Troubleshooting ET 200SP DI LED Status vs TIA Tag Bit Mismatch

Problem Description

A common field issue with SIMATIC ET 200SP distributed I/O involves a divergence between the physical channel LEDs on a Digital Input (DI) module and the corresponding bits in the TIA Portal tag table. A typical symptom is one channel LED illuminated (e.g., LED #5) while the monitored tag bit shows a different index as TRUE (e.g., bit #6). When the field signal is then physically removed (door closed, sensor de-energized), the LED updates correctly but the tag bit remains stuck TRUE in the online watch table.

This is a field-proven hardware defect pattern, not a programming error. The discrepancy almost always points to either:

• A failed DI module where the input front-end is partially damaged, or
• A misinterpretation of the channel-status versus channel-fault LED behavior in TIA Portal diagnostics.

The reproducible test in the field is to swap the suspect module with a known-good module in the same ET 200SP base unit (BU). If the symptom follows the module, the DI module is faulty and must be replaced.

Affected Hardware and Module Families

The behavior is most often reported on the following SIMATIC ET 200SP DI modules:

For the high-feature (HF) variants with channel diagnostics, the green channel status LED and the red channel fault LED provide per-point indication. Reference the ET 200SP Manual Collection – Diagnostics Display of the Channel Status / Channel Fault LED for the canonical LED semantics that apply across the DI/AI module family.

ET 200SP DI Module LED Conventions

Each ET 200SP DI module front panel includes two LED groups:

• DIAG LED (green/red): Module-level diagnostics. Solid green = module OK and cyclic data exchange active. Flashing or red = module fault (no station parameter assignment, configuration error, channel fault, etc.).
• Channel status LED (green), 0 to n: One LED per input channel, indicating the logical state of the input. Numbering is zero-based; channel 0 is the leftmost LED on the module insert.
• Channel fault LED (red), 0 to n: Present on HF modules. Lights when a channel diagnostic is pending (wire break, short circuit, overflow, parameter error).

The channel status / channel fault LED diagnostic description explicitly states: The green channel status LED indicates whether the channel is enabled. The green LED does not light up if a channel is disabled. A lit green LED therefore means the channel is enabled and the input is HIGH; a dark LED means LOW or disabled.

Process Image of Inputs (PII) Behavior

The PLC updates the Process Image of the Inputs at the start of OB1 (and at the configured update points for OB3x, OB6x, and OB1x). When you watch a tag in TIA Portal, you are observing the PII value the controller most recently read from the IM / ET 200SP station.

For an ET 200SP station on PROFINET, the IM cycles input frames from each head module slot at the configured refresh rate. If a DI module's input ASIC is partially damaged:

1. The input comparator may latch HIGH internally on one or more channels.
2. The green LED correctly tracks the field wiring (because the LED driver is fed from the same comparator output stage) — or, in the failure mode most often reported, the LED follows the field signal but the PII word does not update because the SPI / backplane readback returns a frozen value.
3. The TIA Portal tag stays TRUE because the user program reads from the PII, which is the last value the head module reported.

When the field device is de-energized, the LED reflects the new state (LOW), but the corrupted input register is still being read by the IM, so the PII never transitions back to FALSE. This is a clear signature of silicon-level damage, not of a programming or PII access error.

Root Cause Analysis

The diagnostic tree below covers the major causes ranked by frequency in field service:

In the field case that motivates this article, three conditions were met: the green LED and the tag bit disagreed, multiple channels were dark despite 24 V present at the terminal block, and a physical module swap moved the failure to the other slot. That combination is conclusive evidence of a hardware fault on the DI module's input stage.

Step-by-Step Diagnostics

1. Capture the symptom. In TIA Portal, open Online & Diagnostics > Watch Table for the ET 200SP station. Force a refresh and screenshot the tag values, the module's DIAG LED state, and the field-side voltage at the terminal block with a DMM.
2. Rule out user program access. Search the project for PEEK, POKE, direct peripheral accesses (%I with offset), and any FB/FC that writes into the PII. If the user program reads the input only through the symbolic tag (PII) in OB1, the PII is the only source of truth.
3. Check module configuration. In the device view, right-click the DI module > Properties > Module parameters. Confirm that the affected channel is enabled, that diagnostics are enabled, and that the input delay is appropriate for the sensor type.
4. Read the module diagnostics buffer. Online > the DI module > Diagnostics. Look for channel diagnostics (wire break, short circuit, parameter assignment error). Note the timestamp and event ID.
5. Cycle power to the IM and head module. If the symptom persists after a warm restart, the PII is not being refreshed by the DI module — the input stage is the suspect.
6. Swap the DI module with a known-good spare on the same ET 200SP station. Do not rewire the terminals; simply lift the module insert and reseat the spare. Observe whether the failure follows the suspect module or stays with the slot.
7. If the failure follows the module: Replace the module. Capture the serial number and firmware version for the warranty claim.
8. If the failure stays with the slot: Inspect the base unit (BU) contacts, the spring terminals, and the 24 V power feed to the BU. A damaged BU can produce identical symptoms and must be replaced as well.

