CPE Electromagnetic Compatibility (EMC) & Physical Layer

Question 1

What is the characteristic impedance of Profibus Type A cable as specified in EN 50170?

Accepted Answer

135 Ω

Answer

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?

Accepted Answer

200 mV (0.2 V)

Answer

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?

Accepted Answer

Single-point grounding at one end only

Answer

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?

Accepted Answer

100 m

Answer

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?

Accepted Answer

Ferrite bead

Answer

Ferrite beads present high impedance to high-frequency common-mode currents while having negligible effect on the lower-frequency differential Profibus signal.

(CPE) Certified Profibus Engineer Practice Test

(CPE) Certified Profibus Engineer Practice Test

CPE Electromagnetic Compatibility (EMC) & Physical Layer