Wiring SM 1231 4AI 4-20mA 2-Wire and 3-Wire Transmitters

The Siemens SIMATIC SM 1231 analog input module is one of the most widely used signal modules on the S7-1200 platform, but its wiring documentation is a frequent source of confusion when commissioning 2-wire, 3-wire, and 4-wire 4-20 mA field transmitters. This article consolidates the official Siemens wiring guidance, the loop-powering rules for current loops, and the TIA Portal configuration steps required to read a stable process value from a passive loop-powered (2-wire) or actively powered (3-wire / 4-wire) 4-20 mA transmitter on the SM 1231 4AI.

1. SM 1231 4AI Module Identification

Before wiring, confirm the exact module variant in your S7-1200 rack. The SM 1231 family contains several analog input modules with different channel counts, resolutions, and signal-type support. The 4-channel current/voltage variant relevant to most 4-20 mA applications is the SM 1231 AI4.

The 13-bit variants accept voltage (±10 V, 0-10 V, ±5 V, 0-5 V, ±2.5 V) and current (0-20 mA, 4-20 mA). The 16-bit variants add RTD and thermocouple support. For pure 4-20 mA transmitter applications, the 6ES7231-4HD32-0XB0 (4AI, 13 bit) is the most common selection, although the 16-bit modules deliver significantly better noise performance and are recommended for low-level pressure, flow, or analytical signals.

2. The Power Architecture of a 4-20 mA Current Loop

A 4-20 mA current loop is a series circuit. Current flows from a DC power supply through the transmitter, then through the receiver (the SM 1231 input), and back to the supply negative terminal. The transmitter regulates the current to a value proportional to the measured variable. Because the loop is a series circuit, the SM 1231 cannot act as a current source; it is the current-sensing element, not the loop supply.

The classic Ohm's-law check for any 4-20 mA loop is:

V_loop ≥ V_transmitter(min) + V_receiver + (I_max × R_wire_total)

For a 2-wire loop-powered transmitter with a typical 12 V minimum supply requirement, a ~250 Ω input burden on the SM 1231 (~5 V drop at 20 mA), and a wire loop resistance of 20 Ω:

V_loop ≥ 12 V + 5 V + (0.020 A × 20 Ω) = 17.4 V

A nominal 24 V DC supply leaves more than 6 V of headroom, which is the standard design margin. Anything below 18-20 V of total supply at the transmitter terminals will cause saturation or non-linearity at the top of the range.

Why the SM 1231 has no loop power

Each SM 1231 input channel contains a precision current-sensing resistor across the input terminals. The module's internal ADC measures the voltage developed across this resistor. The module does not contain a boost converter, current source, or any active driver that could push current out of the input pin toward the field device. Wiring diagrams in the S7-1200 System Manual show a generic current source or voltage source block; engineers must interpret that block as the field transmitter, not as something the module provides.

3. Transmitter Type Classification

Before touching a wire, identify which transmitter family the field instrument belongs to. The wire count dictates where the 24 V DC enters the loop.

2-wire loop-powered transmitter

The transmitter draws all operating current from the loop. Typical current draw is 3.5-4 mA at the bottom of the range, leaving 4-20 mA for the signal. The total loop current is bounded by the transmitter's own minimum operating current plus the signal current, so a properly designed loop can detect a broken wire (0 mA) and an under-range (≤ 3.6 mA by NAMUR NE 43).

3-wire transmitter

The transmitter receives its operating power on two dedicated wires (V+ and GND) and produces the 4-20 mA signal on a third wire that returns to the same GND. The signal wire and the power negative are NOT isolated.

4-wire transmitter

The transmitter has a completely isolated power input and signal output. The signal output is typically a true floating current source that can drive 0-20 mA or 4-20 mA into a grounded load such as the SM 1231 input.

4. SM 1231 Terminal Layout (AI4 Variant)

The 4AI SM 1231 uses a removable terminal strip with 16 positions. The bottom row carries the analog inputs. The relevant terminal assignments for current inputs are:

5. Wiring Diagrams by Transmitter Type

5.1 Two-wire (loop-powered) 4-20 mA transmitter

The 2-wire loop is the most common configuration in process plants. The SM 1231 input sits in series with the transmitter and the 24 V DC supply.

        +24 V DC
           |
           +---- to transmitter V+ (loop +)
           |
   SITOP / CPU sensor supply
   or external 24 V supply
           |
           |      +-------------+
           |      | Transmitter |
           |      |  (2-wire)   |
           |      |             |
           |      |  V+ -- in   |
           |      |             |
           |      |  SIG -- out |
           |      +------+--------+
           |             |
           |             | signal wire
           |             |
           |      +------+--------+
           |      |   SM 1231    |
           |      |  AI0+ (T2)  |
           |      |  AI0- (T3)  |
           |      +------+--------+
           |             |
           |             | return wire
           |             |
           +-------------+
        24 V DC common (M / GND)

Step-by-step wiring procedure for channel 0:

1. De-energize the field loop. The CPU and 24 V supply can remain powered.
2. Land the transmitter's positive loop terminal on SM 1231 terminal AI0+ (terminal 2 on the 4AI module).
3. Land SM 1231 terminal AI0- (terminal 3) on the 24 V DC common / M terminal at the supply.
4. Verify the transmitter's V+ / loop+ terminal is tied to the 24 V DC + of the supply.