Module Replacement Procedure

ET 200SP modules are hot-swappable on PROFINET when the IM is configured for "module replacement without programming device" (a standard setting). The mechanical and software procedure is:

1. Open the affected ET 200SP station's slot by unlatching the module insert.
2. Note the color code label on the module and the wiring label on the BU terminal block to ensure the replacement matches the I/O type.
3. Lift the suspect module straight out. Do not use tools; the latch is finger-operated.
4. Compare the new module's article number and firmware version against the configured device in the TIA Portal hardware catalog. If firmware is newer, expect TIA Portal to issue a "device version not assigned" warning — assign the configured version in the device properties, or update the project to the new firmware if desired.
5. Seat the replacement module until the latch clicks. The DIAG LED should illuminate green within a few seconds as the IM re-parameterizes the slot.
6. Verify in the online diagnostics that the module reports no channel faults and the DIAG LED is solid green.
7. Force a value test: apply 24 V to each input from a hand-held test source and confirm the tag bits track the LEDs.

Verification and Commissioning Checks

After module replacement, run the following checks to confirm the system is healthy:

• LED/tag parity test: For each channel, apply 24 V and 0 V alternately, and confirm that the corresponding tag bit transitions in the watch table. Disagreement between LED and tag means the replacement module is also defective or the BU is damaged.
• Diagnostic buffer clear: Open the IM's online diagnostics buffer and acknowledge any pending diagnostic events. Confirm no new entries appear after the I/O is exercised.
• PROFINET cycle health: Monitor the IM's port statistics (Online > Diagnostics > Port Statistics). Discarded frames or CRC errors on the PROFINET line will cause the PII to repeat the last good value, which can mimic a stuck input.
• Field wiring audit: Verify that all field devices share the same 24 V reference as the BU and that no field ground is bonded to PE in a way that creates a ground loop through the input.
• Spare module storage: Bag the suspect module with its article number, firmware version, and a short fault description. Send it to the local Siemens service center for RMA analysis to confirm root cause (over-voltage, ESD, thermal).

Common Edge Cases and Field Caveats

1. Mixed BU types in one station. A BU with a different color code (e.g., a BU15-P16+A0+2B vs. a BU20-P12+A0+4B) changes the channel count and the PII offset. If the TIA Portal project has the wrong BU type, the channel numbers and tag bits will be permanently misaligned regardless of module condition.

2. Firmware-downgrade trap. Replacing a 6ES7131-6BF01-0BA0 with a 6ES7131-6BF00-0BA0 (or vice versa) requires a firmware-compatible GSD. TIA Portal will refuse to go online if the device version does not match the project. Always match the article number and the firmware version suffix in the HW catalog.

3. Input filter misconfiguration. The input delay on HF modules can be set from 0.1 ms up to 20 ms. With a very long filter, fast input edges from a door switch can be filtered out — the LED will flicker but the PII stays LOW. This is not a module defect; it is a configuration error.

4. 24 V sensor powered from a different source. If the sensor pulls 24 V from a supply that is not referenced to the BU's 24 V terminal, the input may see a continuous differential of 24 V (LED on) while the BU input comparator reads 0 V (tag LOW) or vice versa. Bonding the supplies resolves this.

5. IRT vs. RT PROFINET timing. On IRT networks with very tight send clocks, a marginal DI module can produce cyclic IO faults that look like "stuck" PII values. Switch the station to RT first to isolate.

Related Diagnostic Standards and References

For formal channel-fault semantics, see the official ET 200SP documentation. The TIA Siemens Cloud ET 200SP Manual Collection – Channel Status / Channel Fault LED page is the canonical reference for the LED matrix. Always cross-check any field-observed LED behavior against the module's own diagnostic buffer rather than relying on a single LED's appearance.

Frequently Asked Questions