Wiring S7-300 SM331 Potentiometer: Fix SF LED, Control ACS880

1. Overview

This reference walks through the full signal chain for a college / pilot build: panel-mounted push-buttons and a 10 kΩ potentiometer -> Siemens SIMATIC S7-300 analog input module SM331 (6ES7 331-7KF01-0AB0) -> SIMATIC S7-300 analog output module SM332 (6ES7 332-5HD01-0AB0) -> ABB ACS880 industrial drive -> three-phase asynchronous motor. The most common failure on first commissioning is the SM331 SF (group fault) LED lighting red and the CPU going to STOP, caused by a mismatch between the physical measuring range module on the side of the SM331 and the channel configuration in STEP 7 HW Config. Resolving the SF LED and producing a clean 0-10 V speed reference for the ACS880 is the focus of this article.

2. Prerequisites

• SIMATIC S7-300 CPU 31x (e.g., 6ES7 314-1AG14-0AB0) with STEP 7 V5.5 or TIA Portal V15 and later.
• SM331 AI8x12Bit, order number 6ES7 331-7KF01-0AB0 (8 channels, 12-bit resolution, isolated).
• SM332 AO4x12Bit, order number 6ES7 332-5HD01-0AB0 (4 channels, 12-bit resolution).
• 10 kΩ panel-mount potentiometer, linear taper, 0.5 W minimum.
• 2 kΩ series resistor on the 24 V feed to the potentiometer (acts as a current limiter and produces a clean 0-10 V sweep across the wiper).
• ABB ACS880-01 drive with firmware ASCI2 v2.32 or later (compat-checked against the analog reference chain in this article).
• Three-phase asynchronous induction motor (e.g., 400 V Δ / 690 V Y, IE2 / IE3).
• STEP 7 library "Standard Library -> TI-S7 Converting Blocks" containing FC105 (SCALE) and FC106 (UNSCALE).

3. Hardware: SM331 (6ES7 331-7KF01-0AB0) Channel Map

The module occupies 16 bytes of input area beginning at the configured start address. With the source project using PEW 288-303, the byte map for channels 0-7 is:

4. Resolving the SF LED: Measuring Range Module Position

The SM331 6ES7 331-7KF01-0AB0 has a physical 4-position rotary selector (the "measuring range module") on the left side of the card. The factory default is position D which enables RTD / thermocouple measurement on all channels. If HW Config is set to "Voltage ±10 V" but the physical selector is on D, the module raises an module / channel fault, the SF LED turns red, and the diagnostic interrupt places the CPU in STOP with OB122 (I/O access error) or OB82 (diagnostic interrupt) pending.

Procedure

1. Power down the S7-300 rack. Remove the 24 V supply from the analog module.
2. Unlatch the SM331 by depressing the top release tab and swing the module off the DIN rail. The module remains wired via the front connector if you leave the connector attached; it is easier to remove the front connector first.
3. Locate the small black rotary selector on the left side of the module housing (when looking at the label).
4. Use a small flat-blade screwdriver to rotate the selector so the arrow points to B for the CH0 potentiometer input. If you also use channels 1-7 for voltage, set the second selector (also visible on the same side) to A or B as required.
5. Re-seat the module on the rail, restore the front connector, re-apply 24 V.
6. Open HW Config, double-click the SM331, click Properties -> Inputs on channel 0, set: Measurement type = V, Range = +/-10 V, Integration = 50 Hz rejection (or 60 Hz if your mains is 60 Hz).
7. Save & compile (Station -> Save and Compile) and download HW Config to the CPU.
8. Switch the CPU from STOP to RUN. The SF LED should extinguish within 2-3 s.

For the official device manual covering the measuring range module, see the Siemens product page: SIMATIC S7-300 SM331 Manual.

5. Potentiometer Wiring (4-Wire Voltage Mode)

The wiring described in the source is the standard 4-wire voltage measurement topology for an external potentiometer fed from the SM331's 24 V sensor supply (terminal L+) through a 2 kΩ series resistor. The 4-wire approach uses the M terminal at the wiper end as a sense reference, eliminating voltage drop on the ground conductor.

Figure 1 — 4-wire potentiometer wiring to SM331 channel 0

6. SM332 Analog Output (6ES7 332-5HD01-0AB0) Configuration

The SM332 is mapped at PQW 304-311. Channel 0 (PQW 304) drives the ACS880 analog speed reference input (typically AI1 on the drive). Configure channel 0 as Voltage 0-10 V in HW Config; this matches the ACS880 default AI1 scaling and avoids bipolar noise on the wiper signal.

