Resolving Blinking Green DIAG LEDs on SIMATIC ET200SP I/O Modules

Problem Overview

When commissioning a SIMATIC ET 200SP distributed I/O station, a common field symptom is the upper status LED on every plugged I/O module flashing green at approximately 2 Hz. The interface module (for example, the IM 155-6 DP HF, 6ES7155-6BA01-0CN0) typically reports a clean PROFINET or PROFIBUS connection, the station powers up, and the device itself appears in the topology, yet every digital input, digital output, analog module, and technology module shows the same flashing-green state.

This condition is not a hardware fault. The I/O module is alive on the backplane bus, it is enumerated, and the backplane is healthy. The flashing-green DIAG LED on the I/O module is the documented Siemens signal that the module has not yet received an assigned parameter set from the IO controller. The configuration in the project tree exists as a slot, but the channel-level parameters have not been assigned, compiled, and downloaded to the interface module's I/O database.

According to the ET 200SP system manual, the I/O module's upper LED (the DIAG/status LED) is interpreted as follows when it is in the green-flashing state: "Module parameters are not assigned." The remedy is not to replace the module or reseat it. The remedy is to assign the module parameters in TIA Portal, compile the hardware configuration, and download the hardware changes to the interface module.

ET 200SP I/O Module DIAG LED State Reference

The Siemens ET 200SP status and error displays documentation defines the upper status LED (the channel/DIAG LED) on the I/O modules with the following states.

The same vocabulary applies to the DIAG LED on the interface module. On the IM 155-6 DP HF, an additional state is documented: "Interface module is not configured" (DIAG LED off, with the corresponding RN/ER/MAINT LEDs giving a more specific combination). The relevant excerpt from the Siemens manual states: "Interface module is deactivated. Activate the interface module with the configuration software or the user program. Interface module is not configured." This state typically indicates that the slot has been inserted in the project but the PROFINET device name or the device configuration has not been downloaded to the interface module yet.

It is critical to distinguish the IM-level DIAG LED from the I/O-module DIAG LED. A flashing-green IM DIAG LED is a different diagnostic than a flashing-green module DIAG LED. The IM 155-6 DP HF DIAG LED reflects the head-end configuration state. Each I/O module's DIAG LED reflects that module's own parameter-assignment state. When every I/O module is flashing green, it is a per-module parameter issue, not an interface-module failure.

Root Cause Analysis

The root cause of every I/O module showing the green-flashing DIAG LED simultaneously is that no module parameter set has been downloaded to those slots. There are several ways an ET 200SP station can end up in this state during commissioning:

1. A new module was physically added to a BaseUnit after the last hardware download. The PLC has a stale configuration that does not yet include the new module's parameter record.
2. A module was replaced with a different article number (for example, a 6ES7131-6BF00-0BA0 replaced with a 6ES7131-6BF01-0BA0) and the new module's parameters differ from the previous module. TIA Portal flags the slot as not equal.
3. The I/O module was inserted into the configuration with default values, and any channel that requires a parameter (input delay, diagnostics enable, substitute value behavior, measurement type) has not been explicitly set.
4. The hardware configuration was never compiled and downloaded to the interface module after the modules were added. The PROFINET connection is up, but the IO controller's process image does not yet have an active record for the slots.
5. A firmware update on the IM 155-6 or the I/O module changed parameter defaults, and the project was opened with an older TIA Portal version that does not know the new defaults.

The Siemens manual is explicit: the green-flashing DIAG LED on the I/O module means the parameters are not assigned. It is not a wire break, not a backplane fault, and not a power-supply issue. Treating it as a hardware fault by reseating modules, swapping them, or replacing the BaseUnit will not resolve the symptom.

Prerequisites

To resolve the green-flashing DIAG LED state, the following are required:

• TIA Portal V16 or later (V17 / V18 recommended for current ET 200SP firmware).
• The original STEP 7 project that contains the ET 200SP station, with the correct interface module article number. Common IM 155-6 variants are: 6ES7155-6AA00-0BN0 (PN ST), 6ES7155-6AU01-0CN0 (PN HF), 6ES7155-6AR01-0CN0 (PN HS), and 6ES7155-6BA01-0CN0 (DP HF).
• A PROFINET or PROFIBUS connection from the engineering station to the IO controller / interface module.
• The I/O module's HSP (Hardware Support Package) installed in TIA Portal, matching the module's firmware version. The module's firmware version is printed on the module label and can also be read in TIA Portal under Online & Diagnostics → Module Information → Firmware.
• The user must have at least the "Configure hardware" right in the TIA Portal project.

Resolution Procedure: Assigning Parameters in TIA Portal

The following step-by-step procedure is the documented Siemens path to clear the green-flashing DIAG LED on an ET 200SP I/O module.

