Siemens 6ES7 332-7ND02-0AB0: Wire Break Diagnostic Failure Fix

Problem Statement: SM 332-7ND02 Wire Break Diagnostic Does Not Report

An ET 200M distributed I/O station connected to a SIMATIC S7-414-5H redundant CPU via PROFIBUS DP is fitted with analog output module 6ES7 332-7ND02-0AB0. Group and channel diagnostics are enabled in HW Config. A field engineer forces a 10 mA setpoint in user code, disconnects the current loop wiring at the channel terminals, and observes the following symptoms:

• SF (group fault) LED on the SM 332 module remains OFF.
• Diagnostic record read via SFC 59 "RD_REC" returns no channel fault.
• EXTF LED on the S7-414-5H does not indicate an external fault.
• OB 82 (diagnostic interrupt OB) does not trigger.

This behavior is reproducible across power cycles, CPU restarts, and re-downloads of HW Config. The same configuration previously worked on the predecessor article number 6ES7 332-7ND00-0AB0. The engineer concludes that something fundamental about the module’s diagnostic capability has changed between revisions, and the project must be migrated to 6ES7 332-5DH01-0AB0 to restore the wire break detection that the application logic expects.

Module Identification and Variant Comparison

Three SIMATIC S7-300 / ET 200M analog output modules are commonly confused in the field. The MLFB (Maschinenlesbare Fabrikatebezeichnung / machine-readable product designation) encodes the resolution, isolation, output type, and diagnostic class.

The visual faceplate differences are minor. All three modules ship as standard S7-300 form-factor plug-in cards for the ET 200M IM 153 interface modules. Distinguishing them in the cabinet requires reading the MLFB label under the door flap, not relying on the SF / ON LED layout.

Diagnostic Architecture in ET 200M over PROFIBUS DP

Before diagnosing the SM 332 specifically, the diagnostic propagation path from the distributed I/O station back to the S7-414-5H CPU must be understood:

1. Channel fault detection – The SM 332 analog front-end measures output current / voltage. If the configured diagnostic detects a fault condition (wire break, short circuit, range violation), the channel ASIC sets an internal bit.
2. Module diagnostic interrupt – The IM 153-x interface module requests a diagnostic interrupt from the PROFIBUS DP master (CPU 414-5H). The interrupt is signalled via PROFIBUS DP “diag” telegram and stored in the DP slave diagnostic buffer.
3. OB 82 trigger – The CPU operating system detects the incoming diagnostic event and calls OB 82 (diagnostic interrupt). The EXTF LED on the H-CPU illuminates. OB 82 start info contains the logical base address of the offending module and the channel number.
4. SFC 59 / SFC 13 – From OB 82, the user program typically calls SFC 59 "RD_REC" with record index 0x0000–0x000F to fetch the standardized diagnostic records (DS0 / DS1) and manufacturer-specific records. A wire break appears in record 0x0080 type “Channel” with channel fault code 0x0006 (wire break) for current outputs.
5. SF LED illumination – The IM 153 sets the SF LED on the SM 332 faceplate as soon as the module-ASIC sets the diagnostic bit, regardless of CPU state.

Failure of any link in the chain results in the symptom pattern reported in the source: SF LED off, no OB 82, SFC 59 returns empty diagnostic buffer. The H-CPU's EXTF indicator reflects the entire DP segment, so a missed diagnostic on a single channel is sufficient to keep the LED dark.

Root Cause Analysis

Three plausible root causes exist for the observed symptom pattern. Each must be eliminated in order.

The decisive evidence in the case documented by the source engineer is that the same project (identical HW Config, identical wiring, identical test procedure) previously triggered wire break detection on 6ES7 332-7ND00-0AB0. After the cabinet was retrofitted with 6ES7 332-7ND02-0AB0 during a parts-replacement event, the diagnostic stopped firing. The Siemens local field office confirmed that the 7ND02 was marketed as the successor to the 7ND00 but dropped wire-break-on-current diagnostic as a hardware design choice.

Configuration Verification Procedure

Before assuming the module is the root cause, walk the configuration in HW Config (STEP 7 V5.x) or in the TIA Portal device view.

1. Open the ET 200M station and select the SM 332 slot.
2. Double-click the channel to open the Properties dialog.
3. On the Outputs tab, set Output type = Current.
4. Set Output range = 4  — 20 mA (do not use ±20 mA or 0 — 20 mA if wire break detection is required).
5. On the Diagnostics tab, enable:
   • ☐ Group diagnostics
   • ☐ Wire break
   • ☐ Short circuit to M (where supported)
6. Confirm the diagnostic address assigned by the configurator (e.g. 256—257 for diagnostic, 512–519 for process outputs).
7. Save, compile, and download to the CPU.
8. Perform a full STOP → RUN transition on the CPU and verify the diagnostic byte returns 0x00 in SFC 59 read of record 0.

If, after this walkthrough, wire break still does not report, the module hardware itself does not support the feature — proceed to migration.

Diagnostic Block Implementation

Standard diagnostic handling in STEP 7 V5.x uses OB 82 for the interrupt entry, and SFC 59 to extract the standardized record. The skeleton below works on any S7-300 / S7-400 CPU pair.

