Resolving S7-300 SM-331 Module 'Configured But Not Available'

Problem Overview

When commissioning or maintaining a Siemens SIMATIC S7-300 station built around a CPU 315-2 PN/DP (MLFB 6ES7315-2EH14-0AB0 or earlier 6ES7315-2EH13-0AB0 / 6ES7315-2AG10-0AB0), an analog signal module can be physically mounted, mechanically latched, and yet still raise a diagnostic message in STEP 7 (SIMATIC Manager) v5.x or TIA Portal V13+ that reads:

"The above-defined module is configured but not available"

This diagnostic is reported from the online view of the hardware configuration (HW Config) when the controller attempts to enumerate the I/O on the backplane bus and cannot reconcile the expected slot population with the actually installed module. The most common occurrences on the affected station are with the SM-331 AI 8x12 (8-channel, 12-bit analog input, MLFB 6ES7331-7KF02-0AB0) and the SM-332 AO 8x13 (8-channel, 13-bit analog output, MLFB 6ES7332-5HF00-0AB0). The same symptom applies to most other SM-331/332/334 variants and to digital SM-321/322/323 modules whenever the backplane bus path is broken or the project configuration drifts from the installed hardware.

Reference: S7-300 Module Specifications Manual (entry ID 8859629).

Hardware Identification and MLFB Reference

Always begin by capturing the full MLFB (Maschinenlesbare Fabrikatebezeichnung) from the front-panel labeling of every module in the rack. The MLFB determines firmware compatibility, channel count, resolution, diagnostics capability, and which slot it may legally occupy. For the modules in this case the relevant catalog data is:

Cross-check each MLFB with the configuration loaded in HW Config. Mismatched MLFBs (for example a 6ES7331-1KF02-0AB0 in the project versus a 6ES7331-7KF02-0AB0 in the rack) will report the same "configured but not available" diagnostic even with a healthy backplane bus, because the slot ID the CPU expects to read does not match the device descriptor returned by the module.

Diagnostic Message: "Module Is Configured But Not Available"

The diagnostic text comes from the online Module Information dialog, accessible by right-clicking the slot in HW Config and selecting Module Information > Diagnostics after going online with the CPU. The buffer entry is logged as event ID 0xC1 / SF (System Fault) in the CPU diagnostic buffer. The accompanying cause text typically reads "Module failure" or "Module does not exist" depending on the STEP 7 version.

STEP 7 decodes the event as follows:

• 0xC1 + 0xBF: "Module error / module not available / module does not respond" — the CPU polled the slot during the cyclic I/O update and received no valid identifier frame on the backplane bus.
• 0xC1 + 0x33: "Wrong module in slot" — the module answered but its device descriptor did not match HW Config (this is the most easily confused sibling diagnostic).

Reference: S7-300 CPU 315-2 PN/DP Operating Instructions (entry ID 13242790).

Root Cause Analysis Matrix

Prerequisites for Troubleshooting

1. STEP 7 V5.5 SP4 / SP5 with HW Config updated, or TIA Portal V13 SP1 or later with the correct HSP (Hardware Support Package) installed for the SM-331/332 variants in use.
2. Approved Siemens MPI or PROFINET PC/PG cable (USB-MPI, 6ES7972-0CB20-0XA0, or Ethernet through the PN interface of the CPU).
3. Source project ("offline" STEP 7 archive) containing the S7 program, the HW Config of the station, and the symbolic table.
4. Appropriate access level on the CPU. The default password for a clean CPU 315-2 PN/DP is "runner"; protected projects may require a project-specific password.
5. Spare U-shaped backplane bus connectors (Siemens MLFB 6ES7390-0AA00-0AA0) and, ideally, a known-good SM-331 or SM-332 module for cross-testing.

Step-by-Step Diagnostic Procedure

Step 1 - Go online and read the CPU diagnostic buffer

1. Open the project in STEP 7 or TIA Portal.
2. Select the CPU 315-2 PN/DP and choose PLC > Accessible Nodes or Online > Go Online.
3. Open PLC > Module Information. Read the Diagnostic Buffer tab and note the most recent event, its event ID (decimal or hex), and the slot number cited.
4. Confirm that the buffer entry references the slot where the SM-331 or SM-332 is physically installed (slot 4 through slot 11 in a typical S7-300 rack, since slots 1, 2, and 3 are reserved for power supply, CPU, and IM).

Step 2 - Compare the installed MLFB to the HW Config

1. Open HW Config.
2. Double-click the slot in question and read the Order Number / MLFB in the properties dialog.
3. Physically read the MLFB on the right side panel of the installed module.
4. If they differ, correct the HW Config to match the installed module and download the configuration again (PLC > Download to Target > To Target System).

