Resolving SF LED on Siemens 6ES7331-7KF02-0AB0 Analog Input

The SIMATIC S7-300 SM 331 analog input module 6ES7331-7KF02-0AB0 (AI 8 x 12 bit) is a high-density, group-isolated module that is widely deployed in S7-300 stations and in ET 200M distributed I/O behind an IM 153-1 interface module. Unlike the older 6ES7331-1KF02-0AB0 module it often replaces, the 7KF02 is divided into four independent channel groups, each requiring its own measuring range module setting and STEP 7 hardware configuration. A persistent SF (System Fault) LED after commissioning is almost always a result of one of three root causes: wrong measuring range module position, mismatch between the transducer type (2-wire vs. 4-wire) and the configuration, or an incompletely downloaded hardware configuration. This reference walks through the diagnostic logic, the part-number specific pinout, the measuring range module matrix, and the field-proven correction procedure.

1. Problem Statement

Field symptoms reported on a station composed of:

• Main CPU: 6ES7 315-2AG10-0AB0 (CPU 315-2DP)
• Distributed I/O head: 6ES7 153-1AA03-0XB0 (IM 153-1)
• Original AI module: 6ES7331-1KF02-0AB0 (8 AI, 13 bit, single group)
• Replacement AI module: 6ES7331-7KF02-0AB0 (8 AI, 12 bit, four groups)

After physically swapping the analog module and re-wiring the 24 V supply, compensation, and a 4–20 mA sensor, the SF LED on the 7KF02 stays lit. Disconnecting the sensor (open input) does not clear the fault, which indicates the SF is not solely a wire-break diagnostic. The downstream effect is that the IM 153-1 and the CPU 315-2DP also report SF, blocking any new hardware download via STEP 7 / SIMATIC Manager. The repair path is: (a) correct measuring range modules, (b) edit HW Config, (c) restore the PG/PC online link, (d) download hardware to the CPU.

2. Hardware Identification & Order Numbers

Verify the exact variant by reading the MLFB on the side of the module. The "7KF02" suffix designates a group-configurable 8-channel module; the "1KF02" is the older single-group 8-channel variant and is not electrically or configuration-compatible. Mixing the two without re-engineering the STEP 7 project is the most common origin of the SF LED on a swap-out.

3. SM 331 AI 8x12 bit - Channel Group Architecture

The 7KF02 module divides its eight inputs into four groups of two channels each:

Each group is driven by its own measuring range module (a small white carrier with four black slider positions labeled A, B, C, D, E, F on the left side of the module housing). Both channels inside a group must be wired to the same kind of transducer and must be configured identically in HW Config. The factory default for measuring range modules is position D (4–20 mA, 2-wire transmitter).

4. Measuring Range Module Position Matrix

The 7KF02 is fundamentally a multi-range module. The measuring range module acts as a hardware "key" - if its position does not match what the STEP 7 hardware configuration says it should be, the module sets SF. The complete position matrix is:

For a 4–20 mA 2-wire transmitter the slider must be in D. For a 4-wire (externally powered) 4–20 mA transmitter the slider must be in C. The difference is not just electrical - in position D the module sources the 24 V loop supply through pin 20, while in position C the loop must be powered externally and the module behaves as a passive current sink on the measurement terminals.

5. Two-Wire vs. Four-Wire Transducer Wiring

A 2-wire transmitter receives its loop power from the analog module itself. Only two wires run from the field: the +24 V supply is sourced on pin 20 (or its per-group equivalent) and the current loop returns on the channel input terminal. Total loop voltage drop budget is typically 18 V (transducer drop) + line drop; the module's internal supply compensates.

A 4-wire transmitter has a separate pair of power wires (commonly brown +24 V / blue 0 V) and a separate signal pair (signal + / signal -) that the AI module reads passively. The module must NOT source loop voltage on pin 20 for a 4-wire device; if it does, the readings will be wrong and the module will detect the configuration mismatch and raise SF.

Front connector pinout for the 6ES7331-7KF02-0AB0 (20-pin version), 4–20 mA 2-wire example on Group 0:

For 4-wire mode, pin 20 is unused; the sensor is powered from an external DIN-rail 24 V supply, and only the signal pair is connected to the channel input pins.

6. SF LED - Diagnostic Decode

The SF LED on SM 331 indicates a module-internal diagnostic event, which is mirrored in the diagnostic buffer of the CPU and in the module diagnostic record (DS0/DS1). Typical 7KF02 diagnostic events that drive SF include:

To read these on-line, open the S7 project in STEP 7 / SIMATIC Manager, go online to the CPU, and use PLC → Module Information on the 7KF02 slot. The Diagnostic Buffer tab and the IO Device / Module Diagnostic tab contain the exact channel-level event and timestamp.

7. Step-by-Step Troubleshooting Procedure

7.1 Restore the Online Link

1. De-energize the ET 200M station.
2. Physically remove the 7KF02 from the rail.
3. Power the IM 153-1 / CPU only - this should clear the cascading SF on the IM and CPU.
4. Confirm online connection between PG and CPU 315-2DP. If the CPU still has SF, see the S7-300 CPU diagnostic buffer FAQ.

7.2 Verify Measuring Range Module Positions

1. Open the front door of the 7KF02.
2. Locate the four measuring range module carriers on the left side.
3. For each used channel group, slide the carrier to the correct position:
   • 2-wire 4–20 mA transmitter → Position D
   • 4-wire 4–20 mA transmitter → Position C
4. For unused groups, either leave at factory D and disable in HW Config, or cover with the dummy carrier so no floating diagnostic appears.

