PLC/SCADA and DCS
PLC/SCADA and DCS tests how control logic is executed, how operators interact with the plant and how alarms and historical data are managed. The exam distinguishes PLC/SCADA architecture from integrated DCS architecture and probes scan-time, alarm and redundancy behaviour.
Core rules and concepts
- A PLC executes a cyclic scan: read inputs, solve logic, update outputs, then service communications and diagnostics. Scan time affects response to short events; a pulse shorter than one scan may be missed unless a high-speed input, interrupt or latch is used.
- IEC 61131-3 defines ladder diagram, function block diagram, structured text, sequential function chart and instruction list. Ladder logic implements seal-in circuits, timers and counters; structured text suits calculations and complex logic.
- Avoid double coils, race conditions and reliance on rung order across programs. Use one owner per output and defined initialisation. Forcing and simulation must be controlled and removed before handover.
- SCADA systems use RTUs or PLCs, a tag database, HMI, historian and alarm server. DCS systems distribute controllers and integrate process control, operator stations and history in one architecture, often with redundant controllers and networks.
- Alarm management follows ISA-18.2: rationalise alarms, set priorities, use deadbands and delay, suppress nuisance alarms and manage shelving. HMI design follows ISA-101: prioritise abnormal situations and avoid colour-only coding.
- Redundancy must be tested by failure injection; a redundant pair that does not transfer cleanly is not redundant.
Worked example
A PLC scan is 20 ms. A limit switch closes for 5 ms. If the pulse occurs just after the input scan, it may be missed entirely. The fix is not simply to reduce the scan time; use a high-speed counter input, an interrupt routine or a hardware latch. If the event must be counted, specify the input frequency and module capability, then prove it with a FAT test.
Traps
Assuming scan time is the same as I/O response time; using the same output coil in two rungs; leaving forces active; confusing an alarm summary with alarm rationalisation; and designing HMI screens that rely on colour alone.