CPE Electromagnetic Compatibility (EMC) & Physical Layer 2 — Questions and Answers
Question 1: What is the correct passive Profibus bus termination network configuration?
- A single 120 Ω resistor across A and B lines
- 390 Ω pull-up to +5V, 220 Ω across A-B, 390 Ω pull-down to GND (Correct answer)
- Two 75 Ω resistors in parallel to ground
- A single 220 Ω resistor from A to B with no biasing
Correct answer: 390 Ω pull-up to +5V, 220 Ω across A-B, 390 Ω pull-down to GND
The Profibus termination network uses a 390 Ω resistor to +5V, a 220 Ω resistor across the differential pair, and a 390 Ω resistor to GND, providing impedance matching and idle-state line bias simultaneously.
Question 2: What is the consequence of operating a Profibus segment with no active bus termination at one or both ends?
- Signal amplitude increases above spec
- Signal reflections cause intermittent data corruption (Correct answer)
- Bus current consumption increases
- Baud rate is automatically reduced
Correct answer: Signal reflections cause intermittent data corruption
Without termination, signal energy is reflected from unterminated cable ends, creating superimposed waveforms that cause bit errors and intermittent communication faults.
Question 3: What measurements does a Profibus bus analyzer perform to assess physical layer signal quality?
- Cable resistance only
- Signal voltage levels, noise margin, timing jitter, and reflection waveforms (Correct answer)
- IP address and subnet mask of each node
- Power consumption of individual field devices
Correct answer: Signal voltage levels, noise margin, timing jitter, and reflection waveforms
A Profibus bus analyzer captures and analyzes signal voltage, noise margin, jitter, and reflection signatures to give a complete picture of physical layer health.
Question 4: What is the primary purpose of a potential equalization bonding conductor in a Profibus installation?
- To carry the differential Profibus data signal as a backup path
- To equalize ground potentials between cabinets and reduce common-mode interference currents (Correct answer)
- To provide +24V supply to field devices along the bus
- To serve as a redundant communication path
Correct answer: To equalize ground potentials between cabinets and reduce common-mode interference currents
The potential equalization conductor minimizes the ground potential difference between interconnected cabinets, reducing common-mode currents that would otherwise flow through the cable shield and cause interference.
Question 5: What physical characteristic of Profibus Type A cable primarily limits the maximum achievable baud rate to 12 Mbit/s?
- Excessive conductor resistance per unit length
- Capacitance per unit length and resulting signal propagation delay (Correct answer)
- Insufficient insulation voltage rating for industrial environments
- Excessive cable weight per meter
Correct answer: Capacitance per unit length and resulting signal propagation delay
The distributed capacitance (≤30 pF/m) combined with cable length creates an RC time constant that limits signal rise times, capping the practical baud rate at 12 Mbit/s.
Question 6: What is the maximum number of devices (stations) allowed on a single Profibus DP segment before a repeater is required?
- 16
- 32 (Correct answer)
- 64
- 126
Correct answer: 32
Profibus DP allows a maximum of 32 stations per segment (including masters and repeaters), limited by the cumulative electrical load of RS-485 transceiver unit loads on the bus.
Question 7: What common-mode voltage range must Profibus RS-485 transceivers tolerate without data corruption?
- ±2 V
- ±5 V
- ±7 V (Correct answer)
- ±15 V
Correct answer: ±7 V
The RS-485 specification requires transceivers to operate correctly with common-mode voltages between -7 V and +12 V, accommodating the ground potential differences typical in industrial systems.
CPE Electromagnetic Compatibility (EMC) & Physical Layer 2
What is the correct passive Profibus bus termination network configuration?