Configuring Analog Input Sensor Fault Detection: 4-20 mA NAMUR NE 43 vs 1-5 V Voltage Signals

In SIMATIC PCS 7, the channel driver block Pcs7AnIn cannot evaluate NAMUR NE 43 sensor fault limits (wire-break below 3.6 mA or short-circuit above 21.0 mA) when the analog input module is configured for 1..5 V voltage signals. The 1..5 V measuring range uses standard asymmetric voltage ADC thresholds, where underflow/overflow occurs only near 0.75 V (approx. 3.0 mA) and 5.75 V (approx. 23.0 mA). To achieve certified NAMUR NE 43 diagnostic trip limits, field transmitters must be connected directly as 4..20 mA current inputs, or explicit software limit comparators must be engineered in CFC.

Principles of Sensor Fault Detection and the NAMUR NE 43 Standard

In process automation systems adhering to IEC 61511 and NAMUR guidelines, analog process instrumentation must provide deterministic self-diagnostics to distinguish between a valid process measurement and a hardware failure (such as a severed field cable or short-circuited transmitter):

The NAMUR NE 43 Recommendation:
Defines standardized current ranges for analog 4..20 mA transmitters:
Measurement Range (Linear Scaling): 3.8 mA <= I <= 20.5 mA
Low Fault (Wire Break / Defective Sensor): I <= 3.6 mA
High Fault (Short Circuit / Sensor Saturation): I >= 21.0 mA
Channel Driver Processing (Pcs7AnIn):
In SIMATIC PCS 7, the Advanced Process Library (APL) analog channel driver block Pcs7AnIn continuously evaluates the hardware diagnostic word (MODE, QUALITY). When a valid 4..20 mA channel detects current dropping below 3.6 mA, the driver immediately asserts Bad = TRUE and sets Quality Code 0x00 (Bad), triggering channel passivation and fail-safe safety interlocks.

Hardware ADC Architecture: Current Loops (4..20 mA) vs Voltage (1..5 V)

In many legacy installations, external signal conditioning barriers, isolators, or legacy transceivers convert standard 4..20 mA field loops into 1..5 V voltage signals before wiring into S7 analog input modules (such as SM 431 6ES7431-1KF00-0AB0 or SM 331 6ES7331-7KF02-0AB0):

The Engineering Intent:
A 250-ohm precision shunt resistor ($V = I \times R$) converts:
$4.0\text{ mA} \times 250\,\Omega = 1.0\text{ V}$
$20.0\text{ mA} \times 250\,\Omega = 5.0\text{ V}$
The Diagnostic Blind Spot:
While the voltage scaling accurately maps the linear span (1 V to 5 V = 0% to 100%), the physical analog-to-digital converter (ADC) on the module is configured for a voltage measuring range (1 to 5 V), completely decoupling the module firmware from current-loop monitoring rules.

Why Pcs7AnIn Channel Drivers Cannot Enforce NAMUR Limits on 1..5 V Signals

Engineers testing 1..5 V systems on a calibration test bench observe a critical failure:
* When test current is reduced below 3.6 mA (e.g., 3.4 mA = 0.85 V), the system asserts no sensor fault (SF).
* The system only detects a fault when test current drops nearly to 1.0 mA (0.25 V).
* Similarly, for high faults, the system ignores 21.2 mA and only trips above 22.5 mA (5.6 V).

The Underlying Cause:
1. In HW Config, when an analog channel is parameterized as Voltage 1..5 V, the module's internal ASIC disables current-mode NAMUR monitoring.
2. The module driver generator (MOD_D1 / CH_AI) parameterizes the channel for standard voltage limits.
3. Because 0.85 V is an electrically valid voltage above true hardware underflow, the ADC treats it as a legitimate value. It does not set the hardware wire-break diagnostic bit, leaving Pcs7AnIn unaware of the field fault.

Underflow and Overflow Thresholds in S7 Analog Input Modules

In S7-400 and S7-300 analog input modules, raw integer digital counts (-32768 to +32767) map differently across measurement types:

S7 Integer Mapping for 1..5 V:
Nominal Range (1.0 V to 5.0 V): 0 to 27648 counts.
Overrange (5.0 V to 5.75 V): 27649 to 32511 counts.
Overflow (> 5.75 V, approx. > 23 mA): +32767 counts (Hardware SF asserted).
Underrange (0.75 V to 1.0 V): -1 to -4864 counts.
Underflow (< 0.75 V, approx. < 3.0 mA down to 0 V): -32768 counts (Hardware SF asserted).
Because underflow triggers at 0.75 V (equivalent to 3.0 mA) rather than the NAMUR limit of 0.90 V (equivalent to 3.6 mA), an open transmitter drifting between 3.0 mA and 3.6 mA generates false valid measurements.

