Programmable Logic Controllers and Automation Systems
What the exam asks
PLC items test the scan cycle, I/O wiring conventions, analog scaling, program structures and network basics. Expect at least one scaling calculation and several questions on where safety functions must live.
Core rules
- Scan cycle. Read inputs into the image table, solve the program top to bottom, then update outputs, plus housekeeping and communications. An output based on an input can lag by a full scan; time-critical interlocks use immediate I/O instructions.
- Languages. IEC 61131-3 defines ladder diagram, function block diagram, structured text, instruction list and sequential function chart. Structured text suits math and string handling; ladder suits interlocks and maintenance readability.
- Discrete I/O. A sourcing (PNP) sensor supplies +24 V to the input, so the input card must sink current. Match sensor type to the card's common terminal, or the input never turns on.
- Analog scaling. EU = (raw − raw_min) ÷ (raw_max − raw_min) × (EU_max − EU_min) + EU_min. Do the arithmetic in REAL; integer math truncates.
- State and memory. Retentive registers survive power loss; one-shots fire for a single scan; watchdogs catch stalled scans.
- Safety. Emergency stop, guard interlocks and safe-stop functions belong in a safety-rated relay or safety PLC (SIL/PL rated), not in standard logic alone. Resetting an E-stop must never restart the machine by itself.
Worked example
A 4–20 mA level transmitter is ranged 0–500 L on a card that reports 0–32767 counts. The card reads 16384. What is the level?
Fraction of span = (16384 − 0) ÷ (32767 − 0) = 0.500. Current = 4 + 16 × 0.500 = 12 mA. Level = 0.500 × 500 = 250 L. Reasoning: convert raw counts to a fraction first, then map to engineering units. The 20% offset between counts and 4 mA matters when the card reports under-range, which is how you detect a failed transmitter.
Traps that cost marks
- Assuming zero counts equals 0% and 0 mA.
- Performing scaling in integer math and losing resolution.
- Expecting an output to change in the same scan as its input.
- Placing safety functions in standard ladder logic.