Troubleshooting S7-400 ET200M Profibus Output Failure on CFC

Problem Overview

A Siemens SIMATIC S7-400 CPU connected over PROFIBUS-DP to one or more ET 200M distributed I/O stations may exhibit a symptom where a digital output (DO) channel—referenced in a CFC (Continuous Function Chart) block as CH_DO—reports a logic value of 1 at the block output, yet the corresponding physical output terminal remains de-energized. The fault is intermittent: only some channels fail, on different ET 200M stations across the plant, while neighboring outputs on the same module respond correctly.

This class of fault is rarely a hardware failure of the output module itself; in most cases it traces back to one of four root causes: missing load-voltage supply to the output group, mismatched I/O addressing between HW Config and CFC, double assignment of a single output address by two logic blocks, or a CFC run-time/OB binding error that prevents the chart from being executed cyclically. The diagnostic procedure below isolates each cause in sequence and prescribes the corrective action.

Affected System Architecture

Reference topology for the fault scenario:

The CPU holds the CFC chart in the user program; the chart produces process image output (PIQ) values that the operating system writes to the PROFIBUS-DP slave during the I/O cycle. The ET 200M head (IM 153-2) forwards the PIQ payload to the slot where the SM 322 digital-output or SM 332 analog-output module sits. Each output group on an SM 322 requires its own 24 V DC load supply on terminals 1L+, 2L+, 3L+ — without it, the module cannot source current even though the bit in the PIQ is set.

Root Cause Matrix

Diagnostic Procedure: Load Power Supply

The single most common reason a CFC output "does nothing" is the absence of the 24 V load voltage on the SM 322 output group. The CPU and the IM 153 backplane bus are powered separately; an SM 322 digital-output module will accept the bit, light no SF LED, and still drive nothing if 1L+ / 2L+ are floating.

1. Open the ET 200M cabinet and locate the affected SM 322 (6ES7322-1BF01-0AA0, 6ES7322-1BH01-0AA0, or similar 32-channel / 16-channel DO module).
2. With a calibrated DMM, measure between terminals 1L+ (or 2L+, 3L+) and 1M (or 2M, 3M). Expected: 20.4 V ≤ U ≤ 28.8 V DC.
3. Verify polarity: L+ must be positive with respect to M. Reversed polarity on a DO module typically blows the per-group fuse.
4. If voltage is absent, trace the feed to the SITOP power supply or the marshalling terminal block. Verify that the line-protection fuse (typically 6 A or 10 A) is intact.
5. Repeat for every output group of every SM in the affected ET 200M. Document on the cabinet single-line diagram.

Diagnostic Procedure: HW Config and Address Mapping

If the load supply is present, the next most likely cause is a mismatch between the physical slot population and the HW Config image that the S7-400 CPU downloaded to the IM 153.

1. In STEP 7 Manager, open the S7 project and double-click Hardware to launch HW Config.
2. Navigate to the PROFIBUS-DP subnet and expand the affected IM 153-2 (order number 6ES7153-2BA10-0XB0 or 6ES7153-2BA70-0XB0).
3. Right-click the slot where the DO module is inserted in real life and select Object Properties. Confirm the order number matches the printed label on the module exactly.
4. In the Addresses tab, note the start address of the outputs (for example 0, 4, 8…). For SM 322 modules, the default is a process image partition (PIP) start address; for SM 332 analog modules the start address is a PQW word boundary.
5. Cross-check that the start address is not the input address of an input module already present on another ET 200M. STEP 7 will compile without warning if you manually rename a slot's address range to overlap with another station.
6. Save & Compile (Station > Save and Compile), then Download to the CPU (PLC > Download to Target). Cycle power to the IM 153 only if a hardware change was made.

Diagnostic Procedure: CFC Chart Installation

A CFC chart is a graphic network of function blocks compiled into STL/FBD and executed only if it is installed into a cyclic organization block (typically OB1, OB35, or OB36–OB38 depending on cycle time). The block-icon editor displays the running block even when the chart is not installed; the online value you see is the last computed value held in the PIQ area.

1. Open the CFC editor and select the affected chart in the chart tree (left pane).
2. Right-click the chart folder and select Chart > Chart Installation.
3. In the dialog, verify that the chart appears under the correct run-time group. Default assignments:

4. Click the Enabled check-box. A chart with the box cleared is compiled but not executed.
5. Confirm the chart's Task is bound: View > Run Sequence shows the run-time groups in execution order.
6. Download the chart (Chart > Download) and the chart installation (Options > Chart Install > Download).

Diagnostic Procedure: Cross-Reference Analysis

The second-writer problem is common when charts are copy-pasted between projects or when a programmer reassigns a tag alias. The CPU honors the last assignment made in the current scan; the first writer's value is silently overwritten.

