Configuring S7-300 SM331 4-Wire Current Input for ABB AC880

1. Problem Overview

An S7-300 station consisting of a CPU 315-2DP/PN (or comparable -2DP/-PN variant) is reading drive speed feedback from an ABB AC880 industrial drive through an SM331 analog input module, part number 6ES7331-7KF02-0AB0 (AI 8x12 Bit). The drive's analog reference output is configured for a 0 to 20 mA signal, but the operator can only see FC105 returning RET_VAL = W#16#0008 and the raw process image word (PIW) sitting at 32767 (0x7FFF), which is the over-range sentinel value for the SM331 channel.

This mismatch between drive signal type, hardware wiring, and STEP 7 hardware configuration is one of the most common field faults encountered when retrofitting legacy 4-20 mA I/O to drives that use 0-20 mA. The module is mechanically capable of measuring 0-20 mA, but only when the rear range module is in the correct slot. Selecting the wrong slot, or selecting 2-wire measurement, will produce exactly the symptoms described here.

2. Affected Hardware and Module Identification

The module in question is the SM 331, AI 8 x 12 Bit, with order number 6ES7331-7KF02-0AB0. The eight channels are organised as two groups of four. Each group of four shares a single range module inserted into a slot on the rear of the module. Removing the front cover / sub-D shell exposes two small rotary switch positions usually labelled A, B, C, D.

Notice the key constraint: the module only supports 0 to 20 mA on position C, in 4-wire mode. If you require 0 to 20 mA into this SM331, position C is the only valid selection, and the transmitter must either be self-powered or be supplied by an external 24 V source. Position D only supports 4 to 20 mA because the internal current sink can only source the loop power for a 4 mA pre-bias on a working 4-20 mA transmitter; there is no equivalent pre-bias support for a 0-20 mA loop.

Drive side, the ABB ACS880 / AC880 family industrial drive provides configurable analog outputs that can be programmed as either 0 to 20 mA or 4 to 20 mA via parameter group 14 (or the equivalent firmware menu, depending on firmware version). The drive hardware itself does not care whether the receiver is wired as 2-wire or 4-wire - the 2-wire / 4-wire distinction is a property of the receiver side and the wiring harness.

3. Root Cause Analysis

The two observed symptoms trace back to a single mechanical configuration problem on the SM331.

3.1 Symptom A - PIW = 32767 (0x7FFF)

The SM331 returns 0x7FFF in the process image when the raw input signal is below the negative over-range or above the positive over-range of the configured span. The configured span is taken from both the HW Config measurement type and the rear range module position. If either is wrong, the ADC ends up looking at the wrong shunt resistor, and 0 mA signal current shows up either as 0x7FFF or 0x8000 on the process image.

Reading 32767 with the drive outputting a perfectly valid 4 mA signal in this case indicates that the range module is currently in position D (2-wire), but the wiring is not configured as 2-wire, or the range module is in position A / B (voltage), so the channel is looking at the wrong resistor divider and the input current is saturating the voltage range rail.

3.2 Symptom B - FC105 RET_VAL = W#16#0008

FC105 (SCALE) returns W#16#0008 when its IN parameter is either greater than 27648 (positive over-range) or less than 0 (negative over-range / open wire). Since the SCALE block passes the raw PIW into IN, an over-range PIW (32767) is reported as FC105 error 0008. The drive is fine. The PLC scan is fine. The fault is structural.

4. 2-Wire vs 4-Wire Current Topology

Understanding the difference between 2-wire and 4-wire transmitters is essential before touching the wiring or the range module. The terms describe the number of conductors that carry both power and signal:

4.1 2-Wire (Loop-Powered) Transmitter

The transmitter is powered by the analog input module over the same two wires that carry the current signal. The receiver (SM331 in position D) sources 24 V through the input terminals and regulates the loop current. The transmitter and receiver share both power and signal wires.

4.2 4-Wire (Self-Powered) Transmitter

The transmitter has its own dedicated 24 V supply (often drawn from the drive's control terminals or from a separate power supply). Two wires carry only the current signal, returning to the SM331 over a separate pair of terminals. With this topology, the SM331 is set to position C and supports both 0-20 mA and 4-20 mA.

4.3 Which to Use with This Drive?

The drive is sometimes called "2-wire" colloquially because the drive terminals present a passive analog output. That does not mean the SM331 channel must be configured as 2-wire / position D. Because the AC880 supports 0-20 mA and the SM331 only accepts 0-20 mA on the 4-wire range (position C), the only correct configuration is:

1. Drive side: any topology that exposes the 0-20 mA signal on two terminals with a common return.
2. SM331: 4-wire measurement, range module position C.
3. External 24 V: supplied to the drive output stage from the drive itself or a dedicated power supply.

5. STEP 7 Hardware Configuration

Open the hardware configuration (HW Config in STEP 7 V5.x, or Device Configuration in TIA Portal) and locate the SM331 module on the rail. Double-click the slot to open the Properties dialog, then go to Inputs. Set each channel used by the drive feedback to the following:

Save, then download the hardware configuration to the CPU. STEP 7 rewrites the diagnostic buffer with event "Hardware updated by STEP 7".

