Fixing ET 200SP 6ES7134-6HD00 AI Module Reading Its Own Sensor Supply Current
Problem Summary
When interfacing a FAGET EM223 intelligent current transformer (a 4-wire measurement device that derives its operating power from the primary current being measured) to a Siemens SIMATIC ET 200SP analog input module 6ES7134-6HD00-0BA1, the input value remains elevated even when the measured primary current is zero. Typical readings are ~2 mA on the channel, despite the sensor transmitting 0 mA on its analog output.
The phenomenon is reproducible: the moment the sensor is physically connected to the AI module terminals Uv0 and I0+, the raw input value rises by a fixed offset that is not present with the sensor disconnected. This offset corresponds to the module's own short-circuit-proof sensor supply current flowing through the input stage.
Root Cause Analysis
The ET 200SP AI module 6ES7134-6HD00-0BA1 is a 2-wire measurement module designed for passive (loop-powered) transducers. Each channel provides a regulated 24 V sensor supply (Uv) on terminal Uv0 (or Uv1) that delivers power to the connected sensor over the same two wires used to measure the 4-20 mA / 0-20 mA current signal.
On a 2-wire module, the input current path is:
- Sensor supply
Uv0sources +24 V through a current-limiting circuit. - Current flows out of
Uv0, through the sensor's internal shunt / electronics, and returns onI0+. - The shunt inside the AI module measures that return current as the input value.
If the sensor does not need this supply (because it is internally powered, as is the case with the EM223 which harvests energy from the primary current of the CT), the supply current from Uv0 still has somewhere to flow. Inside a 4-wire passive-style sensor, the supply current takes an unintended path through the output stage and is interpreted by the AI module as a real measurement signal. The result is an apparent "zero offset" of roughly 2 mA.
Uv0 / Uv1 outputs are hard-wired and feed the input stage shunt. This is by design, not a parameter error.Affected Hardware Identification
Module 6ES7134-6HD00-0BA1 (problematic in this scenario)
- SIMATIC ET 200SP, AI 2×U/I 2-wire Standard
- Resolution: 16 bits incl. sign
- Measuring ranges (per channel): ±10 V, 0-10 V, ±5 V, 0-5 V, 1-5 V, ±20 mA, 0-20 mA, 4-20 mA
- Sensor supply: 24 V DC, short-circuit-proof, max. per channel
- Connection type: 2-wire only
- Integration time / interference frequency: configurable (60 Hz / 50 Hz / 16.6 Hz)
Module 6ES7134-6GD00-0BA1 (correct module for 4-wire sensors)
- SIMATIC ET 200SP, AI 2×I 2/4-wire Standard
- Resolution: 16 bits
- Measuring ranges: 4-20 mA, ±20 mA, 0-20 mA
- Connection type: 2-wire or 4-wire selectable per channel
- Sensor supply: separately fed from the load voltage (no internal loop-power dependency on the AI input)
Reference the official ET 200SP analog input module manual for the full pin-out and configuration table:
SIMATIC ET 200SP Analog Input Module Manual (84133942)
Diagnostic Procedure
Before replacing the module, confirm the failure mode with the following field-proven steps.
Step 1 - Verify the AI channel raw value
Open TIA Portal → Online → Monitoring & Forcing. Read the raw integer (0-27648 for 0-20 mA range) on the affected channel. With the sensor powered (primary current present, even at 0 A), record the value.
Step 2 - Disconnect the sensor output wires
Pull the wire on I0+ only (leave Uv0 connected to nothing). Observe the raw value:
- If the value drops to zero or to overflow (e.g., 32767), the module is correctly "looking" for a current loop. The 2 mA reading when the sensor is connected is real loop current.
- If the value remains at 2 mA with the sensor completely removed, the module is feeding current back into the input through its own supply.
Step 3 - Confirm sensor type
Read the FAGET EM223 datasheet and identify whether the device is:
-
4-wire (active): has dedicated
+Uband-Ubpower terminals, plus separate+/-analog output terminals. Internally powered from primary current. - 2-wire (passive, loop-powered): has only two terminals that carry both supply and signal.
If the EM223 is a 4-wire device, the 2-wire module is electrically incompatible. Confirm by checking that the EM223 output does not share common with its power terminals internally.
Solution: Migrate to a 4-Wire Capable AI Module
The correct fix is to swap the AI module from 6ES7134-6HD00-0BA1 (2-wire) to 6ES7134-6GD00-0BA1 (2/4-wire). The 6GD00 module does not feed a sensor supply onto its analog input lines; instead, the sensor must be powered from an external 24 V DC source on its dedicated +Ub / -Ub terminals.