Step 3 - Verify the U-shaped backplane bus connector

1. Power off the rack.
2. Remove the SM-331 (or SM-332) being investigated by unlatching the top and bottom latches.
3. Inspect the U-shaped bus connector seated in the rear of the module. Verify the contacts are clean, not bent, and that the connector is fully inserted into the plastic guide.
4. Re-seat the connector into the next module's slot and reinstall the affected module. Confirm the latches click and that the module is flush with its neighbors.

Reference: S7-300 Module Specifications - backplane bus wiring rules (entry ID 8859629).

Step 4 - Power-cycle and observe the module LEDs

1. Apply 24 V DC to the PS-307 power supply and the CPU.
2. Watch the front-panel LEDs of the SM-331 / SM-332 during the CPU's startup phase. The expected sequence is SF off, BF (if Profibus) off; a steady SF immediately after startup indicates the module is responding on the backplane bus but reporting an internal fault. No LED activity at all on a fully seated module typically points to a backplane bus or 24 V supply problem.
3. Note: SM-331 AI 8x12 (6ES7331-7KF02-0AB0) does not provide per-channel fault LEDs; instead the SF LED reflects the group-diagnostic state. Check the diagnostic buffer for the channel-level reason.

Step 5 - Module swap test

1. Replace the suspect module with a known-good spare of the exact MLFB.
2. Power-cycle the CPU and go online.
3. If the spare is recognized and the diagnostic clears, the original module is defective. Dispose of or return the defective module per the local warranty procedure (Siemens RMA).
4. If the spare still reports "configured but not available", the problem is on the rack side: re-check the bus connector, the 24 V supply, the slot mechanics, and the project configuration.

Verifying Hardware Configuration in STEP 7 / TIA Portal

For the CPU 315-2 PN/DP, the configuration in HW Config must be downloaded to the CPU before the slot will be recognized. Use the following sequence:

1. Open HW Config in STEP 7 V5.5 or the Device View in TIA Portal.
2. Drag the correct MLFB from the catalog onto each slot. For the analog modules in this case, set the measurement type and range per channel group. For example, for 4-wire RTD on a 6ES7331-7KF02-0AB0 choose RTD under the relevant group, select the platinum standard (Pt100 or Pt1000), and set the smoothing to match the application.
3. Compile and save the project.
4. Select the CPU and choose PLC > Download to Target. In TIA Portal this is Online > Download to Device.
5. After download, select PLC > Module Information and verify the General tab shows the correct order number and the Diagnostics tab is empty.

Reference: S7-300 CPU 315-2 PN/DP manual - configuration download procedure (entry ID 13242790).

Backplane Bus Connector Inspection

The U-shaped backplane bus connector (MLFB 6ES7390-0AA00-0AA0) is the most common single point of failure on a service call. The connector is keyed to prevent reversed insertion, but the plastic housing can crack under torque, and the gold fingers can oxidize after long storage in a humid cabinet. Replace the connector whenever:

• The module was stored for more than 12 months in an uncontrolled environment.
• The connector has been removed and reinserted more than 10 times (the contact plating wears).
• The connector shows any visible deformation of the U shape or any discoloration of the contact springs.

Module Swap Test Procedure

When two identical MLFBs are available, the swap test isolates the module from the rest of the system in under five minutes. Perform the test in this order:

1. Identify a known-good module with the same MLFB and the same firmware version (the firmware version is printed on the front label as FW: x.y).
2. Power off the rack.
3. Swap the suspect module with the known-good module. Keep the wiring on the terminals and transfer it to the spare module.
4. Power the rack back on. Wait for the CPU RUN or STOP indicator.
5. Go online and read the diagnostic buffer. A clean buffer means the original module was defective. A persistent diagnostic means the slot, the bus connector, or the HW Config is at fault.

Online Module Information Interpretation

The Module Information dialog (called from HW Config or from the device view) provides six tabs of useful data:

Commissioning Verification Checklist

1. CPU 315-2 PN/DP transitions to RUN and the SF LED remains off.
2. All configured SM-331 / SM-332 slots respond in Accessible Nodes and report no diagnostic in the Module Information dialog.
3. For each AI channel, force a known signal (for example, a 4 mA / 20 mA current) and verify the raw value in the corresponding input word (IW) of the process image matches within 0.1 percent. The default analog input range for 6ES7331-7KF02-0AB0 is 0 to 10 V or 4 to 20 mA depending on the wiring and the configuration.
4. For each AO channel, set a value in the output word (QW) and measure the output voltage or current at the terminal block.
5. Trigger a power-cycle and confirm the modules enumerate without errors.
6. Document the final HW Config and back it up with the S7 program archive.

FAQ