6. Range Module Physical Switch Setting

Even if HW Config says "4-wire current 0-20 mA", the SM331 will not convert correctly unless the rear range module is mechanically in position C. The following physical switch setting is the one that drives in this scenario must select:

1. De-energise the S7-300 station. Remove the front connector / sub-D shell from the SM331 module.
2. Locate the small rotary switch on the rear of the module. Each group of four channels has its own switch; the group that contains the speed feedback channel must be set.
3. Use a small flat-blade screwdriver to rotate the switch so the arrow / letter points to C.
4. If both groups have channels in use, confirm both switches are in C.
5. Reinstall the front connector. Restore power.

7. FC105 Scaling Configuration

With the hardware configured and the input reading a clean 0-27648 mW mapped signal, FC105 (SCALE) is called to map the raw integer to engineering units. For a 0 to 20 mA speed feedback where 0 mA = 0 rpm and 20 mA = 1500 rpm, the FC105 call is:

CALL FC 105
  IN      := PIW 304          // raw input from SM331 channel 0, group 0
  LO_LIM  := 0.0              // engineering value at 0 mA
  HI_LIM  := 1500.0           // engineering value at 20 mA
  BIPOLAR := FALSE            // 0-20 mA is unipolar
  RET_VAL := MW 100           // store FC105 error code
  OUT     := MD 200            // engineering units (REAL)

7.1 Why BIPOLAR Must Be FALSE

A 0-20 mA signal occupies only the positive half of the converter's bipolar range. Setting BIPOLAR := TRUE would double the apparent resolution without giving any extra information and would also lift the integer zero point to a non-integer boundary. With the AC880 0-20 mA output, leave BIPOLAR off and treat the raw integer as 0-27648 unipolar.

7.2 Bipolar Note for 4-20 mA Applications

If the field application is changed to 4-20 mA (still on position C of the range module), the scaling changes:

• Integer 0 corresponds to 4 mA (4 mA is the integer offset).
• 4 mA = 0 engineering units (typically 0 rpm or live-zero scale).
• 20 mA = 27648 = HI_LIM.
• A reading of 0 or below is indicative of an open wire.

Open-wire detection on 4-20 mA is a useful diagnostic and is one reason industrial standards favour 4-20 mA over 0-20 mA for new installations.

8. FC105 RET_VAL Error Code Reference

The error code returned into the RET_VAL output of FC105 is a compact diagnostics channel. The codes most relevant to this application are listed below; reference the Siemens "Standard Functions" manual for the complete table.

A persistent W#16#0008 with the in-station hardware in position C almost always means the rear range module and the HW Config measurement type still disagree.

9. HMI Display Wiring

Once FC105 writes a clean REAL into MD 200, the HMI tag should reference MD 200 (or a copy in a global DB) and convert it to engineering units on display:

9.1 Watchdog Tag for FC105 Error

For predictive maintenance, expose MW100 (the FC105 RET_VAL) to the HMI as well, masked against W#16#0008. A persistent non-zero RET_VAL indicates that the FC105 call is misconfigured or that the PIW is in over-range. Either symptom should be alarmed at HMI level.

10. Verification Procedure

Confirm the fix on the running machine using the following ordered checks. Capture diagnostic buffer entries at each step.

1. Verify range module position. Look at the SM331 rear slot. The letter exposed should be "C" for the relevant group.
2. Verify HW Config. In STEP 7 or TIA Portal, double-click the module and confirm "Measuring type = 4-wire current" and "Measuring range = 0 to 20 mA".
3. Monitor the PIW online. In a watch table (VAT), force the drive to 0 mA. Expected PIW = 0.
4. Force the drive to 20 mA. Expected PIW ≈ 27648 (or 27646-27649 depending on converter linearity).
5. Force the drive to 4 mA. On 0-20 mA this should give PIW ≈ 5529. On 4-20 mA it should give PIW ≈ 0 plus instrument error.
6. Check FC105 RET_VAL. With the drive at any mid-scale current (e.g. 10 mA), FC105 RET_VAL should be W#16#0000.
7. Verify HMI value. Confirm the HMI numeric field shows the engineering value, e.g. 750 rpm at 10 mA on a 0-1500 rpm scale.
8. Disconnect the signal lead. Trigger diagnostic interrupt by pulling the signal terminal. Confirm "Analog input wire break" appears in the diagnostic buffer with the expected slot and channel.

11. Common Field Pitfalls

Even after the range module is correctly set, certain common mistakes show up repeatedly on site:

12. Related Manual References

For further details, consult the primary Siemens documentation for the AI module and for the AC880 drive. The current versions are maintained on the Siemens and ABB product support portals:

• S7-300 SM331 Analog Input Module Manual (6ES7331-7KF02-0AB0)
• Siemens Industry Online Support for the latest HSP packages for STEP 7 / TIA Portal
• ABB Library - ACS880 / AC880 Drive Firmware Manuals for analog output configuration parameter groups

13. Frequently Asked Questions