Configuring HART AI Modules in Siemens S7-400H Redundant Systems

Overview of HART in S7-400H Systems

The Siemens SIMATIC S7-400H is a fault-tolerant automation platform that pairs two redundant H-CPUs (e.g., 6ES7 414-4HM14-0AB0 or 6ES7 417-4HT14-0AB0) running in hot-standby mode. HART (Highway Addressable Remote Transducer) field-device communication is not a native CPU function in the H-System; it is delegated to HART-capable analog input modules mounted in ET 200M distributed I/O stations. The most common module in this role is the SM 331 HART variant, of which the 6ES7 331-7TF00-0AB0 (8 AI, 0/4-20 mA, HART, 12-bit resolution) is the historical reference part number.

Two architectural constraints determine whether a HART device behind an S7-400H can be parameterized with SIMATIC PDM:

1. The HART module must be installed in an ET 200M station that is connected to the H-System via PROFIBUS DP. S7-400H central racks do not accept SM 331 modules; they only host CPUs, CPs, and interface modules.
2. SIMATIC PDM must reach the ET 200M station through data record routing, an extended routing mode used exclusively by PDM that is not supported by the internal DP and MPI interfaces of the H-CPU.

Hardware Prerequisites

Confirm the following components and catalog numbers before commissioning:

Refer to the S7-300 Module Data Manual (Entry ID 109825772) for the electrical and functional specification of the SM 331 HART variants.

Compatible SM 331 HART Analog Input Modules

The following SM 331 modules support HART and may be installed in ET 200M stations of an S7-400H:

Each HART module multiplexes up to eight 4-20 mA loops and exposes the HART signal on top of the analog current through the HART modem on the module. The ET 200M station must use an active backplane if the design requires module replacement during runtime, which is a typical requirement for H-System availability targets.

PDM Routing Fundamentals: Normal vs Data Record Routing

SIMATIC PDM does not communicate with HART field devices using standard S7 messaging. It uses data record routing, which is documented in Siemens Entry ID 7808062. The two routing modes are not interchangeable:

The base routing concept is described in Siemens Entry ID 584459. In short, data record routing forwards PDM parameter blocks together with the address structure of the target device, which means the addressing depends on the destination field device rather than on the CP.

Network Topology for PDM Access

Two topologies are commonly deployed. Each uses an external CP because the internal DP/MPI interfaces of the H-CPU do not implement DS routing.

Topology A: CP 443-5 Extended in the H-Rack

Topology B: IE/PB Link Gateway in the H-Rack

When the engineering station is connected through Industrial Ethernet, an IE/PB Link Gateway (6GK7 141-5AB10 or 6GK7 141-5BB00) mounted in the H-rack provides data record routing from the PG/PC Ethernet interface to the PROFIBUS segment that hosts the ET 200M stations. In this case, SIMATIC PDM must be integrated inside STEP 7; standalone PDM cannot terminate a data record route through the IE/PB Link.

HART on Redundant AI Pairs

In H-System designs, critical analog inputs are typically wired redundantly: two SM 331 modules read the same 4-20 mA loop from the field device. The HART signal is superimposed on the loop current and must therefore pass through both modules' input protection diodes. Two engineering consequences follow:

1. The redundant wiring topology inserts a series diode network into each branch; the effective loop voltage seen by the transmitter is the supply voltage minus two diode drops (~1.2-1.4 V total for silicon diodes). Verify that the transmitter's minimum terminal voltage still allows HART communication (typically ≥ 17 V at 20 mA for most HART 7 devices).
2. When both modules read the same loop, only one of them should be designated as the HART multiplexer master. The other module reads the analog value but suppresses HART transmission to avoid bus contention. Configure this in STEP 7 HW Config under the AI module properties "Inputs" → "Channel 0..7" → "HART" → "Operating mode".

STEP 7 Configuration Procedure

1. Insert the H-CPUs and CP 443-5 DX04 in HW Config of the S7-400H station. Place the CP in slot 4 or higher of the UR2-H rack; assign PROFIBUS address 2.
2. Add an ET 200M station on the same PROFIBUS subnet. Insert the IM 153-2 in slot 0 and the SM 331 HART modules in subsequent slots.
3. Configure the AI channels of the SM 331: set measurement type to "4DMU" (4-wire current, HART), enable diagnostics, and set the HART operating mode to "Multiplexed, 8 channels".
4. Assign a PROFIBUS address to each HART field device in the device index (HW Config → ET 200M → SM 331 → "HART" tab). PDM uses this address when building the data record route.
5. Open SIMATIC PDM from inside STEP 7 (menu: Options → SIMATIC PDM → Device Configuration). Choose "LifeList" to scan the PROFIBUS segment via the CP 443-5 DX04.
6. Upload or download device parameters via the PDM tree. The DS routing path is built automatically: PC → Ethernet → CP 443-1 → backplane → CP 443-5 DX04 → PROFIBUS → IM 153-2 → SM 331 HART → field device.
7. Repeat for the redundant partner module. PDM will expose two HART nodes per physical transmitter; bind both to the same logical HART tag in the plant view.

For Topology B, insert an IE/PB Link PN IO (6GK7 141-5BB00) in HW Config instead of the CP 443-5 DX04, and ensure the "DS Routing" checkbox is enabled in the link properties. STEP 7 will then add the link to the data record routing chain transparently.

Commissioning and Verification

After configuration, validate the HART path with the following checks:

Troubleshooting Matrix

Engineering Best Practices

• Always use a CP 443-5 Extended (not a CP 443-5 Basic) when PDM access is required. The Basic variant does not implement data record routing.
• Configure redundant SM 331 HART pairs as single-channel redundant (one HART master, one HART mute). Avoid 8-out-of-8 redundancy unless the field device supports simultaneous HART access (rare).
• Keep HART loop wiring short and shielded; HART is sensitive to capacitive loading and the redundant diodes already impose an extra barrier.
• Maintain a one-to-one mapping in PDM between the physical HART index and the logical TAG used by HMI/PCS 7; this avoids confusion after hot-swaps.
• Update the IM 153-2 firmware to V5.x or higher for full HART 7 compatibility, as documented in the ET 200M manual (Entry ID 21928932).