Siemens SIMATIC S7-300 SM 331 Analog Input Module 6ES7331-7KF01-0AB0: Wiring, Configuration, and Field Reference
The 6ES7 331-7KF01-0AB0 is a legacy SIMATIC S7-300 analog input (SM 331) module with 8 inputs at 12-bit resolution, organized in four electrically-isolated channel groups. It was the predecessor to the 6ES7331-7KF02-0AB0 and remains in service across brownfield S7-300 installations worldwide. Engineers maintaining older S7-300 racks frequently encounter this module because spare-part inventories, retrofit projects, and STEP 7 V5.x systems continue to depend on it. This reference consolidates wiring practice, measuring-range-module selection, HW Config parameters, SF LED diagnostics, and the migration path to 7KF02.
1. Module Identification and Order Number Structure
Siemens SIMATIC order numbers follow a structured naming convention. For the SM 331 family:
| Segment | Decoded Meaning | Value in 6ES7331-7KF01-0AB0 |
|---|---|---|
| 6ES7 | SIMATIC S7 product group | SIMATIC S7 |
| 331 | Functional sub-group | SM 331 analog input |
| 7KF01 | Function variant / firmware revision | 8 AI, 12-bit, 4 channel groups (legacy) |
| 0AB0 | Front connector / packaging / release code | 20-pin front connector, standard packaging |
Within the SM 331 family, the 7KF01 belongs to the 8-channel, 12-bit class. The 7KF01 is distinguished from the 7KF02 only by an internal firmware revision and a small set of additional diagnostic features in the successor. The mechanical envelope, slot footprint, and front connector are identical, so 7KF02 can be installed in any rack position previously populated with a 7KF01.
6ES7331-7KFxx-0AB0, which means STEP 7 HW Config accepts either part number when the catalog is loaded. STEP 7 V5.4 SP5 and earlier may not recognize 7KF02 unless the hardware support package (HSP) is installed.2. Technical Specifications
The following table summarizes the published specifications for the SM 331 AI 8x12-bit module (7KF01). Engineers retrofitting this module should verify terminal-specific limits against the panel nameplate and the latest Siemens datasheet.
| Parameter | Value |
|---|---|
| Number of inputs | 8 |
| Resolution | 12 bits (plus sign) |
| Channel groups | 4 groups (A, B, C, D), each with 2 channels |
| Measurement type per group | Programmable (voltage, current, RTD, resistance) |
| Galvanic isolation | Yes, between channel groups and backplane |
| Common-mode voltage | Up to 11 V DC between channels of different groups |
| Update time (per group) | Approximately 10 ms to 140 ms depending on integration mode |
| Front connector | 20-pin screw or spring-loaded |
| Power consumption from backplane | 1.0 W (typical) |
| Measuring range module positions | A, B, C, D (one per channel group) |
| Diagnostic LEDs | SF (group fault), 24 V (backplane / load supply) |
| Operating temperature | 0 °C to 60 °C horizontal mounting |
The 12-bit ADC plus sign provides an effective range of 0 to 27648 counts in STEP 7 normalized engineering units, which corresponds to nominal 0% to nominal 116.03% of the configured range. Overrange is reported up to 32511 counts and wire-break / open-circuit / underrange is reported as 32768 / -32768 depending on signed/unsigned interpretation.
3. Channel Group Architecture
The 8 inputs are divided into four channel groups of 2 channels each. Each channel group has its own measuring range module on the left side of the module. The four groups are:
- Group A: Channels 0 and 1
- Group B: Channels 2 and 3
- Group C: Channels 4 and 5
- Group D: Channels 6 and 7
Both channels within a group share the same measurement type and the same measuring range module position. Mixing voltage and current within a single group is not supported. Adjacent groups are isolated from each other, which permits mixed signal types across the module.
4. Measuring Range Module Positions
The mechanical measuring range module (a small rotary switch accessible from the left side of the module after removing the label flap) determines the hardware measurement range and the wiring convention for that channel group. The four positions are:
| Position | Measurement Type | Wiring Mode | Typical Range |
|---|---|---|---|
| A | Voltage / Resistance / RTD | 4-wire (resistance/RTD) or 2-wire differential | ±50 mV, ±500 mV, ±1 V, ±5 V, ±10 V, 0–600 Ω, Pt100/Ni100 |
| B | Voltage (4-wire RTD) | 4-wire | ±1 V, ±5 V, ±10 V |
| C | Current (4-wire transducer) | 4-wire | 0–20 mA, 4–20 mA, ±20 mA |
| D | Current (2-wire transducer) / Voltage | 2-wire | 0–20 mA, 4–20 mA |
For a 4–20 mA 4-wire transducer, position C is the correct setting. For a 2-wire loop-powered 4–20 mA transmitter, position D is correct and the loop is fed from the 24 V supply on the module terminals. The 7KF02 successor maintains the same position mapping.
