In the field, nothing unnerves commissioning engineers more than watching pulse counts mysteriously drop by half. The Siemens 6ES7138-4DA04-0AB0 High-Speed Input Module, capable of processing signals up to 100 kHz, is a precision instrument — but its accuracy depends entirely on how well electromagnetic interference (EMI) is kept at bay. A loose shield, an ungrounded cable, or a poorly laid route can instantly turn clean pulses into noise.

To maintain reliable operation, engineers must treat EMC (Electromagnetic Compatibility) not as an afterthought, but as an integral part of system design. Below, we combine Siemens’ official recommendations, RS documentation, and hard-earned field lessons into six practical “iron rules” for ensuring signal integrity.

1. Shielding: 360° Grounding Is Non-Negotiable

Always use dual-shielded encoder cables — braided and foil types — for PNP encoders. The outer shield should be bonded 360° to the copper grounding bar at the control cabinet side, while the inner shield must be grounded via the TM-E15S26-A1 metal clip on the module side.
This configuration creates a continuous Faraday tube, blocking both radiated and conducted noise along the cable’s path.

2. Cable Length: Respect the 50 m Rule

According to Siemens’ manual, cable shielding remains effective only up to specific distances.

· With a 200 kHz filter, total shielded cable length should not exceed 50 m.

· For 20 kHz filtering, the limit extends to 100 m.

If longer distances are unavoidable, install a signal repeater, ensuring both ends are properly grounded. Ignoring this rule risks cumulative signal loss and increased susceptibility to EMI.

3. Grounding: Single-Point, Low-Impedance, Same Plane

Inside the cabinet, use a dedicated 6 mm² green-yellow grounding bus that unifies the PE, shield, and DIN rail ground. The overall grounding resistance should stay below 0.1 Ω.
Never share grounding branches with VFDs or welding machines, which are notorious sources of common-mode noise. Proper grounding provides a stable reference potential for high-speed counting and prevents differential disturbances.

4. Cable Routing: The 30 cm Separation Principle

Maintain at least 30 cm of physical separation between 24 V signal lines, motor power cables, and inverter outputs.
If crossing is unavoidable, enforce a 90° angle between them to minimize electromagnetic coupling.
Layered cable trays (power, control, and communication in separate tiers) are a proven way to reduce interference in compact cabinets.

5. Twisting and Filtering: Build the Common-Mode Defense

Add ferrite cores near the encoder side, looping the signal through 2–3 turns. On the module side, press the 0 V and shield together into the grounding clip to establish a defined common-mode return path.
In real-world tests, this setup has been shown to reduce 30 MHz spikes by up to 12 dB, significantly stabilizing pulse detection under noisy conditions.

6. Verification: The 30-Second Pulse Health Check

Before declaring the installation complete, perform a pulse integrity test using TIA Portal:
Run a 1 kHz square wave for 1 second, then observe the Count value — it should vary by no more than ±1.
Next, energize nearby VFDs to 50 Hz; if the deviation exceeds 3, your cabling still leaks EMI. Reinspect the shield terminations — any “pigtail” shield connection is a red flag.

Conclusion: Seal Every Crack in the “Water Pipe”

The 6ES7138-4DA04-0AB0 operates with a minimum pulse width of 2.5 μs, leaving only a few dozen nanoseconds for interference to slip through. Think of your cable shield as a water pipe—even the smallest gap lets “EMI water” leak in.

Follow the six rules above, and your 100 kHz high-speed input will count as steadily as a slow digital input.
In modern automation, precision begins with protection—and EMC cabling is the frontline defense between data accuracy and chaos.