Troubleshooting Siemens ET 200S 4DI Module: LED On, No PLC Input

Problem Statement

A Siemens ET 200S station built on an IM 151-1 HF head module (6ES7 151-1BA02-0AB0) reports the following fault: a 4DI DC24V electronic module (6ES7 131-4BD01-0AA0) on terminal module TM-E15C26-A1 displays the per-channel green status LED for each wired input, but the input image in the connected S7-315F-2 DP CPU never changes. No SF or BF LEDs are lit on the IM 151-1, and no BF is reported on the CPU. The PLC is in RUN. STEP 7 V5.x is used for engineering.

This is a classic ET 200S symptom pattern: local I/O is powered and the channel LED proves the input voltage is reaching the module's threshold detector, but the PROFIBUS-DP slave does not report the change to the master. The root cause is almost always on the configuration side, not on the sensor wiring side, and the discussion below walks through every documented cause and its diagnostic signature.

Affected Hardware and Configuration

The reported configuration in slot order, left to right, is:

Topology Reference

Root Cause Matrix

Five distinct causes have been documented for the symptom "channel LED active, no input to PLC" on an ET 200S DI module. Use the matrix below to triage quickly:

The reported symptom (LED on, no SF, no BF) eliminates causes 1 and 2 as primary causes and points to causes 3, 4, or 5. Causes 1 and 2 must still be verified because they coexist with causes 3 to 5 and can mask them.

ET 200S Configuration Rules That Must Be Honored

The IM 151-1 HF (6ES7 151-1BA02-0AB0) is a PROFIBUS-DP slave with up to 63 module slots and a maximum of 244 bytes of inputs and 244 bytes of outputs on the backplane. To operate correctly the following rules must be observed; each rule corresponds to a class of symptom:

1. Terminating module is mandatory on the rightmost slot of every ET 200S station. The terminator closes the backplane bus and prevents reflections. ET 200S will power modules and even light channel LEDs without the terminator in some cases, but the slave will not enter cyclic data exchange reliably with the master.
2. A PM-E (Power Module) is required to open a new potential group. Modules placed before the first PM draw power from the IM 151-1's load current supply. Modules placed after a PM draw power from the PM. If the PM is missing, the modules after it will not be supplied and will not communicate, even though the slots in HW Config may still be accepted.
3. TM-E15C26-A1 (screw-type, 15 mm wide, 26 terminals) is the correct terminal module for the 4DI DC24V (6ES7 131-4BD01-0AA0). Using a TM of the wrong width forces the electronic module to be inserted in a different slot, breaking the slot-to-address mapping in HW Config.
4. Electronic module order number must match the entry in HW Config. STEP 7 V5.x performs a parameter assignment check at startup; if the configured MLFB does not match the plugged module, the slot is reported as "module not parameterized" and the input image for that slot remains zero even though the LED on the physical module may light up.
5. Address assignment must be consistent. The default is slot-based addressing (slot 1 = IB 0...IB x, slot 2 = next byte). If the user program monitors an offset that does not correspond to the slot, the program "sees" no change.

Diagnostic Procedure

Execute the following checks in order. Each step is independent of the others and can be run while the system is in STOP or RUN as noted.

1. Verify the bus terminator. The terminator (6ES7 193-4JA00-0AA0) must be the rightmost component on the backplane. Power off the station, remove the rightmost electronic module, and confirm that the terminator is seated and locked. Without it, the symptom can appear on the last few slots while leaving earlier slots functional.
2. Verify PM placement. In the reported configuration the suspect DI is the first slot after the second PM. Confirm that the PM is correctly inserted on a PM-E terminal module (TM-P15C23-A1 or equivalent) and that 24 V DC is present on the load voltage terminals (L+ / M) of that PM. An open load-voltage fuse on the PM will cause the modules after it to be de-energized without lighting the SF on the IM.
3. Read the CPU diagnostic buffer. In STEP 7, open PLC > Diagnostic/Settings > Diagnostic Buffer on the online S7-315F-2 DP. Look for entries of class "I/O fault" or "Station failure" referencing DP station number assigned to the ET 200S. Note the slot number reported.
4. Read the slave diagnostics via SFC 13 (DPNRM_DG). Insert the following block in OB 1 (or in a watchdog OB) with the slave's PROFIBUS address (default for ET 200S is set on the IM 151-1 DIP switch):

CALL "DPNRM_DG"
  REQ    := TRUE
  LADDR  := W#16#03FF   // diagnostic address of the DP slave
  RETVAL := MW100       // return code
  RECORD := P#DBX 0.0 BYTE 32
  BUSY    := M110.0

The RECORD area receives the DP slave diagnostic frame (6 bytes of standard diagnostics + module status). A non-zero module status for the suspect slot confirms a configuration mismatch or module fault at the slave level.

5. Compare HW Config to physical build. In STEP 7, open HW Config > ET 200S station. Each slot must list the exact MLFB and version of the electronic module physically installed. Right-click each slot, choose Object Properties, and verify the order number. If the slot is empty in HW Config but a module is physically present, STEP 7 does not assign inputs to that slot.
6. Read the input image directly from the slave. Use a VAT (Variable Table) in STEP 7 to monitor the process input image. For slot 5 (the suspect 4DI), if slot-based addressing is enabled, the inputs should be visible at IB x ... IB x+0 (4DI = 4 bits packed in the low nibble of one byte). Toggle the field input and observe whether the bit transitions.
7. Swap the suspect DI with an identical, known-good module. If the suspect module is the first 4DI in the station, swap it with the second 4DI (slot 6) temporarily. If the problem follows the module, the electronic module is defective. If the problem stays on slot 5, the slot, terminal module, or HW Config entry is at fault.

Step-by-Step Resolution

Apply the following corrective actions in sequence until the input is visible in the PLC.

