Learning outcomes
- Explain the key principles of plc diagnostics, online changes and backups.
- Carry out a structured practical check or configuration task.
- Recognise common faults, risks and evidence needed for diagnosis.
Module 7 · Lesson 4
PLC diagnostics, online changes and backups
Diagnose CPU, I/O, logic and communications faults while controlling the risk of online work.
Before you begin
Approved laptop, current project and backup procedure.
Safety: Use site procedures, approved test equipment and competent supervision. Never bypass a protection or interlock without formal control.
Step-by-step learning
- Confirm the correct PLC identity and project version before going online.
- Upload or compare and preserve the running program before changes.
- Use watch tables and cross-references to locate the cause without uncontrolled forcing.
- Record changes, test results and rollback method.
Ian's practical tips:
- Never download merely because the software says the projects differ. Find out why.
- Forces and temporary bypasses must be logged, visible and removed before handover.
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.