Siemens SM 331 1KF01 to 1KF02 Replacement Compatibility Guide

Overview

The SIMATIC S7-300 SM 331 analog input module family includes two closely related 8-channel, 13-bit variants: the predecessor 6ES7331-1KF01-0AB0 and its spare-part successor 6ES7331-1KF02-0AB0. Both modules occupy one slot in any S7-300 central rack and expose the same logical channel map, the same range selection bits, and the same STEP 7 / TIA Portal hardware catalog entry. Siemens officially designates the 1KF02 as the drop-in replacement for the 1KF01, meaning the two are interchangeable without rewiring or reconfiguration. The single functional delta a field engineer will encounter is the current-loop burden resistance: 50 Ω on the 1KF01 and 100 Ω on the 1KF02. This reference documents that delta in detail, quantifies its effect on every supported measurement type, and gives the commissioning checklist for verifying correct operation after a swap.

Part Number Identification and Lifecycle Status

Both order numbers belong to the SIMATIC S7-300 SM 331 (6ES7 331) family of analog input modules. The MLFB breakdown is identical: 6ES7 = SIMATIC, 331 = SM 331 AI family, 1KF = 8-channel 13-bit variant, 01/02 = hardware revision, 0AB0 = front connector / coating designator. The 1KF01 entered active marketing in the early 2000s; the 1KF02 supersedes it as a fully compatible spare part. The product withdrawal date for the 1KF01 has passed in most regions, and current orders placed with Siemens for the older part number are fulfilled from stock or routed to the 1KF02 unless the customer explicitly insists on the original MLFB.

Reference: SIMATIC S7-300 SM 331 Analog Input Module Manual (8859629).

Technical Specifications Comparison

The two modules share resolution, channel count, group structure, conversion principle (integrating ADC), and the full set of supported measurement types. Differences are concentrated in the analog front-end (input resistance for current mode), basic error limits, and diagnostic capability.

Input Resistance Difference: 50 Ω vs 100 Ω — Engineering Impact

The only measurement-side parameter that differs is the burden (shunt) resistance for current inputs. The 1KF01 uses a 50 Ω shunt between the input pin and signal ground; the 1KF02 uses a 100 Ω shunt. The full-scale burden voltage at 20 mA therefore differs:

• 1KF01: V_burden = 20 mA × 50 Ω = 1.00 V
• 1KF02: V_burden = 20 mA × 100 Ω = 2.00 V

For a 4–20 mA loop powered from 24 VDC at the transmitter, the maximum permissible loop drop is:

V_loop_max = V_supply − V_transmitter_min

With a 24 V supply and a typical 12 V minimum transmitter compliance, you have roughly 12 V of headroom. The extra 1 V burden from the 1KF02 is therefore not a constraint. It only becomes a constraint if the loop supply is unusually low (e.g., 12 V with a 10 V minimum transmitter) or if multiple resistors in series already exist (e.g., intrinsic safety barriers with documented maximum end-to-end resistance). Always re-check loop compliance after a 1KF01 → 1KF02 swap in IS installations.

Effect on 0–20 mA and ±20 mA Loops

The measured current is set by the transmitter. The shunt does not alter the current; it only alters the voltage the module develops internally. As long as the burden voltage remains below the module's compliance and the transmitter can drive the additional 1 V, the raw count returned by STEP 7 is unchanged. No scaling change is required in the user program.

Effect on Voltage Inputs

Voltage inputs use a high-impedance FET front-end (10 MΩ typical) on both modules. The 50 Ω / 100 Ω shunt is not in the voltage path; there is zero measurable difference between the two modules in voltage mode.

Effect on RTD / Resistance Inputs

Pt100 and Ni100 measurements use a constant-current excitation source internal to the module. The internal excitation current is matched to the module's reference resistor network; this is unchanged between the two revisions. Field wiring of a 4-wire Pt100 is therefore electrically identical. The 1KF02 specifies a marginally tighter basic error for the RTD range, so measurement accuracy typically improves slightly after the swap.