5. Two-Wire vs Four-Wire Transducer Wiring
Selecting the correct transducer wiring topology is the single most common source of SF LED faults on this module. The decision tree is:
- If the transmitter is self-powered (i.e., it has its own 24 V supply, or it is being driven by a battery-powered signal generator), wire as a 4-wire transducer. Set the measuring range module to position C for that group.
-
If the transmitter is loop-powered (only 2 wires connect the field device, and the loop is completed through the module), set the measuring range module to position D. The module provides loop power through terminal
M+/M-. - If the source is a voltage output (e.g., ±10 V), use position A or B depending on whether 4-wire differential sensing is required.
For a battery-powered 4–20 mA signal generator being used as a calibration source, the instrument is by definition self-powered. The wiring convention must therefore be 4-wire, and the measuring range module position must be C. Wiring it as 2-wire (position D) will leave the loop open, which the module interprets as an open circuit and reports via the SF LED and the diagnostic buffer.
5.1 Wiring Pinout for 4-Wire 4–20 mA
For a 4-wire current transducer connected to channels 0 and 1 (Group A) on the 20-pin front connector:
| Terminal | Function | Connection |
|---|---|---|
| 1 | CH0+ (signal +) | Transmitter current output + |
| 2 | CH0- (signal -) | Transmitter current output - |
| 3 | CH1+ | (not used for this group) |
| 4 | CH1- | (not used for this group) |
| 5..10 | Group B | (per configuration) |
| 11..16 | Group C | (per configuration) |
| 17..20 | Group D | (per configuration) |
| 19 | MANA | Ground reference for analog section |
| 20 | M (24 V return) | Common 24 V return |
Verify the exact pinout against the wiring diagram printed inside the front door of the module. The above table assumes the standard 7KF01 wiring diagram; refer to SM 331 AI 8x12-bit Getting Started Part 1 and Getting Started Part 2 for the channel-to-pin map specific to 7KF01.
6. STEP 7 HW Config Parameterization
After mechanically setting the measuring range modules, the same selection must be made in software:
- Open SIMATIC Manager > HW Config.
- Locate the SM 331 in the rack (slot 4 to 11 typical).
- Open Object Properties > Inputs.
- For each of the four channel groups (0–1, 2–3, 4–5, 6–7), set the measurement type and range:
Group 0/1: Measurement type = 4-wire current, Range = 4..20 mA
Group 2/3: Measurement type = Voltage, Range = +/-10 V
Group 4/5: Measurement type = Deactivated (or RTD/Pt100)
Group 6/7: Measurement type = Deactivated
- Enable Diagnostic Interrupt and Group Diagnostics under the Diagnostics tab if the program should react to wire-break or overrange events.
- Compile and download the hardware configuration to the CPU.
7. SF LED Diagnostics and Troubleshooting
The SF (red) LED on the front of the SM 331 indicates a group fault. The most common causes for 6ES7331-7KF01-0AB0 are:
| Symptom | Root Cause | Remediation |
|---|---|---|
| SF on, all channels return 7FFF / 8000 | Measuring range module missing or wrong position | Power down, set module to correct position, re-power |
| SF on, only one group affected | Channel-group specific fault (wire break, overrange) | Check field wiring, loop supply, and signal range |
| SF on, all channels read 0 | Backplane 24 V missing | Verify PS 307 / CPU 24 V output and fuse |
| SF intermittent | Noise pickup on long cable runs | Use shielded cable, ground at one end only |
| SF after STEP 7 download | HW Config mismatch with physical module position | Reconcile measuring range module positions to STEP 7 settings |
| SF and group reads 32767 / wire-break bit set | Open circuit on current loop (position C without loop power) | Check transmitter, polarity, and 24 V supply |
To capture the precise diagnostic event, open STEP 7 > PLC > Diagnostic Buffer on the online CPU. The buffer records OB82 / OB83 diagnostic interrupts with timestamp and channel group identification. Cross-reference the entry with the field wiring and measuring range module position to localize the fault.
8. Successor Module 6ES7331-7KF02-0AB0 Comparison
The 7KF02 is the direct successor and a drop-in replacement for the 7KF01 in the vast majority of installations. Engineers sourcing spares for a 7KF01 failure should procure a 7KF02 unless the existing STEP 7 project is locked to a specific hardware catalog.
| Feature | 7KF01 (legacy) | 7KF02 (successor) |
|---|---|---|
| Inputs | 8 | 8 |
| Resolution | 12 bit + sign | 12 bit + sign |
| Channel groups | 4 | 4 |
| Form factor | Single-width S7-300 | Single-width S7-300 |
| Front connector | 20-pin | 20-pin |
| Galvanic isolation | Yes | Yes |
| Diagnostic interrupt | Basic (OB82) | Extended (OB82 with detailed channel info) |
| STEP 7 V5.5 SP3+ | Supported | Supported |
| TIA Portal V13+ | Limited | Supported via HSP |
| Measuring range module | Same positions A–D | Same positions A–D |
| Order number | 6ES7331-7KF01-0AB0 | 6ES7331-7KF02-0AB0 |
The Siemens ID for the successor module is 8859629. The product page lists the 8 inputs in 4 channel groups with programmable measurement type per group, identical to the 7KF01 architecture.