5.2 Three-wire 4-20 mA transmitter

For a 3-wire transmitter, the supply negative is shared with the signal return. The SM 1231 input sees the 4-20 mA flowing from the signal output of the transmitter back to the same 24 V common.

        +24 V DC -----+----- to transmitter V+ (terminal 1)
                       |
                       |      +-------------+
                       |      | Transmitter |
                       |      |  (3-wire)   |
                       |      |             |
                       |      | V+  -- in   |
                       |      | GND -- com  |
                       |      | SIG -- out  |
                       |      +--+-----+----+
                       |         |     |
                       |         |     +----- signal wire to SM 1231 AI0+
                       |         |
                       |         +----- to 24 V common (M) and to SM 1231 AI0-
                       |
        M -------------+----- to transmitter GND (terminal 2)

Step-by-step wiring procedure:

1. Connect the 24 V supply positive to the transmitter's V+ (or PWR) terminal.
2. Connect the transmitter's GND terminal both to the 24 V supply M and to the SM 1231 AI0- terminal (3).
3. Connect the transmitter's signal output terminal to the SM 1231 AI0+ terminal (2).

Many 3-wire transmitters have the signal output internally referenced to GND; check the datasheet. If the signal output is a "sourcing" output (current flows out of the signal terminal and returns via GND), the wiring above is correct. If the signal output is "sinking" (current flows from GND into the signal terminal), the polarity of the SM 1231 input must be reversed.

5.3 Four-wire 4-20 mA transmitter

A 4-wire transmitter has fully isolated power and signal. The signal wires are a floating current source that drives 4-20 mA through the SM 1231 input. Polarity follows the 2-wire rule: the terminal carrying current INTO the SM 1231 goes to AI0+.

1. Power the transmitter from a separate 24 V supply (or the same supply, but with a dedicated fuse and short pigtail).
2. Connect the transmitter's SIG+ output to SM 1231 AI0+ (terminal 2).
3. Connect the transmitter's SIG- output to SM 1231 AI0- (terminal 3).
4. Do NOT bond the signal negative to the supply M at the transmitter end; the signal is already isolated internally.

6. Choosing the 24 V Loop Supply

The loop supply can be:

• The S7-1200 CPU's own sensor supply output (terminals L+ and 1M on the CPU). Convenient because it is monitored and faulted by the CPU, but it is limited in current (typically 400 mA for the CPU 1214C, 1600 mA for the CPU 1217C) and shares the same ground as the analog inputs.
• A dedicated SITOP power supply, e.g. 6EP1332-3BA10 (10 A) or 6EP1331-3BA10 (3 A). The preferred choice for noisy panels or long cable runs.
• The existing 24 V distribution already powering other field devices. Always check polarity and ground referencing.

Use a fuse (typically 1 A slow-blow) on the loop supply rail. Run a separate neutral/return wire from each transmitter to the supply negative; do not daisy-chain return wires through terminal blocks, as the cumulative voltage drop will add error.

7. TIA Portal Configuration

After the physical wiring is complete, configure the analog channel in TIA Portal. The configuration is done in the Device view, selecting the SM 1231, then the Properties tab, then expanding Analog inputs → channel 0.

The analog value is read from the process image input (PII) at the IW address assigned by the system, e.g. %IW64 for the first channel. The integer value is the raw ADC count. The scaling from raw to engineering units is performed in user code using the S7-1200 NORM_X and SCALE_X instructions, or with the legacy SCALE library block.

Raw-to-engineering conversion

For a 13-bit SM 1231 (6ES7231-4HD32-0XB0) the nominal range mapping for 4-20 mA is:

• 4 mA → 0
• 20 mA → 27648
• Underrange (< 1.185 mA) → negative values down to -32768 (overflow / wire break)
• Overrange (> 22.81 mA) → 32767

For a 16-bit SM 1231 (6ES7231-5ND32-0XB0) the range is 0 to 27648, with -32768 reserved for wire-break/underflow and 32767 for overflow. Confirm the exact mapping in the device documentation for the specific MLFB in use.

8. Handling Unused Channels

This is one of the most common commissioning oversights. The Siemens documentation for the SM 1231 wiring diagrams specifies two rules for unused channels:

1. Unused voltage input channels should be shorted. Tie the + and - terminals together at the terminal block. A floating voltage input can pick up induced noise and generate random ADC values or trigger overflow diagnostics.
2. Unused current input channels should be set to the 0 to 20 mA range and/or have broken-wire error reporting disabled. This prevents the channel from generating a permanent wire-break diagnostic alarm when no current is flowing.

Either rule may be applied; applying both provides the cleanest result. In TIA Portal, navigate to the unused channel's properties and uncheck "Wire break" under "Diagnostics" if the channel is intentionally left in 4-20 mA mode.