1. In STEP 7 Manager, right-click the S7 program and choose Reference Data > Display.
2. Select Cross-References and confirm the filter is set to All.
3. Type the suspect address (for example Q 0.0 or PQW 256) into the address field.
4. Step 7 returns a list of every network, CFC block, SFC/SFB, and OB that references the address. Each row shows Access (read/write), Type, Block, Location.
5. If more than one block performs a write access (W) to the same bit, both will appear. You must determine which one is logically correct.
6. To isolate quickly, force the suspected erroneous writer offline (open the CFC block and temporarily mark the output pin "OVR"), then observe whether the physical output follows the remaining writer.

For an S7-400 the cross-reference data is generated at compile time. Always run Program > Compile All after structural changes to refresh the database.

Diagnostic Procedure: PROFIBUS and ET 200M LED Diagnostics

The IM 153-2 front panel and the SM 322 module each carry a row of status LEDs that localize the fault in seconds.

1. Walk the cabinet and read every LED on the affected IM 153 and on each SM.
2. For an IM 153 BF red, read the CPU diagnostic buffer: PLC > Diagnostic/Setting > Diagnostic Buffer. PROFIBUS station failure events include the slave diagnostic word and the slave address.
3. Open PLC > PROFIBUS > Monitor/Modify to see the slave status: "OK", "Fault", "Not Found". A "Fault" state with cyclic I/O still active usually means a parameterization mismatch.
4. Check the terminating resistor on the PROFIBUS connector. A missing or dual termination on the bus segment will produce intermittent BF events that briefly silence every output.

Step-by-Step Resolution

Apply the corrective action matching the diagnosed root cause.

Cause 1: Load supply missing

1. Restore 24 V to the affected output group.
2. Cycle power to the IM 153 to clear the diagnostic latch on the SM.
3. Verify with a VAT force that the output now follows.

Cause 2: HW Config mismatch

1. Insert the correct module in HW Config slot.
2. Adjust the start address to match the wiring diagram.
3. Compile and download HW Config.
4. Cycle the IM 153 power.

Cause 3: Chart not installed

1. CFC > Chart > Chart Installation > enable and assign to run-time group.
2. Options > Chart Install > Download.

Cause 4: Cross-reference double-write

1. Identify the redundant writer with Reference Data > Cross-References.
2. Remove or comment out the erroneous write, or change its destination to a separate tag.
3. Compile all and download.

Cause 6: PROFIBUS slave failure

1. Read the CPU diagnostic buffer and note the slave address.
2. Inspect the PROFIBUS connector and cable for the failed slave.
3. Verify the slave's PROFIBUS address matches HW Config (DIP switches or rotary switch on the IM 153).
4. Replace the IM 153 if hardware fault persists.

Verification and Commissioning

After the corrective action, perform a structured I/O check before returning the system to production.

1. Open a Variable Table with one row per output bit/word of the affected module.
2. Trigger each output from the CFC chart in automatic mode and confirm the VAT row reflects the same value within one scan cycle.
3. Measure the field voltage at the actuator terminal with a DMM; verify the polarity matches the actuator wiring diagram.
4. For analog outputs, set the CFC block to SUBSTITUTE VALUE = 0 on CPU STOP, then to LAST VALUE on CPU STOP and verify each behavior matches the project specification.
5. Cycle the CPU from STOP to RUN three times. Watch the IM 153 SF/BF LEDs during each startup. They must extinguish within 5 s of RUN.
6. Read the CPU diagnostic buffer: PLC > Diagnostic/Setting > Diagnostic Buffer. The buffer must be free of SF events for the affected ET 200M for at least 15 minutes.

Preventive Measures and Best Practices

• Maintain a documented address map per ET 200M station in the cabinet drawing set. Reject any CFC change that overlaps an existing process-image range without a peer review.
• Add diagnostic CFC blocks (CH_DI, CH_AI, STATUS) for every output group. The STATUS block pulls the diagnostic word from the IM 153 and surfaces it on the HMI as a per-channel health indicator.
• Fit each output group with a fused marshalling terminal so that a field-side short does not propagate back into the SM 322.
• Use the optional SFC Visualization in STEP 7 to compare the run-time sequence against a baseline saved in the project archive; any drift indicates a chart that has been installed out of order.
• Store the compiled S7 program with the STEP 7 V5.x archive function on every plant change. The cross-reference data is rebuilt on compile and a previous archive is the fastest way to identify a regression.
• Schedule an annual PROFIBUS cable inspection using an oscilloscope or a BT 200 bus tester. A degraded cable produces intermittent BF events that drop the entire ET 200M and silence every output.

Spare Parts Reference

FAQ