9. Battery-Powered Signal Generator Field Setup
Engineers often commission or troubleshoot an existing S7-300 rack using a battery-powered 4–20 mA signal generator as a calibration source. Apply the following procedure:
- Power down the S7-300 rack or isolate the analog input module slot.
- Identify the channel group that will be exercised (for example, Group A, channels 0–1).
- Set the measuring range module for that group to position C (4-wire 4–20 mA).
- Connect the signal generator:
- Generator (+) → Terminal 1 (CH0+)
- Generator (-) → Terminal 2 (CH0-)
- Generator loop is completed internally to the generator; no external loop supply is required.
- Set the generator output to 4.000 mA and confirm a STEP 7 reading of
0counts. - Set the generator output to 20.000 mA and confirm a reading of
27648counts. - Set the generator output to 12.000 mA and confirm a reading of
13824counts (50% of range).
If the SF LED illuminates at any step, the most likely cause is that the measuring range module is set to position D (2-wire) instead of position C (4-wire). Position D expects the module to source loop power; a battery-powered transmitter cannot complete that loop.
10. Verification and Commissioning Checklist
Run the following checks after installation, parameter download, and field wiring:
- Module seating: Verify the module is fully latched into the backplane and the front connector is screwed tight.
- Measuring range module: Confirm all four module positions match the HW Config selection. Record the position for each group on the wiring diagram.
- STEP 7 online: Open the variable table (VAT) and force a download. Read back the raw input words in INT format. A healthy 4–20 mA loop should report between 0 and 27648 counts.
- Diagnostic buffer: Verify the diagnostic buffer is empty after download. Any OB82 entries indicate a configuration/wiring mismatch.
- SF LED: Should be OFF. If ON, consult Section 7.
- 24 V LED: Should be ON, indicating backplane 5 V and external 24 V are present.
- Wire-break test: Disconnect one field wire at a time. The diagnostic buffer should report a wire-break event for the affected channel group if monitoring is enabled.
- Loop test: Apply 4 mA and 20 mA from the calibrated source and confirm linearity (0 / 13824 / 27648 counts).
11. Spare-Part and Lifecycle Considerations
The 7KF01 was classified by Siemens as phased out as of the early 2010s, with the 7KF02 acting as the official functional replacement. For new installations Siemens recommends the SM 331 family variants with higher resolution (e.g., 6ES7331-7NF00-0AB0 for 8x16-bit) or the S7-1500 equivalent (6ES7531-7KF00-0AB0 in the ET 200MP family). For brownfield S7-300 retrofits, the 7KF02 remains the lowest-risk spare.
12. Frequently Asked Questions
Is the 6ES7331-7KF01-0AB0 still available, or should I source a 7KF02?
The 7KF01 is a legacy product. For new installations and field replacements, source the 6ES7331-7KF02-0AB0. The 7KF02 is mechanically and electrically pin-compatible with the 7KF01 and accepts the same measuring range module positions A through D.
How do I connect a battery-powered 4–20 mA signal generator to the SM 331 7KF01?
Wire the generator as a 4-wire transducer. Set the measuring range module for the affected channel group to position C. Connect generator (+) to the channel positive terminal and generator (-) to the channel negative terminal. The generator provides its own loop power; do not configure the module for position D, which expects the SM 331 to source loop power to a 2-wire device.
Why does the SF LED stay on even though the field wiring checks out?
Three causes account for the majority of SF LED faults on the 7KF01: (1) measuring range module position does not match HW Config, (2) diagnostic interrupt OB82 is enabled but a fault is being held (clear with STEP 7 acknowledge), (3) a channel group is unconfigured in HW Config but the field wiring is floating. Set unused channel groups to "Deactivated" in HW Config to eliminate floating-input faults.
Can I mix voltage and current inputs on the same SM 331?
Yes. The 8 inputs are split into 4 channel groups of 2 channels. Each group shares one measuring range module, so you can configure Group A for 4–20 mA, Group B for ±10 V, and Group C for Pt100, for example. You cannot mix measurement types within a single group.
Where can I find the wiring diagram and the official manual for the 7KF01?
The official Siemens Getting Started for the SM 331 AI 8x12-bit family is split into two parts: Getting Started Part 1 and Getting Started Part 2. The 7KF01-specific wiring and configuration details are in chapter 4.4.1 of the S7-300 Module Specifications Reference manual. The successor module 7KF02 datasheet is at Siemens ID 8859629.