CPE Troubleshooting & Maintenance Techniques 4 — Questions and Answers
Question 1: A repeater is used in a PROFIBUS DP network. Which statement about repeater placement is correct?
- A repeater resets the device count, allowing 32 additional devices per new segment
- A repeater counts as one device on each of the two segments it connects (Correct answer)
- Repeaters can be placed anywhere and do not affect bus timing
- Each repeater adds one master to the network
Correct answer: A repeater counts as one device on each of the two segments it connects
A PROFIBUS repeater occupies one device address on each segment it bridges, so each segment can have a maximum of 31 field devices plus the repeater (32 total).
Question 2: When replacing a PROFIBUS slave device in a live plant, which precaution is MOST important to ensure minimal disruption?
- Power down the entire segment before replacement
- Ensure the replacement device has the same PROFIBUS address and GSD parameters as the original (Correct answer)
- Update the master's configuration to remove the slave before replacement
- Install the new device on a separate segment first
Correct answer: Ensure the replacement device has the same PROFIBUS address and GSD parameters as the original
The replacement device must have the identical PROFIBUS node address and compatible GSD configuration so the master can automatically re-establish communication.
Question 3: What diagnostic information is contained in the 'Manufacturer Specific Diagnostic' portion of a PROFIBUS slave's diagnostic response?
- Standard PROFIBUS error codes defined in IEC 61158
- Device-specific fault codes, status information, and operational data defined by the device manufacturer (Correct answer)
- The slave's current PROFIBUS address and baud rate
- The master's station address and cycle time
Correct answer: Device-specific fault codes, status information, and operational data defined by the device manufacturer
The manufacturer-specific diagnostic area allows vendors to include device-specific information such as sensor faults, calibration status, and internal error codes beyond the standard PROFIBUS diagnostic bytes.
Question 4: A PROFIBUS network exhibits intermittent errors that are correlated with operation of nearby variable frequency drives (VFDs). The MOST effective solution is:
- Reduce the PROFIBUS baud rate to 9.6 kbit/s
- Install ferrite cores on PROFIBUS cables and verify proper cable shielding and grounding (Correct answer)
- Move all PROFIBUS devices to a separate room
- Disable the VFDs during PROFIBUS communication windows
Correct answer: Install ferrite cores on PROFIBUS cables and verify proper cable shielding and grounding
Ferrite cores attenuate high-frequency EMI from VFDs, and proper shielding/grounding prevents inductive and capacitive coupling of noise onto the bus.
Question 5: Which parameter in the PROFIBUS DP master configuration defines how long the master waits for a slave's response before declaring a communication error?
- Tslot (slot time) (Correct answer)
- Tset (setup time)
- Min_Slave_Interval
- Watchdog_Time
Correct answer: Tslot (slot time)
Tslot (slot time) defines the maximum time the master waits for a slave response token; if no response arrives within this window, a communication error is flagged.
Question 6: During PROFIBUS maintenance, you discover a connector with bent pins on the RS-485 port. Before replacing the connector, which measurement should be taken?
- Measure network current consumption with a clamp meter
- Measure the differential voltage between A and B lines to assess signal quality (Correct answer)
- Check the PROFIBUS address with a rotary switch
- Measure the cable impedance with a spectrum analyzer
Correct answer: Measure the differential voltage between A and B lines to assess signal quality
Measuring the differential voltage between A and B lines confirms whether the electrical connection is still functional and reveals the extent of signal degradation.
Question 7: A PROFIBUS DP system is configured with a Data_Control_Time of 5 ms. This parameter primarily affects:
- The maximum baud rate the master will use
- The frequency at which the master verifies that acyclic data exchanges are completed
- How often the master sends global control commands (Correct answer)
- The time the master waits before re-initializing a failed slave
Correct answer: How often the master sends global control commands
The Data_Control_Time defines the interval at which the master sends Global Control commands (such as Sync and Freeze) to control groups of slaves.
CPE Troubleshooting & Maintenance Techniques 4
A repeater is used in a PROFIBUS DP network.
Which statement about repeater placement is correct?