Wiring Topology
Notice that the analog signal wires I+ and I- from the EM223 connect directly to the AI module's AI0+ / AI0- terminals with no shared return with the 24 V supply. Power and signal are physically separated.
Replacement Procedure
- Open TIA Portal project. Note the existing device configuration (slot, baseUnit type).
- Right-click the AI module in the device view → "Change device" → search
6ES7134-6GD00-0BA1. - Confirm the BaseUnit compatibility. The 6GD00 typically requires a Type A0 (light) or Type A1 base unit with separate feed-in for the load voltage. Verify against the ET 200SP BaseUnit manual.
- Re-run "Compile → Hardware detection" to refresh the topology in the project.
- Wire the EM223 to the new module per the 4-wire topology shown above.
- Re-download the project to the IM / CPU.
- Verify input value in online monitoring.
Configuration in TIA Portal for the Replacement Module
Properties of the AI channel
- Measurement type: Current (4-wire)
- Measuring range: 4 to 20 mA (or 0 to 20 mA, depending on EM223 output)
-
Integration time: match to interference frequency of the installation (e.g.,
20 msfor 50 Hz mains) - Smoothing: enabled if the CT output has ripple; select medium or weak setting initially
- Diagnostics: enable "Wire break" if using 4-20 mA; enable "Overflow / Underflow"
Hardware identifier (HW ID) impact
Swapping the AI module changes the hardware identifiers in the PLC tag table. If the original program references the AI input as %IW64, after replacement it will be remapped. Re-link any I/O tags or update symbolic addressing.
Alternative Solutions (If Module Swap Is Not Possible)
Option A - Insert an external isolator
Install a 4-wire signal isolator/repeater (e.g., Phoenix Contact MINI MCR, WAGO 857, or similar) between the EM223 and the existing 6HD00 module. The isolator's input side is fed by the EM223's 24 V and ground; its output side drives a passive current loop that the 2-wire AI module can read. This adds cost and one extra device but keeps the existing AI slot.
Option B - Verify EM223 model variant
Some "intelligent CT" models offer a switch-selectable 2-wire mode. If the FAGET EM223 has a configuration for 2-wire output (passive loop), it can be wired to the 2-wire module. Check the EM223 datasheet and order code suffix; confirm whether the output is loop-powered (2-wire) or source-powered (4-wire) by default.
Option C - Use a different AI channel range
Configure the existing 6HD00 to ±20 mA range with a software subtraction of the offset (approximately 2 mA). This does not fix the underlying measurement error — the offset can drift with temperature and load voltage — and is not recommended for closed-loop control. Use only as a temporary diagnostic workaround.
Verification
After the module swap and reconfiguration, run the following verification checks:
-
Zero check: With the motor de-energized (0 A primary), the AI raw value should read
0for 0-20 mA range, or0for 4-20 mA range scaled to 4 mA. Tolerate ±2 LSB. - Span check: Inject a known primary current (or use a calibrator on the EM223 secondary output) and confirm the AI value matches the expected 0-20 mA / 4-20 mA proportional output.
- Diagnostics: Force a wire break (disconnect I+) and confirm the configured "Wire break" diagnostic fires within one conversion cycle.
- Load voltage stability: Vary the 24 V supply to the EM223 within its spec (typically 20-28 V). Confirm the AI reading does not drift. This proves the measurement is now decoupled from the sensor supply.
- Diagnostic buffer: In TIA Portal online → Diagnostics, verify no module faults are reported and the channel status word indicates "OK".
