Learning outcomes
- Explain the key principles of digital inputs and outputs.
- Carry out a structured practical check or configuration task.
- Recognise common faults, risks and evidence needed for diagnosis.
Module 6 · Lesson 2
Digital inputs and outputs
Wire, configure and test sourcing/sinking digital I/O, relay/transistor outputs and interposing relays.
Before you begin
I/O diagrams, test switches and safe output loads.
Safety: Use site procedures, approved test equipment and competent supervision. Never bypass a protection or interlock without formal control.
Step-by-step learning
- Confirm voltage, common arrangement and sourcing/sinking convention.
- Trace field power through the device to the input terminal and common.
- Force or simulate signals only under an approved test plan.
- Verify output command, module indicator, terminal voltage and final-device action.
Ian's practical tips:
- An LED on the PLC output proves only part of the circuit.
- Use interposing relays where isolation, load current or maintainability requires them.
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.