S7-300F SM 336F: Deactivate Spare Channels to Suppress Wire Break

Problem Overview and Affected Hardware

The Siemens SM 336F failsafe analog input module (order number 6ES7 336-4GE00-0AB0) reports wire break diagnostics on channels that have no sensor wired. The diagnostic event appears in the F-CPU diagnostic buffer, lights the SF (group fault) LED on the module, and - if not handled correctly - can trip the F-CPU into a safe state or force the safety program into a passivated state for the affected channels. This is the expected behavior of a failsafe module, not a hardware defect: the F-module monitors every activated channel continuously and treats an open input as a fault because it cannot verify the safety function on that channel.

The fix is to mark unused channels as deactivated in the failsafe parameter assignment, which removes them from safety monitoring and from the diagnostic event generator. No dummy loads, jumper wires, or termination resistors are required, and - in fact - adding such loads is rejected as a non-compliant workaround for SIL 2 / SIL 3 systems.

The module occupies one slot in the S7-300 / ET 200M rack. It is documented in the SIMATIC S7-300 Module Data manual and the SIMATIC S7 F/FH Systems - Configuring and Programming manual.

Root Cause Analysis

Failsafe analog input modules implement permanent cross-check diagnostics on each activated channel. For a 4-20 mA loop the module sources a small test current and measures the resulting loop voltage. If the measured current falls below approximately 3.6 mA the module asserts a wire-break diagnostic. For 0-10 V inputs a similar test verifies that the input voltage sits inside the configured range; an open input sits at an undefined potential and is also flagged.

When a new project is created, every channel of the SM 336F defaults to activated with a default measurement type (typically 4-20 mA on current pairs). Spare channels have no sensor, so they immediately trigger the wire-break diagnostic on the first I/O update after startup.

Three failure patterns typically appear at first commissioning:

1. Wire break on every unused current channel (most common) - default 4-20 mA range and no field wiring.
2. Wire break on the voltage/RTD pair - default 0-10 V range on CH4/CH5 with no sensor.
3. Wire break on the partner of a single wired channel - sensor wired to CH0 only, CH1 left open, partner not deactivated.

All three patterns resolve with the same fix: set Evaluation of the sensor (or Sensor evaluation) to deactivated for the unwired channel.

Preconditions

1. STEP 7 V5.5 SPx or TIA Portal V13 SP1 / V15.1 / V16 / V17 / V18 with the matching S7 F-ConfigurationPack installed on the engineering station.
2. Access to the offline S7-300 / ET 200M station configuration that matches the running CPU.
3. The F-CPU's safety password (F-password) if the F-CPU is password-protected.
4. An empty / writable path to download the modified HW Config into the target CPU.
5. A current backup of the safety program (F-block container) before any parameter change.
6. An updated I/O list that explicitly identifies which channels are spare and which are active.

Solution: STEP 7 V5.x (HW Config)

1. Open SIMATIC Manager and load the offline S7-300 project.
2. Open HW Config for the S7-300 station (or ET 200M station for distributed I/O).
3. Select the SM 336F (order number 6ES7 336-4GE00-0AB0) in the rack view.
4. Double-click the module (or right-click → Object Properties).
5. Switch to the Parameters tab. Three channel pairs are visible: Pair 0 (CH0/CH1), Pair 1 (CH2/CH3), Pair 2 (CH4/CH5). Each pair has two drop-downs - lower channel and upper channel.
6. For every unused channel, change Sensor evaluation (or Evaluation of the sensor) from activated to deactivated.
7. If both channels of a pair are spare, deactivate both. The pair remains in the project but is excluded from safety diagnostics.
8. Confirm OK to close the Properties dialog.
9. From the menu: Station → Save and Compile.
10. Download to the target CPU: PLC → Download to Target System. Confirm the F-password prompt when it appears.
11. Confirm any safety-mode prompts. The CPU does not STOP for a pure parameter modification, but the safety signature changes.

What NOT to do on spare channels

• Do not install 250 Ω termination resistors across unused 4-20 mA inputs to "trick" the module. The channel is still activated and any drift or noise will eventually cross the wire-break threshold.
• Do not leave the channel at its default settings and ignore the diagnostic. F-modules must be intentionally configured for every channel, including unused ones.
• Do not edit the F-parameters without recording the new safety signature in the project's safety log.

