Wiring ET 200SP AI 4xI 2-/4-wire ST (6ES7134-6GD01-0BA1) for 4-20 mA Field Sensors

Symptom: A 4-20 mA field device is wired to channel 0 of a SIMATIC ET 200SP AI 4xI 2-/4-wire ST module (order number 6ES7134-6GD01-0BA1) on an S7-1500 station, but TIA Portal shows 0 mA at the input. The DIAG LED is off, no diagnostic bit is set, and the raw input word (IW) is stuck at zero. After re-reading the manual section 3 (connection diagrams), the engineer cannot determine whether to use terminals 1-5 (4-wire) or 9-13 (2-wire), and the loop still reads 0 mA after re-wiring.

This is the most common commissioning defect on the AI 4xI 2-/4-wire ST. The root cause is almost always a mismatch between the physical sensor type (active vs passive) and the channel parameter "Measuring type" in TIA Portal, compounded by using the wrong BaseUnit underneath the module. The fix is a three-step alignment: identify the sensor, wire the correct terminal pair, and set the matching channel mode in TIA Portal.

1. Module, Order Numbers, and BaseUnit Identification

Record the full order numbers of every part in the slot stack before doing any wiring. The terminal layout that drives the 2-wire / 4-wire decision is determined by the BaseUnit, not by the AI module itself.

Key rule: The 24 V transducer supply (Uv) needed for 2-wire sensors is only routed to the field-side terminals on a BaseUnit type A1. If the module is plugged into a type A0 BaseUnit, no 2-wire sensor can ever be energized and the channel will read 0 mA regardless of TIA Portal configuration.

2. Root Cause: Active 4-Wire vs Passive 2-Wire Sensor Mismatch

The 4-20 mA standard is electrically ambiguous: it does not specify which side of the loop supplies the energy. The Siemens AI 4xI 2-/4-wire ST supports both cases, but the field device and the channel parameter must agree.

2.1 Active (4-wire) transducer

An active transducer contains its own 24 V supply or is fed by a separate power circuit. It pushes the 4-20 mA signal into the loop. The AI module only needs to sink the current: a low-impedance path from the sensor's positive output to the module's M+ input, with M- as the return reference. No power flows from the module to the sensor.

Typical examples: line-powered process transmitters, panel-mount indicators with isolated 4-20 mA output, frequency inverters with analog current output (for example a SINAMICS V20 current output stage), and laboratory calibrators.

2.2 Passive (2-wire) transducer

A passive transducer has only two leads and draws its operating current from the loop. The AI module must act as both the 24 V source (on Uv) and the current sink (on M+). The sensor is energized through the loop itself.

Typical examples: 2-wire pressure transmitters, 2-wire temperature transmitters (head-mount RTD/TC), 2-wire level and flow probes, and most loop-powered field instruments that list only a 4-20 mA output with no auxiliary power input.

3. Field Test: Identify Active vs Passive in 30 Seconds

1. Disconnect the field device from the ET 200SP BaseUnit.
2. Set a calibrated digital multimeter to mA DC.
3. Connect the meter in series across the two field leads.
4. Power the field device per its datasheet (mains, loop, battery, etc.).
5. Read the meter.
6. If the meter reads 4-20 mA without any external 24 V source: active.
7. If the meter reads 0 mA or open loop: passive. The device needs the 24 V from the AI module.

4. Channel-to-Pin Map for 6ES7134-6GD01-0BA1

Each channel occupies two BaseUnit terminals. The exact terminal numbers depend on whether the channel is wired for 2-wire (passive) or 4-wire (active) operation. The table below is the published channel-to-pin map for the AI 4xI 2-/4-wire ST. Channel 0 is shown first; channels 1, 2, 3 follow in order.

Signal name key:

• M+: positive analog input (current enters the module on this terminal)
• M-: negative analog input (current returns on this terminal; analog ground reference)
• Uv: 24 V transducer supply output (only present on BU type A1 and higher; powers a 2-wire passive loop)

For a passive 2-wire device on channel 0, the field wires land on BaseUnit terminals 9 (Uv) and 13 (M+). For an active 4-wire device on channel 0, the field wires land on BaseUnit terminals 1 (M+) and 5 (M-).

5. Decision Flowchart

6. Wiring Topology

7. TIA Portal Channel Configuration

The hardware wiring decision must be mirrored exactly in the device configuration. A mismatch between the physical loop and the configured measuring mode is the single most common reason an otherwise correctly wired channel reads 0 mA.

1. In the TIA Portal project tree, open Devices & Networks and select the ET 200SP station.
2. Click the AI 4xI 2-/4-wire ST module slot.
3. Open Properties → General → Module parameters → Inputs.
4. For each channel, expand the channel and set Measuring type to one of: Current (4-wire transducer), Current (2-wire transducer), Voltage, or disabled. Pick the option that matches the BaseUnit wiring.
5. Set Measurement range to 4-20 mA (other valid ranges: 0-20 mA, ±20 mA).
6. For a 2-wire passive sensor, verify the Supply voltage 24 V parameter is enabled (default for 2-wire transducer mode).
7. Enable Wire break and Overflow / Underflow diagnostics to catch future wiring faults.
8. Set Interference frequency suppression to the local mains (50 Hz in EU, 60 Hz in US).
9. Compile the project and download to the S7-1500 CPU.

8. Module Parameter Reference

9. SCL Scaling Example for S7-1500

Once the raw input word is available in the process image (for example IW 100 for channel 0 of the AI module at slot 4), scale it to engineering units with an SCL function block. For a 4-20 mA range, raw 0 corresponds to 4 mA and raw 27648 corresponds to 20 mA.

