CPE Troubleshooting & Maintenance Techniques 3 — Questions and Answers
Question 1: A PROFIBUS DP network operates at 1.5 Mbit/s. The maximum allowable cable length per segment at this baud rate is approximately:
- 1200 m
- 400 m (Correct answer)
- 200 m
- 100 m
Correct answer: 400 m
At 1.5 Mbit/s, the maximum segment length is approximately 400 m according to PROFIBUS specifications due to signal propagation and attenuation constraints.
Question 2: What is the purpose of the 'Freeze' mode command in PROFIBUS DP troubleshooting?
- Stop all bus communication for safe maintenance
- Synchronize output updates across multiple slaves simultaneously
- Lock input values from multiple slaves at a single point in time (Correct answer)
- Freeze the master's configuration to prevent accidental changes
Correct answer: Lock input values from multiple slaves at a single point in time
The Freeze command causes all slaves in a defined group to simultaneously latch their current input data, enabling synchronized diagnostics or process snapshots.
Question 3: During a PROFIBUS segment audit, you measure a resistance of 220 Ω between the two bus lines (A and B) with the network powered off. What does this indicate?
- A short circuit between A and B lines
- Correct termination with two 110 Ω terminators in parallel (Correct answer)
- An open circuit in one of the bus lines
- Incorrect baud rate configuration
Correct answer: Correct termination with two 110 Ω terminators in parallel
Two 110 Ω termination resistors (one at each end) appear as 55 Ω in parallel — a reading of 220 Ω between A and B lines confirms two properly installed terminators.
Question 4: A PROFIBUS PA segment has 8 field devices consuming an average of 15 mA each. The segment coupler provides 400 mA. What is the remaining current budget?
- 280 mA (Correct answer)
- 300 mA
- 320 mA
- 400 mA
Correct answer: 280 mA
8 devices × 15 mA = 120 mA consumed; 400 mA − 120 mA = 280 mA remaining, which can accommodate additional field devices.
Question 5: Which PROFIBUS DP error condition is indicated when the 'Watchdog_Active' bit in Station_Status_2 is set to 1?
- The master's watchdog has timed out and stopped scanning
- The slave's watchdog timer is enabled and actively monitoring the master (Correct answer)
- A second master has taken control of the slave
- The slave has entered fail-safe mode
Correct answer: The slave's watchdog timer is enabled and actively monitoring the master
When Watchdog_Active is set in Station_Status_2, it means the slave has enabled its watchdog timer and is actively monitoring for periodic data exchange with the master.
Question 6: What is the recommended action when PROFIBUS bus analysis shows that the signal eye pattern is significantly degraded but no communication errors are yet occurring?
- Take no action since communication is still working
- Perform preventive maintenance — inspect connectors, cable bends, and termination (Correct answer)
- Increase retries in the master configuration to compensate
- Switch to a lower baud rate immediately
Correct answer: Perform preventive maintenance — inspect connectors, cable bends, and termination
A degraded eye pattern indicates reduced noise margin that will eventually cause errors; proactive inspection and repair prevents future downtime.
Question 7: In PROFIBUS PA, the function block status code 'BAD – Not Connected' most likely indicates:
- The device has a hardware failure
- The field device is not communicating with the process connection or sensor (Correct answer)
- The DP/PA coupler has lost power
- The GSD file is missing from the master
Correct answer: The field device is not communicating with the process connection or sensor
'BAD – Not Connected' in PROFIBUS PA function blocks indicates the transducer block has no valid connection to its physical sensing element or process.
CPE Troubleshooting & Maintenance Techniques 3
A PROFIBUS DP network operates at 1.5 Mbit/s.
The maximum allowable cable length per segment at this baud rate is approximately: