CPE Installation & Commissioning Procedures 3 — Questions and Answers
Question 1: Which PROFIBUS DP baud rate requires the use of cable Type A to achieve the full specified segment length?
- 9.6 kbit/s
- 500 kbit/s
- 1.5 Mbit/s
- 12 Mbit/s (Correct answer)
Correct answer: 12 Mbit/s
At 12 Mbit/s, only Type A cable (shielded, 135–165 Ω, low capacitance) can reliably cover the specified 100 m maximum segment length.
Question 2: When connecting a PROFIBUS PA device to a DP/PA coupler, what is the typical trunk line impedance used?
- 50 Ω
- 100 Ω
- 100 Ω balanced
- 100 Ω (IEC 61158-2 Type A) (Correct answer)
Correct answer: 100 Ω (IEC 61158-2 Type A)
PROFIBUS PA uses IEC 61158-2 Type A cable with 100 Ω balanced impedance for the trunk and spur connections.
Question 3: What is the recommended action when a PROFIBUS repeater is added to extend a segment beyond the maximum node count?
- Disable the termination on both sides of the repeater
- Terminate each segment independently at both ends (Correct answer)
- Move all terminators to the master end
- Increase baud rate to compensate
Correct answer: Terminate each segment independently at both ends
Each segment separated by a repeater must be independently terminated at both physical ends to prevent signal reflections.
Question 4: During PROFIBUS installation, what does a 'Retry_Error' counter incrementing on the master indicate?
- A slave is responding with wrong data length
- The master is retransmitting requests due to no or corrupt responses (Correct answer)
- The watchdog timer has expired
- A token pass error has occurred
Correct answer: The master is retransmitting requests due to no or corrupt responses
An incrementing Retry_Error counter means the master is not receiving valid responses and is retransmitting, indicating communication failures on the bus.
Question 5: What is the effect of setting a slave's watchdog timeout too short during commissioning?
- The slave ignores the master longer
- The slave frequently enters safe state unnecessarily during normal communication delays (Correct answer)
- The baud rate increases automatically
- The slave sends more diagnostics
Correct answer: The slave frequently enters safe state unnecessarily during normal communication delays
A watchdog timeout set shorter than the maximum master cycle time will cause the slave to enter safe state during normal operation, triggering unnecessary safety shutdowns.
Question 6: When a PROFIBUS GSD file specifies 'Freeze_Mode_supp = 1', what commissioning capability does this enable?
- Freezing baud rate during hot-swap
- Synchronizing input data capture across multiple slaves simultaneously (Correct answer)
- Locking parameterization against changes
- Freezing the slave address in hardware
Correct answer: Synchronizing input data capture across multiple slaves simultaneously
Freeze_Mode allows the master to issue a global freeze command, causing multiple slaves to simultaneously latch their input data for synchronized acquisition.
Question 7: Which physical layer test is performed first when troubleshooting a PROFIBUS segment that shows intermittent communication failures?
- GSD file validation
- Termination voltage measurement (Correct answer)
- Software configuration review
- Baud rate recalibration
Correct answer: Termination voltage measurement
Measuring termination voltage (typically 3.7–5.0 V between A and B lines with no traffic) quickly confirms whether bus termination is correctly applied.
CPE Installation & Commissioning Procedures 3
Which PROFIBUS DP baud rate requires the use of cable Type A to achieve the full specified segment length?