Comparison Table: 4..20 mA Current Measurement vs 1..5 V Voltage Measurement

The following table contrasts operational and diagnostic performance between current and voltage modes:

Diagnostic MetricStandard 4..20 mA Current ModeExternal 1..5 V Voltage ModeMeasuring Range in HW Config4..20 mA (2-wire or 4-wire)1..5 V Voltage RangeNAMUR NE 43 ComplianceFully compliant at hardware levelNot compliant (hardware ignores 3.6 mA)Wire Break Low Fault Trigger<= 3.6 mA (<= -4864 counts)<= 0.75 V (approx. <= 3.0 mA)Short Circuit High Fault Trigger>= 21.0 mA (>= 29030 counts)>= 5.75 V (approx. >= 23.0 mA)Pcs7AnIn Channel DriverAutomatically asserts Bad = TRUEReports Bad = FALSE between 3.0 and 3.6 mAExternal Resistor RequirementNone (internal precision shunt)Requires external 250-ohm resistor/barrierRecommended DeploymentStandard industrial DCS instrumentationLegacy retrofit only (requires software bounds)

Step-by-Step Procedure to Implement Software Sensor Fault Bounds in CFC

When plant hardware mandates 1..5 V voltage inputs, engineers must implement software-based NAMUR NE 43 bounds in CFC:

Option A: Rewire as Direct 4..20 mA (Recommended):
* Remove the external 250-ohm shunt resistor or voltage isolator.
* In HW Config, set the measuring range to 4..20 mA (4DMU or 2DMU).
* Save, compile HW Config, and download to the AS. Hardware NAMUR NE 43 diagnostics are restored immediately.
Option B: Implement CFC Software Limit Monitoring:
* In CFC, open the chart containing the Pcs7AnIn block.
* Set the channel driver scale limits matching the transmitter:
- Scale.Low = 0.0
- Scale.High = 100.0
- Place an analog monitoring block (MonAnL) or comparison block (CompAn02) on the chart.
- Connect Pcs7AnIn.Out to the comparator input.
- Configure software bounds:
- High Fault Threshold: 106.25% (equivalent to 21.0 mA).
- Low Fault Threshold: -2.50% (equivalent to 3.6 mA).
- Wire the comparator fault output to the interlock blocks or force Pcs7AnIn.CSF (Control System Fault) to mimic hardware passivation.

Verification

Verify that sensor fault detection operates reliably across all test points:
1. Connect a calibrated mA loop calibrator to the transmitter input loop.
2. Inject a test signal of 3.5 mA; verify that the system immediately asserts a Sensor Fault, sets Bad = TRUE, and trips fail-safe interlocks.
3. Inject a test signal of 3.8 mA; verify that the system clears the fault and returns to healthy measurement mode.
4. Inject a test signal of 21.5 mA; verify that the system detects an upper sensor fault and logs an alarm.
5. In WinCC Runtime, open the faceplate and verify that the Quality Code transitions from 0x80 (Good) to 0x00 (Bad) at the exact 3.6 mA / 21.0 mA boundaries.

Limitations and Operational Precautions

Voltage Drop on Field Cables: In 1..5 V systems, long cable runs between the shunt resistor and the analog card can introduce lead resistance voltage drops, causing measurement drift and premature low-fault trips. Always locate shunt resistors directly at the I/O terminal block.
Resistor Tolerance: Using standard 5% carbon resistors for 4..20 mA to 1..5 V conversion introduces massive temperature drift. Always use 0.1% tolerance, 15 ppm/°C metal film resistors.
Safety Instrumented Systems (SIS): For fail-safe safety instrumented functions (S7 F Systems), 1..5 V voltage measurement without certified hardware wire-break detection violates IEC 61511 functional safety standards. F-I/O modules (such as 6ES7336-4GE00-0AB0) must be wired as direct 4..20 mA current loops.
Commercial Accuracy: Standard analog input modules (6ES7431-1KF00-0AB0 or ET 200M 6ES7331-7KF02-0AB0) and PCS 7 engineering licenses (6ES7658-2CX58-0YB5) must be configured to manufacturer standards. Obtain quotation verification from an authorized Siemens distributor.

Engineer FAQ

Can I configure NAMUR NE 43 limits in HW Config for a 1..5 V module?

No. The HW Config parameterization for 1..5 V voltage channels does not include NAMUR threshold options. The module hardware ASIC applies fixed voltage underflow/overflow thresholds (0.75 V and 5.75 V).

Why does Pcs7AnIn have a 'MODE' input pin?

The MODE input is automatically connected by the module driver generator (Generate module drivers) to supply the channel block with hardware range configuration and diagnostic mask settings.

Does a 1..5 V signal support HART communication?

No. HART digital frequency shift keying (FSK) signals require a continuous 4..20 mA current loop. Voltage-converted 1..5 V signals strip out or attenuate HART communication waveforms.

What happens to Pcs7AnIn output when Bad = TRUE?

When Bad = TRUE, Pcs7AnIn outputs the pre-configured substitute value (SubsPV_In) or holds the last valid value, depending on the configured feature bit (Feature.Bit28).