CPE Electromagnetic Compatibility (EMC) & Physical Layer 1 — Questions and Answers
Question 1: What is the characteristic impedance of Profibus Type A cable as specified in EN 50170?
- 75 Ω
- 100 Ω
- 135 Ω (Correct answer)
- 150 Ω
Correct answer: 135 Ω
Profibus Type A cable has a specified characteristic impedance of 135 Ω ±20% as defined in EN 50170, which determines the correct termination network values.
Question 2: What is the minimum differential input voltage that an RS-485 receiver used in Profibus must recognize as a valid signal?
- 200 mV (0.2 V) (Correct answer)
- 500 mV (0.5 V)
- 1.0 V
- 1.5 V
Correct answer: 200 mV (0.2 V)
The RS-485 standard requires receivers to reliably detect differential signals as low as 200 mV, providing high noise immunity over long cable runs.
Question 3: Which grounding practice is recommended for Profibus cable shields to prevent ground loop interference?
- Ground at both ends
- Single-point grounding at one end only (Correct answer)
- Leave the shield floating
- Ground at every junction box
Correct answer: Single-point grounding at one end only
Single-point grounding prevents circulating ground loop currents that would otherwise flow through the shield and induce noise onto the differential signal pair.
Question 4: What is the maximum allowed segment cable length for Profibus DP operating at 12 Mbit/s?
- 100 m (Correct answer)
- 200 m
- 400 m
- 1200 m
Correct answer: 100 m
At 12 Mbit/s the signal rise-time budget limits the Profibus DP segment length to 100 meters due to cable propagation delay and capacitive loading.
Question 5: Which passive component is commonly integrated into Profibus bus connectors to suppress high-frequency common-mode noise?
- Electrolytic capacitor
- Ferrite bead (Correct answer)
- Metal oxide varistor
- Zener diode
Correct answer: Ferrite bead
Ferrite beads present high impedance to high-frequency common-mode currents while having negligible effect on the lower-frequency differential Profibus signal.
Question 6: At what angle should Profibus signal cables cross AC power cables to minimize electromagnetic coupling?
- 0° (parallel routing)
- 30°
- 45°
- 90° (perpendicular) (Correct answer)
Correct answer: 90° (perpendicular)
Crossing at 90° minimizes the coupling area between the Profibus cable and the power cable, reducing both capacitive and inductive interference transfer.
Question 7: What is the maximum capacitance per unit length permitted for Profibus Type A cable?
- 10 pF/m
- 30 pF/m (Correct answer)
- 60 pF/m
- 100 pF/m
Correct answer: 30 pF/m
Profibus Type A cable is specified with a maximum capacitance of 30 pF/m, which limits signal attenuation and ensures adequate rise times at 12 Mbit/s.
CPE Electromagnetic Compatibility (EMC) & Physical Layer 1
What is the characteristic impedance of Profibus Type A cable as specified in EN 50170?