Step 1 — Open Device View, Not Network View

A common operator mistake is to look for module parameters in Network View, where the device is shown as a graphical node on the PROFINET or PROFIBUS bus. Network View allows the engineer to set the device's IP address, PROFINET device name, and topology, but it does not expose the per-channel module parameters. Open the project, navigate to the ET 200SP station in the project tree, and double-click Device View rather than Network View. The Device View shows the IM 155-6 head-end on the left and each I/O slot to the right, with the BaseUnit (light or dark) shown beneath each slot.

Step 2 — Select the I/O Module

Click on the slot that contains the I/O module whose DIAG LED is flashing green. The module's Properties pane opens in the inspector window below the device view.

Step 3 — Navigate to Module Parameters

In the Properties pane, expand General. The first sub-node is Module parameters (also labeled "Module parameters" in TIA Portal V17/V18; in older versions the section may appear under the "IO tags" navigation for a digital module).

For a digital input module, the Module parameters page shows the following parameter groups.

For a digital output module, the Module parameters page shows the following parameter groups.

For analog input modules (for example, AI 4xU/I/RTD/TC ST, 6ES7134-6HD00-0BA1), the Module parameters page includes:

• Measurement type per channel: voltage, current, RTD, thermocouple, disabled
• Measuring range per channel: e.g., ±10 V, 0–10 V, 4–20 mA, PT100, type K
• Diagnostics enable: overflow, underflow, wire break, overtemperature
• Smoothing: none / weak / medium / strong
• Interference frequency suppression: 50 Hz / 60 Hz

For analog output modules (for example, AO 2xU/I ST, 6ES7135-6GB00-0BA1), the Module parameters page includes:

• Output type per channel: voltage / current
• Output range: ±10 V, 0–10 V, 4–20 mA
• Reaction to CPU STOP: substitute value / hold last value
• Diagnostics enable: wire break, short-circuit, overload

Step 4 — Activate Each Channel

The most common reason for a green-flashing DIAG LED on a freshly inserted module is that none of the channels have been activated. In the Channel assignment section (or "Channel" tab in older TIA Portal versions), each channel has an "Active" or "Activated" checkbox. TIA Portal ships new I/O modules with all channels deactivated by default, so that the engineer explicitly opts in to the parameter record for the field wiring that exists.

Click each channel that is wired to a sensor or actuator and set Active = true. Set the matching measurement type or output type. For a digital input module that is going to read 24 V proximity switches, channel 0 through channel 7 should be active with input delay 3 ms and diagnostics disabled (or enabled per site standards).

Step 5 — Compile and Download Hardware Changes

After the channels are activated and the parameters are set:

1. Right-click the ET 200SP station in the project tree.
2. Choose Compile → Hardware (rebuild all). Confirm that the compile completes with 0 errors and 0 warnings. Warnings about unsupported parameter combinations should be addressed.
3. Right-click the IO controller (the S7-1500 CPU in the rack, for example, 6ES7515-2AM02-0AB0) and choose Download to device → Hardware configuration. Alternatively, in the device view of the ET 200SP station, click the IM 155-6 slot and choose Download to device → Hardware configuration to push only the distributed I/O configuration.
4. Confirm the download dialog. TIA Portal will offer to assign the PROFINET device name if it is unset. Click Assign PROFINET device name if prompted.
5. Once the download completes, the I/O modules reset, re-enumerate, and the green DIAG LEDs transition from flashing to solid green.

Step 6 — Verify in Online View

Open the Online view of the ET 200SP station. The Diagnostics status column should show a green check mark for every slot. Expand the I/O module and confirm that the channel values are updating. For a digital input module, energize a channel with 24 V and confirm the input value toggles in the watch table or in the module's online view.

Module-Specific Parameter Examples

The exact parameter layout differs by module. The following table maps the most common ET 200SP I/O modules to their default-off-channel behavior and the parameter path that activates them.

The article numbers in the table are the standard ET 200SP catalog references. Always cross-check the article number printed on the module label against the article number in the project before commissioning — a mismatch is a common source of "module not recognized" faults that show up as a red-flashing DIAG LED, not the green-flashing state this article addresses.

BaseUnit Types and Potential Groups

Each I/O module plugs into a BaseUnit (BU) that provides the mechanical mount, the backplane bus, and the wiring terminals. The BaseUnit determines the load-voltage potential group and the field-wiring terminals.

