CPE Troubleshooting & Maintenance Techniques 5 — Questions and Answers
Question 1: A PROFIBUS DP slave remains in the 'WAIT_PRM' state and never transitions to data exchange. What is the most likely cause?
- The slave's PROFIBUS address is 0
- The master is sending parameterization data that does not match the slave's GSD-defined requirements (Correct answer)
- The slave has reached its maximum number of connected masters
- The network baud rate exceeds 12 Mbit/s
Correct answer: The master is sending parameterization data that does not match the slave's GSD-defined requirements
The WAIT_PRM state persists when the slave rejects the Set_Prm telegram because the parameter data length or content does not match what the slave expects per its GSD file.
Question 2: What is the significance of the 'Deactivated' slave status in a PROFIBUS DP master's live list?
- The slave has been physically removed from the bus
- The slave is configured in the master but excluded from cyclic data exchange by the user (Correct answer)
- The slave's watchdog has expired and it entered safe mode
- The slave requires a firmware update before it can participate
Correct answer: The slave is configured in the master but excluded from cyclic data exchange by the user
A 'Deactivated' slave is intentionally excluded from the master's active polling cycle by configuration, often used for devices installed but not yet commissioned.
Question 3: When performing insulation resistance testing on PROFIBUS cable for maintenance purposes, which precaution is essential?
- Apply the test voltage while the network is powered and communicating
- Disconnect all devices and terminators from the cable before applying the test voltage (Correct answer)
- Test only one conductor at a time while the other remains connected to the bus
- Use an AC voltage source at 50 Hz for the insulation test
Correct answer: Disconnect all devices and terminators from the cable before applying the test voltage
All connected devices and terminators must be disconnected before applying insulation test voltage to prevent damage to sensitive electronic components in field devices and terminators.
Question 4: In PROFIBUS PA, what does the 'Simulate' function block parameter enable during maintenance?
- It forces the device into a power-saving sleep mode
- It allows the output value and status to be manually set, bypassing the actual process measurement (Correct answer)
- It automatically runs self-diagnostics on all internal sensors
- It enables remote configuration changes without local access
Correct answer: It allows the output value and status to be manually set, bypassing the actual process measurement
The Simulate parameter in a PROFIBUS PA function block allows maintenance personnel to inject test values and statuses into the function block output without requiring actual process conditions.
Question 5: A PROFIBUS segment experiences communication errors only during the day shift. Plant data shows heavy welding operations nearby. Which root cause is most likely?
- Temperature expansion of the cable causing increased resistance
- Electromagnetic interference from welding equipment coupled onto unshielded or improperly grounded cable (Correct answer)
- Increased network traffic from day-shift control programs
- Power supply voltage sag from heavy electrical loads
Correct answer: Electromagnetic interference from welding equipment coupled onto unshielded or improperly grounded cable
Welding equipment generates significant high-frequency electromagnetic interference that can be coupled onto PROFIBUS cables with inadequate shielding or improper grounding.
Question 6: What is the purpose of the 'Ident_Number' field in a PROFIBUS slave's diagnostic response?
- It uniquely identifies the specific physical device on the bus
- It identifies the device type and must match the Ident_Number in the master's GSD-based configuration (Correct answer)
- It reports the firmware revision level of the slave device
- It indicates the manufacturer's production batch number
Correct answer: It identifies the device type and must match the Ident_Number in the master's GSD-based configuration
The Ident_Number is a manufacturer-assigned type identifier that must match between the slave hardware and the master's configuration derived from the GSD file; a mismatch prevents data exchange.
Question 7: After replacing a PROFIBUS PA pressure transmitter, the new device shows process value 'BAD – Sensor Failure' immediately after power-up. The most likely cause is:
- The PROFIBUS address was set incorrectly
- The device requires configuration or calibration before its sensor measurement becomes valid (Correct answer)
- The DP/PA segment coupler is overloaded
- The GSD file for the new device is missing from the master
Correct answer: The device requires configuration or calibration before its sensor measurement becomes valid
A newly installed PA transmitter may report 'BAD – Sensor Failure' until it is properly configured and calibrated with the appropriate range, units, and sensor-specific parameters.
CPE Troubleshooting & Maintenance Techniques 5
A PROFIBUS DP slave remains in the 'WAIT_PRM' state and never transitions to data exchange.
What is the most likely cause?