Programmable logic controllers: programming, wiring, and troubleshooting
This domain tests ladder logic, I/O wiring, scan behaviour, and safe online diagnosis. Expect seal-in circuits, timers, counters, addressing, and input/output device checks.
Core rules
- The PLC repeats a scan: read inputs, execute the program, update outputs. Ladder rungs are evaluated left to right, top to bottom.
- A normally open (NO) contact is true when its bit is 1; a normally closed (NC) contact is true when its bit is 0. Field device NO/NC and program contact type are separate choices.
- A seal-in branch keeps a motor output on after a momentary start button is released; a stop button is wired or programmed as NC for fail-safe behaviour.
- TON delays turn-on; TOF delays turn-off; RTO retains accumulated time. Counters need reset logic.
- Forcing and online edits change behaviour; record and remove forces before handover.
Worked example
A motor output Y0 must start from X0 (NO pushbutton), stop from X1 (NC pushbutton), and seal in. The rung is: X0 parallel with Y0, in series with X1, driving Y0. Pressing X0 makes the path true; Y0 energises and its parallel contact keeps the path true after release. Pressing X1 opens the rung and drops Y0. If the motor does not start but the X0 input LED is on, check the program address, forced bits, and whether X1 is open. The reasoning: separate field wiring, input status, and program logic.
Traps
Assuming an input LED proves the program sees the right address; leaving a force active; using a program NC contact when the field device is also NC (double inversion); ignoring scan-time effects; and editing online without a backup. These are common causes of phantom faults.