• Light BaseUnit (BU15-P16+A0+2B, 6ES7193-6BP00-0BA0) — opens a new potential group. The 24 V load supply is fed into the terminals on the BU and is passed to all subsequent modules until the next light BaseUnit.
• Dark BaseUnit (BU15-P16+A0+2D, 6ES7193-6BP00-0DA0) — passes the potential group of the left neighbor. Used when the engineer wants to continue an existing 24 V group without breaking it.

The I/O module's parameter record contains the potential group selection. When the project is first created, TIA Portal assigns a light BaseUnit by default to the first slot of the station and a dark BaseUnit to each subsequent slot. If the physical wiring does not match the configured BaseUnit, the I/O module will still flash green (parameters not assigned) because the slot's parameter record has not been committed via a hardware download. Always confirm that the BaseUnit type in the project matches the BaseUnit type physically installed under each module.

Verification Checklist

After the hardware download, run the following verification steps:

1. Visual confirmation — every I/O module's upper LED is solid green. No flashing green, no red.
2. IM 155-6 DIAG LED — solid green (or off, depending on whether the IM is configured for diagnostics).
3. TIA Portal Online view — Diagnostics column shows green check marks for every slot.
4. Channel test — force a digital input high with a hand-held signal source, observe the input tag in the watch table. For an output, force the output tag in the watch table and measure the terminal voltage at the BaseUnit.
5. Diagnostic buffer — open the CPU's diagnostic buffer. There should be no "Module parameters not assigned" entries after the download.
6. PROFINET diagnostics — open the IO controller's online > PROFINET diagnostics. The ET 200SP station should be in "Connected" state with no station-level errors.

If any module still shows a flashing-green DIAG LED, re-enter the Device View, click the slot, and confirm that the Module parameters page has an explicit "Active" check on at least one channel. The most common oversight is to scroll past the Channel activation row and to leave every channel in the default "Inactive" state. A module with zero active channels is still a "parameters not assigned" condition by the Siemens definition.

Related Diagnostic States on the ET 200SP

To help the field engineer distinguish the green-flashing state from other similar states, the following extended reference is provided. Each LED is on the module face, and the colors are interpreted per the Siemens ET 200SP status and error displays documentation.

IM 155-6 PN / DP DIAG LED

I/O Module Channel Status LEDs

The row of small green LEDs on the front of digital input and digital output modules reflects the per-channel state. They are independent of the upper DIAG LED.

• On — channel is logic 1 (input energized / output driven)
• Off — channel is logic 0 (input low / output off)
• Flashing — for a digital input, wire-break diagnostics pending; for a digital output, the module has reported a short-circuit or overload on that channel

I/O Module PWR LED

The PWR LED on the left of each I/O module (and on the IM) reflects the load-voltage supply at the BaseUnit. A missing PWR LED is a power-bus problem, not a parameter problem. Recheck the light BaseUnit (BU...D) that feeds the load group and the 24 V supply to that BaseUnit's screw terminals.

Extended Troubleshooting Matrix

The following matrix covers the most common ET 200SP commissioning faults that an engineer might initially mistake for a "blinking green" issue.

Why "Network View" Is Not Where Parameters Live

Module parameters live in the Device View of the ET 200SP station. The Device View is opened by double-clicking the station in the project tree, and it shows the head-end IM, every BaseUnit, and every I/O module in slot order. When the I/O module is selected, its properties — including module parameters, IO tags, and the channel-level parameters — appear in the inspector window.

If a user is searching for "module parameters assignment" in the wrong view, the option will not appear. Confirm that the I/O module slot is selected in the Device View and that the inspector pane is visible (it can be toggled with View → Inspector in older TIA Portal versions, or it appears by default in V17/V18).

Common Pitfalls After the Download

Once the green-flashing DIAG LED has been resolved, three common pitfalls are worth pre-empting:

1. Re-inserting a module after a download — if the user pulls a module and re-seats it, the module's volatile configuration is preserved only if the slot was already configured. The DIAG LED should return to solid green. If it flashes green again, the slot may have lost its parameter assignment because the module was reseated while the PLC was in STOP. Re-download the hardware configuration.
2. Swapping a module for a higher-revision variant — a 6ES7131-6BF00-0BA0 swapped for a 6ES7131-6BF01-0BA0 may carry a new firmware version. The new module may have additional diagnostic features. The DIAG LED may transition from solid green to red-flashing because the new module's default parameter set enables diagnostics that the project has not yet defined. Open the Device View, accept the new module's parameters (or pin to the old article number), recompile, and redownload.
3. HSP not installed — if TIA Portal does not know the new article number, the slot will show a "?" in the device view, and the DIAG LED on the physical module will be red. Install the matching HSP via TIA Portal's "Options → Support Packages" and restart TIA Portal. The HSP that matches the module's firmware version must be installed, otherwise the project cannot be compiled with that module.

FAQ