CPE Installation & Commissioning Procedures 2 — Questions and Answers
Question 1: What is the maximum allowed voltage drop across a PROFIBUS DP segment cable at full load?
- 1.5 V
- 3.0 V (Correct answer)
- 5.0 V
- 7.5 V
Correct answer: 3.0 V
The allowed voltage drop across a PROFIBUS DP segment cable must not exceed 3.0 V to ensure reliable communication.
Question 2: Which tool is commonly used during PROFIBUS commissioning to verify signal quality without disrupting live traffic?
- Oscilloscope only
- PROFIBUS tester/analyzer (Correct answer)
- Multimeter in AC mode
- Network sniffer
Correct answer: PROFIBUS tester/analyzer
A PROFIBUS tester or analyzer can capture and decode bus traffic non-intrusively, allowing signal quality assessment during live operation.
Question 3: When installing PROFIBUS cable in a cable tray alongside power cables, what is the minimum recommended separation distance?
- 5 cm
- 10 cm
- 20 cm (Correct answer)
- 50 cm
Correct answer: 20 cm
A minimum separation of 20 cm between PROFIBUS cables and power cables is recommended to reduce electromagnetic interference.
Question 4: What happens if a PROFIBUS DP slave device is powered on while the master is already in Data_Exchange mode?
- The slave immediately joins data exchange
- The master detects the new slave and adds it to the active list automatically
- The slave remains in Wait_Prm state until the master re-initializes it (Correct answer)
- The entire bus resets
Correct answer: The slave remains in Wait_Prm state until the master re-initializes it
A newly powered slave waits in Wait_Prm state until the master sends parameterization and configuration telegrams to initialize it.
Question 5: Which PROFIBUS DP diagnostic flag indicates that a slave requires new parameterization data?
- Prm_Fault
- Stat_Diag
- Prm_Req (Correct answer)
- Cfg_Fault
Correct answer: Prm_Req
The Prm_Req (parameterization request) flag in the slave diagnostic byte signals that the slave needs fresh parameterization from the master.
Question 6: During segment commissioning, the bus monitor shows many CRC errors concentrated at one time of day. What is the most likely cause?
- Wrong baud rate setting
- Cyclic electromagnetic interference from nearby machinery (Correct answer)
- Incorrect termination resistor value
- Wrong GSD file version
Correct answer: Cyclic electromagnetic interference from nearby machinery
Time-correlated CRC errors often indicate periodic EMI sources such as motors, drives, or welding equipment that operate on a schedule.
Question 7: What is the purpose of the PE (Protective Earth) connection on PROFIBUS cable shielding?
- To provide bus power return path
- To create a low-impedance path for noise currents away from signal conductors (Correct answer)
- To increase cable capacitance
- To set the cable impedance to 150 Ω
Correct answer: To create a low-impedance path for noise currents away from signal conductors
Connecting the cable shield to PE provides a low-impedance return path for induced noise currents, preventing them from corrupting signal conductors.
CPE Installation & Commissioning Procedures 2
What is the maximum allowed voltage drop across a PROFIBUS DP segment cable at full load?