Wiring Siemens 6ES7331-7KF02-0AB0 SM331 for 2-Wire 4-20mA Inputs

The SIMATIC S7-300 SM331 analog input module with order number 6ES7331-7KF02-0AB0 (AI8x12Bit) is one of the most widely deployed AI modules in S7-300 racks. Its flexibility comes from four plug-in measuring range modules on the side of the housing, one per channel group, that mechanically set the input type (voltage, current, RTD, or thermocouple) and the wiring topology (2-wire vs. 4-wire). Misconfiguring those switches is the single most common cause of an out-of-range, pegged, or dead channel when a field device is connected.

This article documents the correct wiring of a 2-wire, loop-powered 4-20 mA pressure transmitter to the SM331, including the rules that govern when you must use position C (4-wire) instead of position D (2-wire) and when an external signal isolator becomes mandatory.

1. Module Identification and Channel Topology

The 6ES7331-7KF02-0AB0 is the 8-channel, 12-bit resolution variant of the SM331 family. Its eight inputs are arranged in four channel groups of two channels each:

Each measuring range module is a small PCB inserted into a guide slot on the left side of the module. It is keyed to four positions (A, B, C, D), and the same physical position selects both the measurement type and the wiring topology for both channels in that group. The setting is read by the module's microcontroller and the value is also reported to the CPU in the diagnostic buffer if it disagrees with the STEP 7 hardware configuration.

2. Measuring Range Module Positions – Detailed Function

The mechanical position of the measuring range module is the source of truth at runtime. The setting you enter in STEP 7 / TIA Portal under "Measuring range" must match the physical switch, or the module will return a wire-break or overrange diagnostic and the analog value will read 7FFF₁₆ (overflow) or 8000₁₆ (wire break in 4-20 mA mode).

Position D is the only setting that converts the SM331 into an active 4-20 mA current source and sinks the loop through its internal sense resistor. Position C is a strictly passive current sink at the input terminals; it does not energize the loop.

3. 2-Wire vs 4-Wire Transmitter Fundamentals

A 2-wire (loop-powered) transmitter uses the same pair of wires that carry the 4-20 mA signal to also receive its operating power. The current drawn by the transmitter is the measurement variable. A typical 2-wire pressure transmitter has only two terminals: + and -, and the loop is completed through the AI module's internal 24 V supply and sense resistor.

A 4-wire (externally powered) transmitter has separate pairs for power supply and signal output. It produces a regulated 4-20 mA output sourced by an internal amplifier; the AI module only needs to provide a low-impedance return path for the current. The transmitter is electrically independent of the analog input.

4. The Compatibility Rule – Why the Position Matters

The fundamental rule when interconnecting a transmitter and an AI module is that two active current sources cannot be placed in the same series loop. Doing so results in contention, out-of-range readings, possible damage to one or both devices, and is detected by the SM331 as a wire-break or overflow.

In short: a 2-wire transmitter can be connected to a 4-wire AI input, but a 4-wire transmitter cannot be connected to a 2-wire AI input without an isolator. This is the central rule that determines whether to move the measuring range module from position D to position C.

5. Prerequisites for Wiring a 2-Wire 4-20 mA Pressure Transmitter

1. Confirm the transmitter type. Inspect the data sheet: 2-wire pressure switches and pressure transmitters list a single pair of signal terminals and a supply voltage of typically 12-30 V DC. If the device also lists dedicated power terminals (often labeled P+ / P- or V+ / V-), it is a 4-wire device.
2. Verify the available conductors in the field cable. The user in the source case had only one free core carrying 4-20 mA. This is consistent with a 2-wire transmitter whose loop is already closed by a 24 V source elsewhere in the circuit (e.g., a barrier or another AI module). For a direct connection to the SM331, you need exactly two cores plus a shield drain.
3. Set the measuring range module to position D for the channel group that will accept the 2-wire transmitter. Slot D corresponds to channels 6-7 on the 6ES7331-7KF02-0AB0.
4. Verify the load voltage (L+) on the backplane. The SM331 draws loop power for 2-wire mode from the S7-300 backplane load voltage (terminals L+ / M on the power supply module, typically 24 V DC). Without this voltage, 2-wire transmitters read 0 mA.
5. Configure STEP 7 / TIA Portal accordingly. In HW Config set the channel to "4-wire current" measurement type only if the measuring range module is in position C. If the module is in position D, set the channel to "2-wire current". A mismatch generates a parameter assignment error on the CPU diagnostic buffer.

6. Step-by-Step: Connecting a 2-Wire Transmitter (Position D)

1. De-energize the S7-300 rack and remove the 20-pin front connector from the SM331. Note the connector keying (color code 1-20) to maintain polarity convention.
2. Reposition the measuring range module for the appropriate channel group. The slot is on the left side of the module, accessible after the door is opened. Slide the small PCB out, rotate so the arrow points to "D", and re-insert until it clicks.
3. Wire the field cable to the front connector terminals. On the 6ES7331-7KF02-0AB0, each channel group uses two terminal pairs. For channels 6-7 (group 3, position D):
   • Terminal 1 = M+ of CH0 (group 0) — not used here
   • Terminal 2 = M- of CH0 — not used here
   • ...continuing the standard pinout...
   • For a 2-wire transmitter connected to CH6, route the + wire of the transmitter to terminal 15 (CH6+ in position D) and the - wire to terminal 16 (CH6- / Mana).
4. Connect the shield drain to the shield bar of the S7-300 subrack on at least one end, ideally both ends through a 1 nF / 1 MΩ parallel network for HF grounding while maintaining DC isolation.
5. Reinstall the front connector and torque the mounting screws.
6. Restore power and observe the channel in STEP 7 online: open the AI module's variable table and read PIW. A healthy 2-wire loop with the transmitter at 0% pressure should read approximately 4 mA, equivalent to a STEP 7 raw value of 0. At 100% pressure, the raw value should reach 27648 (0-20 mA scaled range) or 27648 (4-20 mA scaled range, with 0 mapped to 4 mA).

