PLC Interview Prep

A PLC repeats a fixed cycle: (1) read all physical inputs into the input image table, (2) execute the user program rung-by-rung top-to-bottom, left-to-right, writing results to the output image table, (3) update the physical outputs from that table, then housekeeping/comms — and repeat. Outputs do not change mid-scan, which is why scan order matters and why a coil written low later in the program overrides an earlier rung.

For fail-safe operation. A normally-closed (NC) Stop is examined with an XIO instruction so the rung is true when the button is not pressed. If the wire breaks or a terminal comes loose, the circuit sees an open — the same as a press — and the machine stops. A normally-open Stop would fail dangerously: a broken wire would leave the machine running with no way to stop it.

A seal-in is a parallel holding contact of the output placed around the momentary Start contact. Once Start energises the coil, the output’s own contact keeps the rung true after Start is released; an NC Stop in series breaks it. It is the basis of three-wire motor control and appears in nearly every real control program.

A TON (on-delay) starts timing when its rung goes true and sets its Done bit when accumulated time reaches the preset; if the rung goes false the accumulator resets to zero. A TOF (off-delay) keeps its output on for the preset after the rung goes false. An RTO (retentive) holds its accumulated value when the rung goes false and needs a separate RES instruction to reset — used where elapsed time must survive power or logic interruptions.

Work systematically rather than randomly: check the PLC and fault indicators, look at the program status (which rung is not making true), then divide and conquer — confirm the input the rung needs is present (force/monitor it), trace back to the field device with a meter (24 VDC present? sensor switching? wiring intact?). Isolate to the smallest failing element before changing anything, then verify the fix.

A normally-open contact (XIC, "examine if closed") passes power when its referenced bit is 1/true. A normally-closed contact (XIO, "examine if open") passes power when its bit is 0/false. The contact symbol describes how it behaves in the program, which is independent of whether the physical field device is wired NO or NC — a common source of confusion that interviewers probe.