// FB_Scale_4_20mA
// Input  : iRaw   : INT    -- raw 16-bit value from analog input (0..27648)
// Output : rEng   : REAL   -- engineering value (e.g. 0.0..100.0 %)
//           rMilli : REAL   -- current in mA (4.0..20.0)
//           bOK    : BOOL   -- quality flag

IF iRaw > 32511 AND iRaw < 32767 THEN
    // Overflow
    rMilli := 20.0;
    rEng   := 100.0;
    bOK    := FALSE;
ELSIF iRaw < 0 THEN
    // Underflow / wire break on 4-20 mA range
    rMilli := 4.0;
    rEng   := 0.0;
    bOK    := FALSE;
ELSE
    rMilli := (INT_TO_REAL(iRaw) / 27648.0) * 16.0 + 4.0;
    rEng   := (INT_TO_REAL(iRaw) / 27648.0) * 100.0;
    bOK    := TRUE;
END_IF;

For 0-20 mA inputs, replace the mA formula with rMilli := INT_TO_REAL(iRaw) / 27648.0 * 20.0;. For ±20 mA inputs, use rMilli := INT_TO_REAL(iRaw) / 27648.0 * 40.0 - 20.0;.

10. Verification Procedure

1. Go online with the S7-1500 CPU in TIA Portal.
2. Open the AI 4xI module → Monitoring → I/O → Inputs.
3. Confirm the raw value (IW) sits in the expected range. For 4-20 mA with the sensor at 4 mA, expect a raw value close to 0. For 12 mA, expect 13824. For 20 mA, expect 27648.
4. Apply a known current from a calibrator (12.000 mA is a good midpoint) and verify the raw value is within ±0.3% of full scale (about ±80 LSB at 25 °C).
5. Disconnect the positive field wire. If wire-break diagnostics are enabled, the channel should report a wire-break diagnostic and the value should underflow to 0 (4-20 mA range).
6. Check the module DIAG LED; it should remain off when the loop is healthy.
7. Record the raw value at 4 mA, 12 mA, and 20 mA for the commissioning log.

Normalization formulas for an S7-1500 AI module:

Process value (mA) = (raw value - 0) / 27648 * (range max - range min) + range min

For a 4-20 mA range, this reduces to mA = (raw / 27648) * 16 + 4. For a 0-20 mA range, mA = (raw / 27648) * 20. For ±20 mA, mA = (raw / 27648) * 40 - 20.

11. Troubleshooting Matrix

12. Channel-to-Terminal Pitfall

The most common cross-wiring mistake is to assign channel 0 to terminals 2 and 6 (the channel 1 row of the pin map). The symptom is that a sensor physically wired to terminals 1 and 5 appears as channel 1 in the PLC, with a stale or zero value on channel 0. Always confirm that the channel number TIA Portal reports in the device view matches the printed terminal numbers on the BaseUnit. A printed terminal number "9" on the BaseUnit is hard-wired to Uv of channel 0 on this module; it is not channel-selectable. The same applies to terminal pairs 1-5, 2-6, 3-7, 4-8 for the 4-wire mode and 9-13, 10-14, 11-15, 12-16 for the 2-wire mode.

13. Mixing 2-Wire and 4-Wire Sensors on One Module

Channel independence allows combining 2-wire passive loops and 4-wire active loops on the same AI 4xI 2-/4-wire ST module. The requirements are:

• The BaseUnit must bring out Uv terminals (BU type A1 or higher).
• Each channel's measuring type in TIA Portal must match the physical wiring of that channel.
• The supply voltage group on the BaseUnit must be powered from the same 24 V DC system feeding the rest of the ET 200SP station.

If the customer is migrating from all-4-wire to a mix that includes 2-wire, a BaseUnit swap is the most common retrofit step. Confirm the BU type before ordering sensors.

14. Connecting a VFD (e.g. SINAMICS V20) Analog Output

A frequency inverter such as the SINAMICS V20 with a 4-20 mA analog output is an active source. It contains its own 24 V supply and drives the loop itself. The correct configuration on the ET 200SP is the 4-wire transducer selection with M+ on terminal 1 (channel 0) and M- on terminal 5. Selecting 2-wire on the module will read 0 mA because the module's 24 V Uv rail will not be connected to the V20's powered output, and the channel may report wire-break if diagnostics are enabled. The V20 analog output common (terminal 12 on the V20 control board) goes to the ET 200SP M- terminal; the V20 analog output (terminal 13 on the V20 control board) goes to the ET 200SP M+ terminal.

15. Shielding, Grounding, and EMC

Analog signal cables should be shielded and grounded at one end only to avoid ground loops. The recommended practice on ET 200SP is to use the shield clamp (6ES7193-6SC00-1AM0) on the BaseUnit and to ground the cable shield at the cabinet entry plate. Run analog signal cables in a separate tray, at least 200 mm away from VFD power cables and switching contactors. The module's M- terminal is internally connected to the backplane ground through the BaseUnit; do not bond M- to ground at the field device as well, as this creates a parallel ground path that injects noise into the measurement.

16. Safety Considerations

The 24 V Uv supply on the AI 4xI 2-/4-wire ST is not isolated from the station 24 V power bus. It shares the same reference ground as the rest of the ET 200SP station. Loop-powered field devices that expect a galvanically isolated supply must use an external isolator. For installations in hazardous areas, confirm that the AI module and BaseUnit are approved for the zone (see the module manual for ATEX/IECEx ratings) and that the field device is loop-powered within the entity parameters. Always de-energize the station 24 V supply before inserting or removing the AI module on a live BaseUnit.

17. Related Documentation

SIMATIC ET 200SP analog input module AI 4xI 2-/4-wire ST - System Manual (Siemens, 59768161)

SIMATIC ET 200SP Manual Collection - Analog Input Modules (TIA Portal cloud docs)

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