Spare-Part Compatibility and Configuration Impact

Siemens classifies the 1KF02 as spare-part compatible with the 1KF01 in the official product master data. This classification means:

• STEP 7 (V5.x) and TIA Portal hardware catalog entries for "6ES7 331-1KF01-0AB0" and "6ES7 331-1KF02-0AB0" produce identical slot configurations.
• The wiring diagram is identical: same 20-pin front connector pin-out, same terminal assignments, same channel group structure.
• The measured-value representation (S7 format: 0–27648 for unipolar, −27648 to +27648 for bipolar) is identical.
• No STEP 7 / TIA Portal hardware reconfiguration or PLC restart beyond a normal cold restart is required.

If you have the original HW Config entry pointing to the 1KF01, the project continues to download and run without modification. If you explicitly want the HW Config entry to read the new MLFB, double-click the module in STEP 7 and re-select "6ES7 331-1KF02-0AB0" from the catalog; the slot configuration is preserved.

Wiring and Channel Group Configuration

Both modules divide the eight inputs into four groups of two channels: CH0/CH1, CH2/CH3, CH4/CH5, CH6/CH7. Each group shares a single measurement-type and range selection, configured in STEP 7 under the slot properties or in TIA Portal under "Properties → Analog inputs → Channel x". Selecting a voltage range for CH0 forces CH1 to the same range; you cannot mix ranges inside a group.

The analog ground terminal MANA must be tied to the field-side ground reference at one point only. Both modules require this same topology. Reference: SM 331 Manual, Section on Terminal Assignment.

Measurement Type and Range Mapping

The following table maps each measurement type to the channel group configuration word used in STEP 7 / TIA Portal:

Range selection codes are identical for both modules. A STEP 7 project written against a 1KF01 will deploy without change to a rack populated with 1KF02 modules.

Conversion Time, Resolution, and Accuracy

Both modules use an integrating ADC with selectable integration time. The integration time directly controls both conversion time and mains-frequency noise rejection:

Total scan time for all 8 channels at the 100 ms integration time = ~820 ms (102 ms × 8). The 1KF02 specifies the same scan times; no firmware update or rescaling is required.

Basic Error Limit and Repeatability

The 1KF02 publishes a tightened basic error limit of ±0.5 % of full scale (vs ±0.6 % on the 1KF01) for voltage and current ranges per the published Siemens manual. Repeatability (the spread of consecutive readings at a stable input) is typically improved due to a redesigned input multiplexer. If your process control loop is sensitive to noise, you will typically observe a small but measurable improvement in steady-state output jitter after the swap.

Installation and Verification Procedure

The following procedure covers a typical 1KF01 → 1KF02 swap on a powered-down S7-300 station. If the application is safety-related or part of a SIL-rated loop, follow your site change-management procedure.

1. Place the CPU in STOP via the mode selector or via STEP 7 online. Document the current AI values from the diagnostic buffer for post-swap comparison.
2. De-energize the S7-300 power supply (PS 307 or equivalent). Wait 60 s for the module's internal capacitors to discharge.
3. Record the wiring at the existing 1KF01: which terminal pair feeds each signal, and the configured measurement type. Although the pin-out is identical, a written record prevents terminal-by-terminal mistakes during reinstallation.
4. Remove the front connector from the 1KF01 by pressing the ejector latch. Label each wire or transfer the front connector to the 1KF02 if the connector is reusable.
5. Unlock the module coding key on the backplane; this key prevents accidental insertion of the wrong module type and is identical between 1KF01 and 1KF02. Slide the 1KF01 out.
6. Insert the 6ES7331-1KF02-0AB0 into the same slot. Verify the module seats flush and the front connector latch closes with a positive click.
7. Re-apply power to the PS 307. Place the CPU in RUN. Open the online AI diagnostic view in STEP 7 / TIA Portal.
8. For each channel, force a known reference input (precision voltage source for voltage ranges, decade resistance box for RTDs, calibrator for current loops) and compare the raw count to the expected value. Acceptable deviation is within the published ±0.5 % basic error limit of the 1KF02 plus the reference standard uncertainty.
9. Reset the diagnostic buffer, document the swap in the maintenance log, and return the system to production.

