Learning outcomes
- Explain the key principles of plc architecture and scan cycle.
- Carry out a structured practical check or configuration task.
- Recognise common faults, risks and evidence needed for diagnosis.
Module 6 · Lesson 1
PLC architecture and scan cycle
Understand processor, memory, scan, tasks, watchdogs, modules and deterministic control behaviour.
Before you begin
PLC rack or simulator and hardware manual.
Safety: Use site procedures, approved test equipment and competent supervision. Never bypass a protection or interlock without formal control.
Step-by-step learning
- Identify CPU, power supply, communications and I/O modules.
- Explain input update, logic execution, output update and communications within the scan.
- Check task periods, watchdog settings and CPU loading.
- Relate scan behaviour to fast pulses, interlocks and alarm timing.
Ian's practical tips:
- A PLC input can change between scans; use high-speed hardware where pulse capture matters.
- Avoid assuming program order is the same as electrical current flow.
Fault-finding questions
- What should be happening, and what evidence proves it?
- Where is the last known correct signal, voltage, state or process condition?
- What changed immediately before the fault?
- Can the system be divided into smaller testable sections?
Practical activity
Create a one-page test record for this lesson. Include expected values, measured results, equipment used, risks controlled, defects found and the final proven condition.