Troubleshooting Redundant Analog Input Modules (6ES7331-7TF01-0AB0) and Marshalled Termination Assemblies (MTA) in SIMATIC PCS 7

When redundant analog input modules (6ES7331-7TF01-0AB0) connected via an Analog Marshalled Termination Assembly (MTA 6ES7650-1AA61-2XX0) return 0 counts in a VAT table despite forcing 4-20 mA signals, the failure is caused by an open circuit or configuration mismatch on the secondary redundant partner channel. In redundant mode, S7-400H operating systems evaluate discrepancies between both cards; if one card detects an open circuit, missing auxiliary power, or incorrect MTA jumpering, the CPU flags a fatal discrepancy and clamps the redundant process value to 0. In non-redundant mode, the channel functions because discrepancy evaluation is disabled.

Operating Principles of S7-400H Redundant Analog Inputs

In high-availability SIMATIC PCS 7 installations, mission-critical 4-20 mA analog measurements are digitized using redundant analog input modules (such as 6ES7331-7TF01-0AB0 8xAI HART):
* The Redundancy Pair:
Two identical modules are installed in separate ET 200M distributed I/O racks (e.g., Rack 7 Slot 4 and Rack 9 Slot 7).
* Controller Redundancy Management:
Configured in HW Config under module properties -> Redundancy tab:
* The S7-400H operating system continuously reads both modules.
* It executes discrepancy evaluation: comparing the digitized counts from Card A and Card B.
* If both cards agree within configured tolerance, the CPU passes the averaged or lower value into the primary process image address.

The Marshalled Termination Assembly (MTA) Signal Distribution Architecture

To avoid dual field wiring from a single transmitter:
* The Analog MTA (6ES7650-1AA61-2XX0):
Field cables land on the MTA terminal blocks. The MTA splits the single 4-20 mA current loop into two isolated outputs routed via two 25-pin D-sub cables to Module 1 (Rack 7) and Module 2 (Rack 9).
* Hardware Jumpering:
The MTA features hardware configuration jumpers for each channel:
* 2-Wire Transmitters (MTA provides loop power).
* 4-Wire Transmitters (Field instrument powers the loop).
* Single vs Redundant Card Operation.

Why Redundant Channels Return 0 Counts While Non-Redundant Works

When an engineer forces 4-20 mA and observes normal counts in non-redundant mode but 0 counts in redundant mode:
1. The Discrepancy Failure Clamping:
In non-redundant mode, the CPU reads Module 1 directly; Module 2 is ignored. In redundant mode, the CPU cross-checks Module 1 against Module 2.
* If Module 2 has a disconnected D-sub cable, a loose terminal, a blown fuse on the MTA, or an incorrect jumper, Module 2 reads 0 mA (Wire Break / Underrange).
* The discrepancy between Module 1 (e.g., 27,648 counts / 20 mA) and Module 2 (0 counts / 0 mA) exceeds the tolerance limit (typically 3% to 5%).
* The S7-400H OS declares the redundant pair Faulted and immediately clamps the output to 0 (or substitute value) to prevent unsafe operation.
2. Channel Range Configuration Mismatch:
If Module 1 is configured in HW Config as 4-wire 4..20 mA while Module 2 is left on default 2-wire or voltage, parameter assignment fails.

Comparison Table: 2-Wire vs 4-Wire Configuration on Module and MTA

The table below outlines proper alignment between field devices, MTA jumpers, and HW Config:

Parameter / Element2-Wire Loop Powered Instrument4-Wire Self Powered InstrumentPower SourceMTA provides 24V DC loop powerExternal instrument power supplyMTA Jumper SettingSet to 2-Wire positionSet to 4-Wire positionHW Config Measuring Type4DMU (4-wire current) / 2DMU4DMU (4-wire current 4..20 mA)Module Measuring Range CardPosition D (Current)Position C or D (Current)Common Failure SymptomBlown MTA channel fuseGround loop voltage offset

Step-by-Step Procedure to Diagnose and Calibrate Redundant MTA Channels

Inspect Module Diagnostic Buffer and LEDs:
* Open SIMATIC Manager -> HW Config.
* Connect online to the CPU and open Module Information (Ctrl+D) for both AI modules.
* Check if Module 2 (Rack 9 Slot 7) displays SF (System Fault) or reports wire break on the channel.
Verify Hardware Jumper Parity on MTA:
Inspect the physical MTA board (6ES7650-1AA61-2XX0). Confirm that channel jumpers for both Cable 1 and Cable 2 are configured identically for 4-wire operation.
Measure Voltage Drop Across Both AI Cards:
With a 4-20 mA signal injected at the MTA field terminal:
* Use a digital multimeter (mV range) to measure across pins M+ and M- on Module 1 connector. Confirm ~250 mV to 1000 mV (50-ohm internal shunt).
* Measure across pins M+ and M- on Module 2 connector.
* If Module 2 reads 0 mV, the D-sub cable is defective, unseated, or the MTA channel fuse is blown.
Synchronize HW Config Settings:
* In HW Config, verify that Module 1 and Module 2 have identical measuring ranges (4..20 mA HART), identical smoothing, and identical interference frequency suppression.
* In the Redundancy tab, ensure the partner rack and slot addresses are set correctly.
Download Hardware Configuration:
Download the compiled hardware configuration to the AS and verify in VAT table that raw counts now display between 0 and 27648.

Verification

To verify redundant analog channel operation:
1. Open a Variable Table (VAT) in SIMATIC Manager monitoring the process image input address (e.g., PIW 512).
2. Inject 12.0 mA at the MTA field terminals. Verify that PIW 512 reads approximately 13824 counts (50% span).
3. Unplug the D-sub cable to Module 1. Verify that PIW 512 continues reading 13824 counts from Module 2 without jumping to 0.
4. Reconnect Module 1 and unplug Module 2. Confirm continuous, bumpless process value delivery.

Limitations and Operational Precautions

Discrepancy Time Settings: Configure a reasonable discrepancy time (e.g., 50 ms to 100 ms) in HW Config to prevent transient noise spikes from declaring redundant channel faults.
HART Pass-Through in Redundant Mode: When using HART communication with 6ES7331-7TF01-0AB0, only the active master module processes acyclic HART commands via SIMATIC PDM.
Licensing Disclaimer: S7-400H redundant hardware (6ES7410-5HX08-0AB0), ET 200M interface modules, and MTA components must be validated against the applicable Siemens catalog and authorized distributor quotation.