Verification Checklist

• CPU transitions from STOP to RUN without SF (system fault) on the module.
• Module SF LED is OFF.
• All eight channels return valid raw counts (no 7FFFh overflow, no 8000h wire-break on current ranges with a healthy loop).
• Channel-by-channel deviation against reference standard within ±0.5 % FS.
• If the original project referenced the 1KF01 MLFB in HW Config, optionally update the entry to the 1KF02 MLFB and re-download the hardware configuration. This step is cosmetic but improves spare-part inventory traceability.

Diagnostic and Error Handling

The 1KF02 adds a configurable diagnostic interrupt that the 1KF01 does not offer. In STEP 7 / TIA Portal, the slot properties expose a "Diagnostics" tab where you can enable interrupts for:

• Wire break (current loop below 3.6 mA in 4–20 mA mode)
• Overflow / underflow of the ADC
• Loss of the internal reference
• Channel group configuration mismatch

When a diagnostic interrupt fires, OB82 is called. Inside OB82, read the channel status word (local data LW8…LW11) to identify the offending channel, then apply your plant-specific response. If your application does not require diagnostic interrupts, leave the diagnostic enable off — the module behaves identically to the 1KF01.

Fault Code Reference

Field-Proven Cautions and Edge Cases

Case 1: Intrinsic Safety Barrier in the Loop

If the 4–20 mA loop passes through a Zener barrier or galvanic isolator, the barrier's maximum end-to-end resistance plus the module's burden must not exceed the transmitter compliance. The extra 1 V drop on the 1KF02 (compared to the 1KF01) reduces available compliance by 1 V. For most 24 V loops this is irrelevant; for 12 V or 13 V loop supplies in hazardous areas, verify before swapping.

Case 2: Existing Module Calibration Constants

If your PLC program applies per-channel gain and offset multipliers in FB105 or in a user-written scaling block (common when the module is part of a weigh scale or precision temperature loop), those constants are tied to the 1KF01's specific analog front-end trim. Recapture the calibration values after the swap or accept the small inaccuracy delta from the improved 1KF02 spec.

Case 3: Mixed Population on the Same Backplane

Mixing a 1KF01 and a 1KF02 on the same rack is supported but not recommended. STEP 7 / TIA Portal will treat them as the same logical module, but a maintenance technician diagnosing a fault will not be able to identify which module is the 1KF02 from a quick visual scan. Replace all instances of the 1KF01 with the 1KF02 during scheduled shutdowns.

Case 4: S7-300 to ET 200M Migration

Both modules are also usable in an ET 200M distributed I/O station behind a PROFIBUS or PROFINET head. The spare-part compatibility rule extends to this configuration. No GSD / GSDML file change is required.

Related SM 331 Variants (Brief Reference)

The 1KF01 / 1KF02 sit in the middle of the SM 331 family. Adjacent part numbers include:

• 6ES7331-1KF00-0AB0: earlier 8 AI × 13 Bit variant, generally not spare-part compatible with 1KF01/1KF02.
• 6ES7331-7HF01-0AB0: 8 AI × 14 Bit, higher resolution but different channel mapping.
• 6ES7331-7NF00-0AB0: 8 AI × 16 Bit, with full diagnostic interrupt support.
• 6ES7331-1KF01-0AB0 and 6ES7331-1KF02-0AB0: subject of this article.

When in doubt about cross-family replacement, consult the official spare-part list published in the Siemens Industry Online Support portal.

FAQ