reference

When deploying the SM 336F in a SIF, the safety function must be verified to the target SIL per IEC 61511 (process industries) or ISO 13849-1 (machinery). Relevant inputs:

• IEC 61508 / IEC 61511 - functional safety of electrical/electronic/programmable electronic safety-related systems
• ISO 13849-1 - safety of machinery, safety-related parts of control systems
• NFPA 85 - boiler and combustion systems hazards code (sugar-mill boiler context)
• HART Communication Protocol specification (FieldComm Group, formerly HCF)

Verify PFH, PFDavg, and SFF values from the SM 336F safety manual against the SIL target of the loop. Do not assume SIL 3 without calculation.