ET 200SP AI Module Comparison Table
| Article number | Description | Inputs | Resolution | Connection | Sensor supply | Use with EM223? |
|---|---|---|---|---|---|---|
| 6ES7134-6HD00-0BA1 | AI 2xU/I 2-wire Standard | 2 (U or I) | 16 bit | 2-wire only | 24 V internal, on Uv0/Uv1 | No — creates offset |
| 6ES7134-6GD00-0BA1 | AI 2xI 2/4-wire Standard | 2 (I only) | 16 bit | 2-wire or 4-wire selectable | External 24 V required | Yes — recommended |
| 6ES7134-6HB00-0CA1 | AI 2xU/I 2-wire High Feature | 2 (U or I) | 16 bit, oversampling | 2-wire only | 24 V internal | No — same offset behavior |
| 6ES7134-6JD00-0CA1 | AI 4xU/I 2-wire Standard | 4 (U or I) | 16 bit | 2-wire only | 24 V internal | No — same offset behavior |
Always cross-check the latest MLFB / firmware compatibility in the TIA Portal hardware catalog before ordering. Reference the official ET 200SP system manual:
SIMATIC ET 200SP System Manual
Troubleshooting Matrix
| Symptom | Probable Cause | Verification | Fix |
|---|---|---|---|
| AI reads ~2 mA with sensor output at 0 mA | 2-wire module Uv supply feeding into 4-wire sensor input stage | Disconnect sensor I+, observe if reading drops | Swap to 6GD00 module; use external 24 V for sensor |
| AI reads negative small value at zero current | Polarity reversed on I+/I- | Swap wires, retest | Correct polarity per module manual |
| AI reads 0 mA but motor current is flowing | Sensor not powered; CT open secondary | Measure EM223 +Ub terminal voltage | Verify load voltage is present and within spec |
| AI reads fluctuating / noisy value | Interference, incorrect integration time | Check motor VFD switching frequency vs. integration time | Increase integration time, enable smoothing |
| AI diagnostic "Wire break" immediately on power-up | Sensor not connected; cable break; wrong BU type | Loop check with mA source | Restore wiring; verify BU matches module |
| AI overflow (32767) with sensor connected | Wrong range configured (e.g., 4-20 mA but sensor outputs 0-20 mA at full scale) | Compare EM223 full-scale output to configured range | Switch range to 0-20 mA |
| AI reading drifts when 24 V PSU load changes | Sensor supply current coupling (Uv varies with input voltage) | Vary 24 V supply within spec, observe drift | Use 4-wire module to decouple signal from supply |
Field Commissioning Notes
- Always check the EM223 datasheet before wiring. Some intelligent CTs share the same MLFB but differ in output type (active 4-wire vs. passive 2-wire). Confirm by measuring: with sensor power removed, a 4-wire device will continue to output on its signal terminals (it harvests from primary); a 2-wire device will output zero.
- If the EM223 is mounted on the motor supply line, observe CT polarity: P1 must face the source, P2 toward the load for the documented phase convention.
- The secondary output of the EM223 is typically rated for a defined burden (e.g., 100 Ω). Do not exceed it. The ET 200SP AI input impedance is < 100 Ω in current mode, so the burden is usually within spec, but verify on long cable runs.
- Shield the analog signal cable and ground at one end only (typically the cabinet end) to avoid ground loops. The ET 200SP base units provide dedicated shield terminals for this purpose.
- Route the analog cable physically separated from VFD output power cables (minimum 200 mm parallel, crossings at 90°).
FAQ
Why does my Siemens ET 200SP 6ES7134-6HD00 show ~2 mA when the FAGET EM223 outputs 0 mA?
The 6HD00 is a 2-wire AI module that sources a 24 V sensor supply on terminals Uv0/Uv1. That supply current flows through the 4-wire EM223 input stage even when its signal output is zero, producing a false ~2 mA reading. The fix is to use the 4-wire-capable module 6ES7134-6GD00-0BA1 and power the EM223 from an external 24 V source.
Can I disable the sensor supply on the 6ES7134-6HD00 module?
No. The Uv outputs on the 2-wire AI module are hard-wired and cannot be turned off in TIA Portal. The 24 V short-circuit-proof supply is part of the 2-wire measurement topology and is always present when the channel is configured for current.
How do I tell if my FAGET EM223 is a 2-wire or 4-wire device?
Check the EM223 datasheet for terminal labels. A 4-wire device has four terminals: +Ub, -Ub (power) and I+, I- (analog output). A 2-wire (loop-powered) device has only two terminals that carry both supply and signal. The 4-wire variant harvests operating power from the primary current and does not need the module's Uv supply.
Will the 6ES7134-6GD00 fit in the same BaseUnit as the 6HD00?
Not always. The 4-wire AI module typically requires a base unit type that brings in a separate load voltage feed for the external sensor supply. Verify the BU part number in the ET 200SP system manual and order the correct BU before the physical swap to avoid a configuration mismatch.
Can I subtract the 2 mA offset in the PLC program as a workaround?
Only as a temporary measure. The offset varies with the ET 200SP power supply voltage (20.4-28.8 V), so a software zero-shift will drift as the cabinet supply changes. For closed-loop or billing-grade current measurement, the 4-wire module swap is the correct and only reliable solution.