1. Seat the bus terminator on the rightmost slot (6ES7 193-4JA00-0AA0). Re-power the station and check whether the suspect input now appears in the input image. If yes, the terminator was the cause.
2. Verify 24 V on the second PM by measuring L+ to M at the PM terminal block. If voltage is absent, trace the feeder (circuit breaker, fuse, terminal) feeding that PM and restore it.
3. Reconcile HW Config with the physical build. In HW Config, right-click the ET 200S station and choose Compare with Actual Configuration. STEP 7 will flag any slot whose MLFB does not match. Correct the entry to the exact MLFB shown on the front of the electronic module (e.g., 6ES7 131-4BD01-0AA0, firmware version if displayed). Save, compile, and download HW Config to the CPU. Restart the CPU (STOP > RUN).
4. Download the slave parameters to the ET 200S if the CPU was not power-cycled after the HW Config change. PROFIBUS-DP slaves only re-read their parameters on slave startup (power-on or bus reset). In STEP 7, PLC > PROFIBUS > Initialize Slave forces a bus reset on the affected station.
5. Verify the user-program address. If the slot is configured but the program reads I 0.0 for an input that lives at I 8.0, the program will see no change. Use a VAT to display the entire process input image (IB 0 through IB 63) and identify the byte that toggles when the field input is triggered.
6. Replace the electronic module if Steps 1-5 do not resolve the issue. The LED being lit is a strong indicator that the input threshold detector and the local status indicator work, but a fault in the module's backplane ASIC or PROFIBUS-DP slave controller can prevent the channel state from being reported upstream. Use a module with the same MLFB and, if possible, the same or higher firmware version.

Module Swap Verification Procedure

The module swap is the single most discriminating test. Use it whenever Steps 1-5 are inconclusive.

1. Power off the ET 200S station. Wait for the SF/BF LEDs to extinguish on the IM 151-1.
2. Unlatch and remove the suspect DI electronic module (slot 5). Note that the terminal module (TM-E15C26-A1) and the wiring stay in place.
3. Insert a known-good, identical 4DI DC24V (6ES7 131-4BD01-0AA0) electronic module into slot 5.
4. Power on the station. Wait for the IM 151-1 to complete its startup (SF off, BF off).
5. Trigger the field input and observe the process input image in the VAT. If the input now updates, the original module is defective and should be returned under warranty. If the input still does not update, the fault is upstream (HW Config, address, or PM) and the original module may be functional.
6. As a control, swap the suspect module into slot 6 (the second 4DI) which is known to work. If the suspect module now fails in slot 6, the module is defective.

STEP 7 V5.x HW Config Inspection

Open the ET 200S station in HW Config and verify the following on the suspect slot:

• MLFB matches: 6ES7 131-4BD01-0AA0. STEP 7 V5.4 SP5 and later reject obviously wrong modules on download, but a slot left empty or populated with a different module family (e.g., 2DI) silently produces no input image.
• Slot is not empty: an empty slot in HW Config is a frequent commissioning error. The slot must be populated with the same electronic module type as installed in the field.
• Diagnostic interrupt is enabled if you want module-level diagnostics to be delivered to the CPU. Object Properties > Diagnostics: enable "Diagnostic Interrupt". With it enabled, channel faults surface as OB 82 calls. With it disabled, a faulty module may produce no visible event in the diagnostic buffer.
• Address assignment: by default, slot-based addressing is in effect. If you have used User-defined addresses, the start address of the slot is shown next to the slot in HW Config. Use that start address, not the slot number, when reading the input image from the user program.

Diagnostic Buffer Analysis

For an S7-300 / S7-400F CPU connected to an ET 200S over PROFIBUS-DP, the diagnostic buffer entries that are relevant to this symptom are:

If event 0x350A appears referencing the suspect slot, the HW Config is wrong. If no relevant event appears at all, the slave is not reporting any fault, which strongly points to either a defective module (cause 4) or an address mismatch in the user program (cause 5).

Verification

After each corrective action, perform the following verification before declaring the issue resolved:

1. Open a VAT in STEP 7 Online. Add the process input address for the suspect slot (e.g., IB 8 if the slot starts at byte 8).
2. Trigger each of the four input channels of the DI module in turn. The corresponding bits in the VAT must transition from 0 to 1 within one PROFIBUS cycle (typically a few ms, well under the OB 1 cycle).
3. Inspect the CPU diagnostic buffer for new entries of class "Diagnostic interrupt" or "Module OK" for the suspect slot.
4. Force the input to 0 (open the contact) and confirm that the bit in the VAT returns to 0 within one cycle.
5. If the station is safety-related (CPU 315F-2 DP), confirm that the safety signature of the PROFIBUS-DP slave is unchanged and that the F-I/O is not affected.

Preventive Measures

• Pre-commissioning checklist: verify terminator, PM placement, and HW Config reconciliation before applying 24 V to the station. This eliminates causes 1, 2, and 3 in a single pass.
• Spare module kit: keep at least one spare electronic module per MLFB used in the plant. The 30-second module-swap test is the fastest way to discriminate between a defective module and a configuration error.
• Diagnostic interrupt enable: enable diagnostic interrupts on every slot. This populates the diagnostic buffer with meaningful events and avoids the "no SF, no BF, no PLC reaction" blind spot that is the subject of this article.
• Address documentation: print the HW Config with address overlays and store it in the cabinet. Operator-level troubleshooting of "input not seen" faults then becomes a 2-minute VAT check instead of a multi-day investigation.
• Firmware consistency: record the firmware version of the IM 151-1 and of every electronic module. Newer firmware on the IM 151-1 occasionally changes the behavior of older electronic modules; the symptom can be introduced by a mixed-vintage station.