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HART, Modbus and 4 to 20 mA Quiz

Test yourself with 20 multiple choice questions. Check your answers to get a score and an explanation for every question.

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HART, Modbus and 4 to 20 mA Quiz: 20 Questions

Choose one answer for each question, then press Check answers. Explanations appear after checking.

1. 12 mA in a 4 to 20 mA loop means:

Answer: 50 % of range. (12 − 4) ÷ 16 = 0.5. Learn more: 4 to 20 mA Current Loop

2. The main reason for a 4 mA live zero is:

Answer: To detect broken wires and power the transmitter. 0 mA means a fault. Learn more: Live Zero

3. NAMUR NE43 failure low current is:

Answer: ≤ 3.6 mA. Measurement 3.8 to 20.5 mA; failure high ≥ 21 mA. Learn more: NAMUR NE43

4. 24 V supply, transmitter minimum 10.5 V, 22 mA: maximum loop resistance is about:

Answer: 614 Ω. (24 − 10.5) ÷ 0.022. Learn more: Loop Resistance

5. HART needs a loop resistance of at least:

Answer: 250 Ω. To develop a readable signal voltage. Learn more: HART Protocol
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6. HART uses FSK frequencies of:

Answer: 1200 and 2200 Hz. 1200 Hz is 1, 2200 Hz is 0. Learn more: HART Protocol

7. Modbus function code 03 is:

Answer: Read holding registers. 04 reads input registers; 06 and 16 write registers. Learn more: Modbus RTU

8. Modbus RTU uses which error check?

Answer: CRC 16. ASCII mode uses LRC. Learn more: Modbus RTU

9. Modbus TCP normally uses port:

Answer: 502. TCP port 502. Learn more: Modbus TCP

10. RS485 needs termination resistors of about:

Answer: 120 Ω at each end. Matches cable impedance. Learn more: RS485
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11. Maximum standard unit loads on one RS485 segment:

Answer: 32. Modern transceivers may use fractional loads. Learn more: RS485

12. FOUNDATION Fieldbus H1 speed is:

Answer: 31.25 kbit/s. Same physical layer as PROFIBUS PA. Learn more: Fieldbus

13. A ground loop occurs when:

Answer: A circuit is earthed at two points with different potential. Current flows in the signal or shield. Learn more: Ground Loop

14. Analog cable shields are usually earthed:

Answer: At one end, the control system end. Avoids ground loops. Learn more: Cable Shield Grounding

15. A 16 bit ADC over 0 to 20 mA has a step of about:

Answer: 0.31 µA. 20 mA ÷ 65,536. Learn more: ADC Resolution
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16. Siemens S7 analog inputs scale 0 to 100 % as:

Answer: 0 to 27648. Headroom above 27648 shows overrange. Learn more: Analog Scaling

17. A PNP sensor output switches:

Answer: +24 V to the load. Sourcing output, used with sinking inputs. Learn more: Sinking and Sourcing

18. Gray code changes how many bits between consecutive values?

Answer: One. Avoids glitches in encoders. Learn more: Gray Code

19. BCD 0001 0010 0101 equals decimal:

Answer: 125. Each 4 bit group is one digit. Learn more: Binary Coded Decimal

20. An encoder with 1024 PPR at 25,600 Hz turns at:

Answer: 1500 RPM. RPM = f × 60 ÷ PPR. Learn more: Encoder
0 of 20 answered
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About this Quiz

20 multiple choice questions on hart, modbus and 4 to 20 ma, the kind asked in instrumentation interviews, technician exams and plant training. Each answer comes with a short explanation, so the quiz works as a revision sheet too.

Show answer key
  1. 50 % of range: (12 − 4) ÷ 16 = 0.5. Learn more: 4 to 20 mA Current Loop
  2. To detect broken wires and power the transmitter: 0 mA means a fault. Learn more: Live Zero
  3. ≤ 3.6 mA: Measurement 3.8 to 20.5 mA; failure high ≥ 21 mA. Learn more: NAMUR NE43
  4. 614 Ω: (24 − 10.5) ÷ 0.022. Learn more: Loop Resistance
  5. 250 Ω: To develop a readable signal voltage. Learn more: HART Protocol
  6. 1200 and 2200 Hz: 1200 Hz is 1, 2200 Hz is 0. Learn more: HART Protocol
  7. Read holding registers: 04 reads input registers; 06 and 16 write registers. Learn more: Modbus RTU
  8. CRC 16: ASCII mode uses LRC. Learn more: Modbus RTU
  9. 502: TCP port 502. Learn more: Modbus TCP
  10. 120 Ω at each end: Matches cable impedance. Learn more: RS485
  11. 32: Modern transceivers may use fractional loads. Learn more: RS485
  12. 31.25 kbit/s: Same physical layer as PROFIBUS PA. Learn more: Fieldbus
  13. A circuit is earthed at two points with different potential: Current flows in the signal or shield. Learn more: Ground Loop
  14. At one end, the control system end: Avoids ground loops. Learn more: Cable Shield Grounding
  15. 0.31 µA: 20 mA ÷ 65,536. Learn more: ADC Resolution
  16. 0 to 27648: Headroom above 27648 shows overrange. Learn more: Analog Scaling
  17. +24 V to the load: Sourcing output, used with sinking inputs. Learn more: Sinking and Sourcing
  18. One: Avoids glitches in encoders. Learn more: Gray Code
  19. 125: Each 4 bit group is one digit. Learn more: Binary Coded Decimal
  20. 1500 RPM: RPM = f × 60 ÷ PPR. Learn more: Encoder
Tip: take the quiz before an interview, note the questions you missed and read the linked article for those topics.
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Frequently Asked Questions

Is this quiz free?

Yes. No sign up is needed and you can retake it as often as you like.

Are the answers explained?

Yes. After you check your answers, each question shows the correct answer and a short explanation.

Where can I find more questions?

See the Interview Q&A section of instrumentationblog.in and the linked article below.

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