7.3 Verify Field Wiring

1. Trace the field device power supply origin:

• If the only conductor pair leaving the cabinet is connected to the AI terminal block, it is a 2-wire loop and pin 20 must be the supply.
• If the device has a separate M12 or terminal block for 24 V and a second pair for signal, it is 4-wire and pin 20 is unused.

2. With the module de-energized, measure on the front connector:

• Pin 20 against ManA (17/18/19): must read +24 V (when the module is back in and energized) for 2-wire groups.
• Channel input pin against ManA: must read a current loop of 4…20 mA when a transmitter is connected (use a loop calibrator in series).

7.4 Edit STEP 7 Hardware Configuration

1. Open SIMATIC Manager and double-click Hardware.
2. Locate slot for the 7KF02 in the IM 153-1 rack. If the project still references the 1KF02, replace the order number 6ES7331-1KF02-0AB0 with 6ES7331-7KF02-0AB0 in the catalog.
3. Double-click the module to open its properties. For each group:

• Set Measurement type = I (current)
• Set Measurement range = 4…20 mA
• Set 2-/4-wire = 2-wire (Position D) or 4-wire (Position C) to match the slider

4. Set the integration time / interference frequency (60 Hz / 50 Hz) to the local mains frequency to maximize noise rejection.
5. Enable the Diagnostics checkbox for each group you want to monitor (wire break, overflow).
6. Save and compile (Station → Save and Compile).

7.5 Download Hardware to the CPU

1. Go online: PLC → Download to Target with the target CPU set to 315-2DP/AG10.
2. Confirm the prompt that the system will go to STOP during download.
3. After download completes, place the CPU in RUN.
4. Insert the 7KF02 into the slot and observe the LED status. SF should extinguish within a few seconds once the module completes its self-test against the loaded SDB.

8. Verification & Commissioning Tests

After the SF LED extinguishes, perform the following to verify the repair is complete and the process values are valid:

1. Module Status Read: In STEP 7 online, double-click the 7KF02 in HW Config and read the module status. All four groups should report "OK" and the diagnostic byte should be 0x00.
2. Wire-break test: Disconnect the field transmitter and confirm the corresponding diagnostic bit in the I/O area toggles, the input value falls to 0, and the SF LED remains OFF (only the wire-break diagnostic is reported, not a hard SF). If SF returns on wire break, the "Diagnostics - Wire break" checkbox is enabled in HW Config and the channel is configured correctly.
3. Loop current verification: Inject 4.000 mA, 12.000 mA, and 20.000 mA with a calibrator. The engineering value in the PA field of the input word (PIW) should read the corresponding linearized value per the configured scale (default 0–27648 for 4–20 mA on S7-300).
4. Channel group consistency check: Verify both channels in each configured group report identical measurement type and range in the HW Config. The 7KF02 rejects an SDB that contains different ranges within a group and raises a "parameter assignment error" diagnostic (0x04 channel fault).

9. Diagnostic Buffer Analysis - Sample Event Trace

A typical event sequence during the fault scenario from the original case study:

12:01:04.123  CPU 315-2DP  Distributed I/O fault - station failure
              IM 153-1 (6ES7 153-1AA03-0XB0)  PROFIBUS DP slave diagnostics
12:01:04.218  SM 331 (6ES7331-7KF02-0AB0)  Module fault - parameter error
              Group 0: configuration "2-wire" but measuring range module = C
12:01:04.240  CPU 315-2DP  STOP due to distributed I/O fault
12:14:11.020  CPU 315-2DP  Hardware reconfiguration downloaded (target = RUN)
12:14:14.450  SM 331 (6ES7331-7KF02-0AB0)  Module OK - all groups operational
12:14:14.456  IM 153-1                    Station OK
12:14:14.460  CPU 315-2DP  RUN

The exact diagnostic events depend on the STEP 7 version and the IM 153-1 firmware. With IM 153-1AA03 firmware ≥ V3.x and STEP 7 V5.4 SP5 or later, the parameter mismatch is also surfaced in the DP slave diagnostic buffer in addition to the module's own SF. The CPU may remain in RUN if the module's diagnostic class is configured as "non-disabling" in HW Config, but a fresh swap-in of a different MLFB (1KF02 → 7KF02) will by default be a "disabling" diagnostic and forces the CPU to STOP until the configuration is corrected and downloaded.

10. Wire-Break Behavior and 4-20 mA Diagnostic Limits

The 4–20 mA current range has well-defined diagnostic thresholds set by the S7-300 analog subsystem:

This is why disconnecting a 4-wire 4–20 mA sensor in the original incident did not clear the SF: the SF was being driven by the configuration mismatch (diagnostic 0x04) and the missing external supply (which prevented the module from completing its parameterization), not by wire-break.

11. Cross-Reference: 1KF02 vs. 7KF02 Swap Considerations

The 1KF02 predecessor is a single-group module. The 7KF02 replacement is a four-group module with different channel pinout and different resolution (13-bit on 1KF02 vs. 12-bit on 7KF02). Engineers commonly assume the modules are drop-in compatible because they have the same number of channels and the same front-connector footprint. The actual considerations are:

Always treat a 1KF02 → 7KF02 swap as a hardware reconfiguration and re-derive the I/O address map, the diagnostic behavior, and the conversion scaling. The number of analog input words (PIW) is identical (8), but their meaning relative to physical channels may differ if the project had previously assumed single-group uniformity.

12. FAQ