9. Verification and Commissioning Procedure

After wiring and configuration, perform the following checks in order before declaring the channel healthy:

1. Power check. With the loop powered and the transmitter connected, measure the voltage at the transmitter terminals. It must be at least 2 V above the transmitter's published minimum operating voltage (typically 12 V for a loop-powered pressure transmitter).
2. Current check. Clamp a multimeter (in mA mode) in series with the loop or use a current shunt. Confirm 4.00 mA at the low end of the process variable and 20.00 mA at the high end.
3. Voltage check at the SM 1231 terminals. With the loop active, measure DC voltage between AIx+ and AIx-. For a 4-20 mA input, this should be approximately 1.0 V to 5.0 V (proportional to the current × ~250 Ω burden). A reading of 0 V indicates the loop is open.
4. Raw value check in TIA Portal. Go online, open a watch table, and force a refresh of the input word (e.g. %IW64). At 4 mA the value should be 0 ± 5 counts; at 20 mA it should be 27648 ± 5 counts. Negative values or 32767 indicate a configuration or wiring fault.
5. Diagnostic buffer check. Open the CPU's diagnostic buffer (Online → Diagnostics → Diagnostic buffer) and confirm no "Analog input wire break" or "Analog input overflow" entries.
6. End-to-end calibration. Apply a known process input to the transmitter (e.g. a calibrated pressure source) and verify the engineering value in the HMI matches the expected scaled value within the instrument's accuracy spec.

10. Troubleshooting Matrix

11. Common Wiring Mistakes

The following errors appear repeatedly on commissioning sites:

1. Assuming the SM 1231 provides loop power. It does not. The 2-wire transmitter will not work without an external 24 V source.
2. Reversing polarity on the SM 1231 input. The module is reverse-polarity tolerant, but the ADC reads zero because the internal diode clamps the negative input.
3. Mixing voltage and current ranges on the same channel. Each SM 1231 channel must be configured for either voltage or current in TIA Portal. Mixing hardware jumpers and software range selection leads to out-of-range readings.
4. Leaving unused voltage channels floating. A floating voltage channel picks up noise and may latch overflow diagnostics, potentially blocking the diagnostic scan of the entire module.
5. Configuring unused 4-20 mA channels with wire-break enabled. This floods the diagnostic buffer with spurious wire-break events.
6. Sharing the 24 V supply return across multiple transmitters via a single daisy-chained wire. The accumulated IR drop and noise coupling cause cross-talk between channels.
7. Routing analog signal cables in the same conduit as VFD output cables or three-phase power. Capacitive coupling induces common-mode noise that the SM 1231's ADC rejects poorly without proper shielding.

12. Shielding, Grounding, and EMC Best Practices

For installations in electrically noisy environments, apply the following:

• Use shielded twisted pair cable, e.g. Belden 8761 or Lapp UNITRONIC LiYCY.
• Ground the shield at one end only, typically at the panel entry, to avoid ground loops.
• If the cable run exceeds 50 m, prefer the 16-bit SM 1231 (6ES7231-5ND32-0XB0) for better common-mode rejection.
• Separate analog signal cables from power cables by at least 200 mm. Cross at 90° if intersection is unavoidable.
• Bond the panel backplane to a clean ground bus, NOT to the building steel.

13. Example SCL Scale Block

The following Structured Control Language (SCL) snippet scales a raw SM 1231 input (channel 0, IW64) into a 0.0-100.0 % engineering range. Adjust the IW address and engineering range for the project.

// SCALE_X block: raw 0..27648 -> engineering 0.0..100.0
// Use NORM_X to convert the INT raw value to a REAL fraction 0.0..1.0,
// then SCALE_X to multiply by the engineering range.

"dbScale".rNormIn   := INT_TO_REAL("iwAI0");        // %IW64
"dbScale".rNormMin  := 0.0;
"dbScale".rNormMax  := 27648.0;
"dbScale".rNormOut  := "dbScale".rNormIn;          // re-used below

"dbScale".rScaleIn  := "dbScale".rNormOut;
"dbScale".rScaleMin := 0.0;
"dbScale".rScaleMax := 100.0;
"dbScale".rScaleOut := "dbScale".rScaleIn;        // result in %

// Optional: detect underrange/overrange before scaling
IF "iwAI0" < 0 THEN
    "dbScale".bWireBreak := TRUE;
ELSIF "iwAI0" > 27648 THEN
    "dbScale".bOverflow := TRUE;
ELSE
    "dbScale".bWireBreak := FALSE;
    "dbScale".bOverflow  := FALSE;
END_IF;

14. Extended Reference: Loop-Powering Topologies

Siemens support entry 40914972 consolidates wiring examples and answers common questions about 2-wire and 4-wire sensor connections to SIMATIC analog input modules. The same source confirms the principle that the SIMATIC analog input does not contain an internal 24 V supply and that the loop voltage must be provided externally, regardless of whether the field device is 2-wire, 3-wire, or 4-wire.

→ Siemens Support Entry 40914972 — Connecting 2-wire and 4-wire sensors to SIMATIC analog input modules

15. Frequently Asked Questions