Solution: TIA Portal

1. Open the project in TIA Portal (V15.1 or later recommended for current F-ConfigurationPack support).
2. Open Devices & Networks and switch to Device View.
3. Click the SM 336F module in the rack of the S7-300 / ET 200M station.
4. In the Inspector window, navigate to Properties → Module parameters.
5. Expand the channel groups (Pair 0, Pair 1, Pair 2). Each pair exposes parameters for the lower and upper channel.
6. For every unused channel, set Sensor evaluation to deactivated.
7. Save the project, then Compile → Hardware (rebuild all).
8. Establish an online connection to the F-CPU and download. Authenticate with the F-password when prompted.
9. Confirm the safety signature prompt and the automatic safety program recompile.

Verification and Diagnostic Clearing

After the parameter change has been downloaded to the F-CPU, verify the fix in four steps.

1. Diagnostic buffer: Open Online & Diagnostics → Diagnostic buffer (TIA) or PLC → Module Information → Diagnostic Buffer (STEP 7). Confirm no further wire-break entries for the previously reported channel numbers.
2. Module LEDs: The SF (group fault) LED on the SM 336F extinguishes. The green active / ready LED remains on. If the module is on an ET 200M station, also confirm the IM 153-4 SF LED is off.
3. CPU mode: The F-CPU remains in RUN. The safety program (F-runtime group) continues to execute normally.
4. Live channel readout: In the F-I/O DB (F-channel-driver DB) or via the F-shared DB, the deactivated channel returns a substitute value (typically 0) and the quality code indicates substitute / deactivated.

Channel Pairing (1oo1 / 1oo2) and SIL Modes

The SM 336F always evaluates channels in pairs to deliver failsafe integrity. There is no true single-channel safety mode; the closest equivalent is a 1oo1 evaluation with the partner channel deactivated. Supported modes:

When wiring only one sensor per pair (the most common configuration), the partner channel must be deactivated - exactly the procedure in this article.

Wire-Break Detection Parameters and Diagnostic Events

The wire-break check is enabled by default for every activated channel and is tied to the configured measurement type:

If wire-break detection must be disabled on an active channel for technical reasons (e.g., a sensor cannot source 4 mA minimum), the parameter Wire break / Short circuit can be cleared individually. This is a deliberate safety-engineering decision and must be documented in the safety validation report, because removing the wire-break check on an active channel reduces the diagnostic coverage.

Diagnostic Buffer Event Reference

The SM 336F writes standardized diagnostic events readable in the F-CPU diagnostic buffer. Typical entries:

The exact event IDs and language are visible in the F-CPU's diagnostic buffer regardless of the HMI language setting.

S7 F-ConfigurationPack and Engineering Tool Versions

The S7 F-ConfigurationPack is the software extension that exposes the safety parameters (sensor evaluation, channel pairing, discrepancy time, etc.) inside HW Config / TIA Portal. Without it, the Parameters tab for the SM 336F either does not appear or is greyed out. Versions must match the base STEP 7 / TIA Portal version:

If the F-ConfigurationPack is missing, install it from the Siemens Industry Online Support entry for the F-ConfigurationPack, restart the engineering software, and re-open HW Config. Mismatched versions between the F-ConfigurationPack and the base TIA Portal are the most common reason for the safety Parameters tab not being visible.

Troubleshooting Matrix

Safety Program Re-Validation and Migration to S7-1500F

Any change to an F-module's parameters counts as a modification to the safety-related system per IEC 61511 / IEC 61508 / IEC 62061. After the parameter change:

1. Document the new safety signature and parameter set in the project's safety validation report.
2. Perform a proof test of every safety function that uses the SM 336F, including a deliberate wire-break test on each activated channel to verify that the diagnostic still trips the safety shutdown.
3. Update the as-built documentation and re-issue the safety manual to the operations team.
4. If the installation is regulated (e.g., functional safety per Machinery Directive 2006/42/EC), notify the notified body as required.
5. Re-issue the PFD / PFH calculation for the safety function with the new activated channel count.

Migration path to S7-1500F

For new installations on S7-1500F or ET 200SP, the equivalent F-AI module is the 6ES7 536-1MF00-0AB0 (F-AI 4xI) in ET 200SP. Parameter assignment is entirely inside TIA Portal; no separate F-ConfigurationPack is needed for safety parameter visibility. The same deactivation workflow applies: mark unused channels as deactivated in the channel properties.

Related F-modules

FAQ