// In OB 82 (Diagnostic Interrupt OB)
// OB_82_PARAM: pre-defined local tags of OB 82
//   OB82_EV_CLASS    : B#16#39 (incoming) / B#16#38 (outgoing)
//   OB82_MDL_DEF_ADDR : Base diagnostic address of the module
//   OB82_IO_FLAG     : 0 = input, 1 = output
//   OB82_FLT_ID      : B#16#05 = channel fault, B#16#06 = module OK

L     #OB82_MDL_DEF_ADDR        // Diagnostic address
T     #wDiagAddr                // Buffer for SFC 59 call

CALL  "RD_REC"                  // SFC 59
REQ   := TRUE
IOID   := B#16#54               // 'S' output area (W#16#54)
LADDR  := #wDiagAddr
RECNUM := B#16#00               // Record 0 = standardized diagnostic
RET_VAL:= #iRetVal
BUSY   := #bBusy
RECORD := P#DBX 0.0 BYTE 32     // 32-byte receive buffer in instance DB

NOP   0

For channel-level decoding, read record B#16#80 (manufacturer-specific channel diagnostic). The fault byte at offset 4 of that record contains the channel number; offset 5 holds the fault code, where 06h = wire break on current output.

On the S7-414-5H redundant CPU, both CPUs subscribe to diagnostic events from the DP line. Whichever CPU is in master role services the OB 82 call; the partner CPU receives the same event as a passive read-only indication. Diagnostic interrupts on ET 200M propagate over PROFIBUS DP “diag” telegrams without DP master interaction.

Replacement Module Migration Path

When the deployed 7ND02 module cannot report wire break in the existing project, three engineering options are available. Choose by application criticality and spare-parts strategy.

Option A — Replace with 6ES7 332-5DH01-0AB0 (preferred)

The 5DH01 module is an 8-channel 12-bit analog output with full diagnostic support on 4–20 mA ranges. Migration steps:

1. Update the BOM and MLFB in HW Config to 6ES7 332-5DH01-0AB0.
2. Re-wire the front connector: channels 0–3 of the 7ND02 align with channels 0–3 of the 5DH01 (same pin layout for AO0–AO3 on the 40-pin front connector).
3. Channels 4–7 of the 5DH01 remain disconnected or are wired to new field devices.
4. Re-process the output process image (PAA) allocation: the 5DH01 occupies 8 output words, vs 4 output words for the 7ND02. Adjust any direct PAA access in the user program.
5. Reload HW Config and user program. Re-verify wire break with the procedure below.

Option B — Source a remaining 7ND00 stock

If process image and terminal assignment must be retained, a spare 6ES7 332-7ND00-0AB0 keeps the 4-channel footprint. Note that 7ND00 is factory-discontinued; long-term spares availability is the controlling constraint. Confirm with Siemens Industry Online Support spare-part status before basing a project on this variant.

Option C — Accept no wire break detection

For non-safety, non-critical loops, accept that the 7ND02 will not detect broken wires on current outputs and add a redundant process-level detection (e.g. read back the current at a paired analog input). This is not a path to a SIL-compliant installation.

Verification Tests

After applying the migration, perform the following five-step verification on at least one channel:

1. Loop continuity check. Force a 12 mA output from the user program. Measure current at the load with a calibrated multimeter. Expected: 12 mA ± 0.1 mA.
2. Wire break simulation. Disconnect the +ve wire at the field terminal. Expected within 1.5 s: SF LED on the SM 332 illuminates, OB 82 fires on the CPU, EXTF LED illuminates on the H-CPU, SFC 59 record 0x80 returns channel fault 0x06.
3. Reconnect. Re-attach the wire. Expected within 1.5 s: SF LED clears, OB 82 fires with outgoing event, EXTF LED clears once all faults clear.
4. Short circuit test. Short the output to M with a test lead. Expected: channel reports short-circuit diagnostic (fault code differs by module, typically 0x07).
5. Process value check. Confirm that the analog output value present in PAE reflects the value set by the user program. No diagnostic should be raised when the loop is intact.

Related Modules and Compatibility Notes

Engineers working on ET 200M Profibus stations may encounter similar diagnostic-related regressions on adjacent modules. The following notes capture recurring field findings.

• SM 332-7ND01-0AB0 – alternative 4×12 bit analog output. Verify wire-break support against the current edition of the SM 332 manual before commissioning. Some variants support wire-break on voltage outputs only.
• SM 332-5HF00-0AB0 – 8-channel 16-bit high-resolution analog output. Diagnostic support is broader but the module requires TIA Portal V13+ for full configuration.
• SM 332-1BL00-0AB0 – older 2-channel module with limited diagnostic range. Generally not interchangeable with the 7ND family.
• ET 200SP / ET 200MP alternatives – modern successors in the SIMATIC portfolio. The diagnostic philosophy is documented in the ET 200SP / ET 200MP system manuals, available via Siemens Industry Online Support. A representative entry point for analog wire break diagnosis on the ET 200SP F-AI module is the TIA Portal help collection, e.g. TIA Portal help: Diagnosis Wire break (F-AI 8xI 0/4..20 mA).

For general SM 332 documentation, retrieve the latest manual from the Siemens Industry Online Support portal (support.industry.siemens.com) by searching the MLFB. The product folder for the 7ND02 contains a section on “Diagnostics” that explicitly enumerates which output ranges support wire break. Treat the manual revision date as the authoritative version — older revisions may still describe wire break as supported, even though the latest hardware release does not.

Frequently Asked Questions