7. Step-by-Step: Using Position C (4-Wire Mode) Universally

If the panel mix includes both 2-wire and 4-wire transmitters and the channel groups cannot be re-keyed to match, the safer practice is to set all four measuring range modules to position C and provide loop power externally. In position C the SM331 input is a passive current sink (~250 Ω to internal ground), so it accepts the output of any 2-wire or 4-wire transmitter as long as a 24 V loop supply is wired in series.

1. Set the measuring range module to position C for the affected group.
2. Set the STEP 7 channel to measurement type 4-wire current, range 4-20 mA.
3. Wire the loop: 24 V DC + → transmitter + → transmitter output + → SM331 channel M+ (terminal 13 for CH6, group 3) → SM331 channel Mana (terminal 14 for CH6) → 24 V DC - supply return.
4. For a 2-wire transmitter, omit the separate power terminal pair and simply connect the two transmitter wires in series with the 24 V source and the SM331 input.

Position C is the recommended "safe" default because the module is purely passive at the input and will not conflict with any field device, regardless of whether the field side is active (4-wire) or passive (2-wire). The cost is one external 24 V supply or DC-DC isolator per loop.

8. When an Isolator is Mandatory

An isolator (also called a signal conditioner, loop isolator, or current repeater) is required in two cases:

1. The transmitter is 4-wire (active output) and the SM331 group is keyed to position D. Two active sources would otherwise be in series. Insert a passive-input / passive-output isolator (e.g., Phoenix Contact MACX MCR-EX-SL, Wago 857-402, or Weidmüller ACT20X) in series. The isolator accepts the active 4-20 mA from the transmitter and produces a passive 4-20 mA output that the SM331 in position D can power and read.
2. Galvanic isolation is required between the field and the S7-300 ground. A signal isolator breaks ground loops, protects against transients, and is often required in hazardous-area installations where the SM331 sits on the safe side of an intrinsic-safety barrier.

9. Wiring Diagrams

9.1 2-Wire Transmitter to SM331 Position D (Module Supplies Loop)

9.2 4-Wire Transmitter to SM331 Position C (External Loop Power)

9.3 4-Wire Transmitter to SM331 Position D via Passive Isolator

9.4 Measuring Range Module – Position Selector Detail

10. Commissioning and Verification Procedure

1. Visual inspection. Confirm the front connector is keyed, screwed down, and the shield drain is bonded to the S7-300 shield bar.
2. Check the load voltage. Use a multimeter at the SM331 L+ / M terminals. Expect 24 V DC ±5%.
3. Apply a known current. Disconnect the field cable and inject a calibrated 4.000 mA, 12.000 mA, and 20.000 mA from a portable loop calibrator in series with the SM331 input. Verify the STEP 7 raw value reads 0, 13824, and 27648 (with the 4-20 mA range selected, normalized to 0-27648). Tolerance ±0.5% of full scale (138 LSB).
4. Reconnect the field device and verify the live value tracks the process variable (e.g., pressurize the sensor with a hand pump and watch the PIW count increase monotonically).
5. Force a wire-break test. Disconnect one wire at the field device. The SM331 should set the channel diagnostic bit (diagnostic interrupt enabled in STEP 7) and the raw value should fall to 0x8000 = -32768 in 4-20 mA mode. Reconnect and clear the diagnostic.

11. Troubleshooting Matrix

12. Loop Voltage and Cable-Length Budget

For long cable runs, the loop voltage at the transmitter terminals is:

V_terminal = V_supply - (I_loop × R_loop_total) - V_isolator_drop

where R_loop_total = 2 × R_cable (out and return) and V_isolator_drop is the burden of any in-line isolator. A typical 2-wire pressure transmitter needs at least 12 V (some models need 14 V) at its terminals to maintain 4 mA output. The SM331 in position D sources 24 V minus the internal drop, so the practical cable length is limited by:

L_max (m) = (V_supply - V_min_TX - V_isolator) / (2 × I_max × R_per_km)

For a 24 V supply, 14 V minimum at transmitter, 0 V isolator, 22 mA worst case, and a 0.5 mm² copper cable at 36 Ω/km: L_max = (24 - 14) / (2 × 0.022 × 36) ≈ 63 m. With 1.0 mm² cable (19 Ω/km), this rises to ~120 m. For longer runs, switch to position C with a local 24 V transmitter supply at the field end.

13. Documentation Trail and Reference Material

• Siemens Support entry 8859629 – 6ES7331-7KF02-0AB0 manual (S7-300 / S7-300 analog modules) – primary reference for wiring diagrams, terminal assignments, and input impedance vs. MRM position.
• Siemens Support entry 8859629 (localized) – equivalent entry used to cross-check regional manual revisions.

14. Frequently Asked Questions