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PLC, 4 to 20 mA and Modbus Quiz
Test yourself with 15 multiple choice questions. Check your answers to get a score and an explanation for every question.
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PLC, 4 to 20 mA and Modbus Quiz: 15 Questions
Choose one answer for each question, then press Check answers. Explanations appear after checking.
1. 4 mA in a 4 to 20 mA loop represents:
Answer: 0 % of range (live zero). Live zero lets broken wires (0 mA) be detected.
2. 50 % of span equals:
Answer: 12 mA. 4 + 16 × 0.5 = 12 mA.
3. Under NAMUR NE 43, a downscale failure signal is:
Answer: ≤ 3.6 mA. Failure: ≤ 3.6 mA or ≥ 21 mA.
4. 4 to 20 mA through a 250 Ω resistor gives:
Answer: 1 to 5 V. 0.004 × 250 = 1 V, 0.02 × 250 = 5 V.
5. HART communication needs a loop resistance of at least about:
Answer: 230 Ω. At least 230 Ω, typically 250 Ω.
6. Siemens S7 analog input full range (4 to 20 mA) corresponds to:
Answer: 0 to 27648. Siemens nominal range is 0 to 27648.
7. A 12 bit ADC has how many counts?
Answer: 4096. 2¹² = 4096.
8. A PLC scan cycle is:
Answer: Read inputs, execute program, write outputs. Input scan, logic execution, output update, housekeeping.
9. Holding register 40001 is read with Modbus function code:
Answer: 03. FC 03 reads holding registers; FC 04 reads input registers.
10. A TON timer:
Answer: Delays turning on. TON is an on delay timer.
11. A 2 wire transmitter is:
Answer: Loop powered by the 4 to 20 mA signal. Power and signal share the two wires.
12. Instrument cable screens should normally be earthed:
Answer: At one end (control room). Single point earthing avoids earth loops.
13. A sinking digital input (PNP sensor) means the sensor:
Answer: Sources current into the input. A PNP sensor sources current into a sinking input.
14. Modbus RTU uses which error check?
Answer: CRC 16. RTU uses a CRC 16; ASCII uses LRC.
15. IEEE 754 single precision float uses:
Answer: 32 bits (two Modbus registers). A 32 bit float occupies two 16 bit registers.
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About this Quiz
15 multiple choice questions on plc, 4 to 20 ma and modbus, 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
- 0 % of range (live zero): Live zero lets broken wires (0 mA) be detected.
- 12 mA: 4 + 16 × 0.5 = 12 mA.
- ≤ 3.6 mA: Failure: ≤ 3.6 mA or ≥ 21 mA.
- 1 to 5 V: 0.004 × 250 = 1 V, 0.02 × 250 = 5 V.
- 230 Ω: At least 230 Ω, typically 250 Ω.
- 0 to 27648: Siemens nominal range is 0 to 27648.
- 4096: 2¹² = 4096.
- Read inputs, execute program, write outputs: Input scan, logic execution, output update, housekeeping.
- 03: FC 03 reads holding registers; FC 04 reads input registers.
- Delays turning on: TON is an on delay timer.
- Loop powered by the 4 to 20 mA signal: Power and signal share the two wires.
- At one end (control room): Single point earthing avoids earth loops.
- Sources current into the input: A PNP sensor sources current into a sinking input.
- CRC 16: RTU uses a CRC 16; ASCII uses LRC.
- 32 bits (two Modbus registers): A 32 bit float occupies two 16 bit registers.
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
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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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