7. STEP 7 Programming: FC105 (SCALE) for the Wiper Input

FC105 converts the 12-bit raw input word (0-27648 for unipolar, -27648 to 27648 for bipolar) into an engineering range. With the SM331 configured for ±10 V, FC105 in bipolar mode is appropriate so that a small negative wiper offset (e.g., -0.2 V from thermal drift) maps cleanly to 0 rpm rather than producing a sign-extension glitch.

// FB100 / OB1 - wiper input scaling
CALL  "SCALE"  // FC105
     IN     := PEW288              // Raw 12-bit input from CH0
     HI_LIM := 1.0E+002            // Engineering max = 100.0 %
     LO_LIM := 0.0E+000            // Engineering min = 0.0 %
     BIPOLAR:= TRUE                // Bipolar for noise headroom
     RET_VAL:= MW100               // Return value (error code)
     OUT    := MD104               // LREAL scaled 0.0 - 100.0 %

// Clamp to physical ACS880 limit (e.g. 1500 rpm / 50 Hz)
L     MD104
LIMIT 0.0, 100.0                  // Clamp 0-100 %
T     MD112                        // Clamped speed demand %

FC105 error codes are written to RET_VAL:

8. Mapping to the ACS880 Speed Reference

Two paths exist: (a) write the scaled value into PQW 304 so the SM332 generates 0-10 V to the drive, or (b) skip the SM332 and feed PEW 288 directly through PROFINET to the ACS880. Path (a) is the one used in the source and is the most portable for a college panel.

// Linear mapping 0.0 - 100.0 % to 0 - 27648 raw at SM332 CH0
L     MD112                       // 0.0 - 100.0 %
L     2.764800e+004               // 100.0 % = 27648 raw
*R    
RND
T     PQW304                      // SM332 channel 0 output

Equivalent STL without floating point (suitable for older 312 / 314 CPUs without real-time math):

// Integer scaling: 0-100 % in MW120, output 0-27648 in PQW304
L     MW120                       // 0 - 100
L     276                         // 27648 / 100
*D    
T     MW124
L     MW124
L     27648
>I    
T     PQW304

9. ABB ACS880 Parameter Configuration

With the SM332 output wired to ACS880 terminal block XAI (AI1+ = pin 2, AI1- = pin 3 on the BMIO-01 board), the drive must be told to interpret the 0-10 V input as a speed reference. Default values are shown; the full manual is at the ACS880 Firmware Manual.

10. Push-Button Logic (Direction + Start/Stop)

The wiper sets the speed setpoint; the push-buttons gate the run command and direction. A minimal logic block in LAD/ST is shown in ST:

// Latching start / stop (edge-triggered)
A     I 0.0                       // Start NO pushbutton
AN    I 0.1                       // Stop NC pushbutton
O     M 50.0                      // Latched run
=     M 50.0

// Direction select (1 = forward, 0 = reverse)
A     I 0.2                       // FWD pushbutton
S     M 50.1
AN    I 0.2
A     I 0.3                       // REV pushbutton
S     M 50.2
A     I 0.1                       // Stop clears direction
R     M 50.1
R     M 50.2

// Scale 0-100 % to ACS880 speed; inhibit when wiper < 5 % to avoid creep
L     MD112
L     5.0
>R    
JCN   NOOUT
L     MD112
T     PQW304
NOOUT: NOP 0

11. End-to-End Signal Flow

Figure 2 — End-to-end analog reference path

12. Troubleshooting Matrix

13. Verification Procedure

1. With the drive in Ready and the wiper at minimum, verify PEW 288 = 0 and PQW 304 = 0 V with a multimeter on the SM332 terminals.
2. Move the wiper to mid-scale. PEW 288 should read ~13824 (unipolar 0-10 V) or 0 (bipolar ±10 V). PQW 304 should read 5.00 V ±0.05 V.
3. Move the wiper to full scale. PEW 288 should read 27648. PQW 304 should read 10.00 V ±0.05 V.
4. On the ACS880 keypad, navigate to Group 01 and confirm 01.01 (Motor speed rpm) tracks 0 -> ~1500 -> 3000 rpm (depending on motor and parameter 12.16).
5. Release the wiper to zero. The drive should ramp to 0 rpm along the deceleration ramp (23.02) and not fault.
6. Toggle the FWD/REV push-buttons. The drive should respond within 50-100 ms (DTC cycle) and reverse without overcurrent trip (verify 01.10 = Motor torque stays below 150 % during reversal).

14. Frequently Asked Questions