Deep Dive: Technical Challenges and Best Practices with Siemens AI Cards

In process automation, the Analog Input (AI) card is more than just a piece of hardware—it’s the critical bridge between the physical plant and the control system. For Siemens systems, AI cards within SIMATIC S7-1200, S7-1500, ET 200 remote I/O, and PCS 7 architectures are widely deployed in industries ranging from oil & gas to chemicals and power generation.

At Navon Logic, we’ve encountered and solved numerous technical challenges with AI cards during programming, commissioning, and troubleshooting phases. Let’s explore the deeper technical aspects that project teams need to be aware of.

1. Signal Scaling and Engineering Units

AI cards read raw values—whether that’s 0–27648 counts for a 4–20 mA input or voltage ranges like ±10 V. A common issue arises when these raw counts are not properly scaled to engineering units in the control logic.

• Example: A level transmitter sending 4–20 mA should correspond to 0–10 meters. If scaling is incorrect, operators may see nonsensical values (like negative levels).

• Siemens TIA Portal and PCS 7 provide scaling functions (FC105, SCL blocks, or CFCs), but misapplication of these leads to control errors and commissioning delays.

Best Practice: Always verify scaling during loop checks with simulated signals, and document the engineering ranges in the I/O database.

2. Noise and Grounding Problems

Industrial sites are electrically noisy environments. We’ve seen AI channels reading unstable values due to:

• Poor shield termination on transmitter cables.

• Incorrect grounding practices causing ground loops.

• Long cable runs without proper isolation.

These issues manifest as “jittery” or drifting readings, which can cause PID loops to oscillate unnecessarily.

Best Practice: Use Siemens AI cards with galvanic isolation where possible, ensure shields are terminated at one end only, and follow Siemens grounding manuals strictly.

3. Thermocouple and RTD Measurement Pitfalls

Siemens AI cards support direct temperature inputs, but they come with their own technical traps:

• Cold junction compensation (CJC): If the CJC sensor in the terminal block fails or is bypassed, thermocouple readings drift significantly.

• Lead resistance in RTDs: Long wiring runs on 2-wire RTDs introduce error. Using 3-wire or 4-wire configurations compensates, but only if properly configured in hardware settings.

Best Practice: During commissioning, simulate known temperature points with calibrators to validate configuration.

4. Broken Wire and Open Loop Detection

One major strength of Siemens AI cards is their diagnostic capability—they can detect broken wires, out-of-range signals, or short circuits.

• However, enabling diagnostics incorrectly can cause nuisance alarms in PCS 7/WinCC.

• Conversely, not enabling them leads to dangerous blind spots where operators see a frozen “last value” even though the transmitter is disconnected.

Best Practice: Always configure diagnostic interrupts correctly and test them by physically disconnecting a loop during FAT/SAT.

5. Update Time and Scan Cycle Effects

AI cards introduce latency in signal processing, often in the 10–250 ms range depending on configuration and filter settings. In fast control loops (like compressor surge control), this delay can destabilize the process.

Best Practice: Adjust hardware filter times to balance noise rejection and response speed. For critical loops, use high-speed AI cards or hardware with reduced conversion times.

6. Calibration and Drift

Even high-quality AI cards are subject to ADC drift over years of operation. Without periodic calibration, small errors accumulate, leading to process inaccuracies—particularly in custody transfer or compliance-critical applications.

Best Practice: Implement a preventive maintenance plan with periodic loop calibration, comparing field instruments against reference calibrators, and documenting deviations.

7. Integration with Safety Systems (SIS)

When Siemens AI cards are used in failsafe systems (F-Systems), special care is required:

• Wrong configuration can lead to unsafe values being passed into the safety logic.

• Redundancy setups (e.g., H-Systems in PCS 7) must be properly tested to ensure seamless switchover without loss of AI data.

Best Practice: Always validate AI behavior under fault conditions (open loop, short circuit) as part of SIL validation testing.

Navon Logic Perspective

At Navon Logic, we’ve seen projects delayed not because the equipment was unavailable, but because AI cards were misconfigured or misunderstood. Small oversights—like reversed scaling, incorrect grounding, or disabled diagnostics—can cause weeks of troubleshooting during commissioning.

Our approach ensures:

• Early verification of AI configurations in FAT.

• Thorough loop checks with documented scaling.

• Simulation and stress testing of diagnostics.

• Ongoing support for maintenance teams to reduce lifecycle issues.

Final Word:
Siemens AI cards are the eyes of your automation system. When properly configured, they deliver precise, reliable data for advanced process control. But when overlooked, they become a hidden source of costly delays.

At Navon Logic, we bridge the gap between theory and field reality, ensuring your AI cards—and your entire automation system—deliver reliable performance from day one.