What you will learn
Explain PLC architecture and the scan cycle; wire and scale digital and analog I/O; build interlocks, timers and counters in ladder logic.
Course syllabus
- 1. PLC architecture
Rack, CPU, power supply and I/O · The scan cycle
- 2. Digital I/O
Digital inputs: sinking, sourcing and diagnosis · Digital outputs and load types
- 3. Analog I/O
Analog inputs and 4-20 mA scaling · Analog outputs and final control elements
- 4. Tags and program organisation
Tags, data types and naming
- 5. Ladder logic
Contacts, coils and rung logic · Interlocks and permissives
- 6. Timers and counters
Timers, counters and sequence steps
Free lesson · 18m
Rack, CPU, power supply and I/O
A PLC system consists of a power supply, a CPU, I/O modules and a communication path, usually mounted on a rack or DIN rail inside a control cabinet.
The power supply converts plant supply to the internal logic voltage and often provides field power. The CPU holds the program and executes it cyclically. I/O modules translate between field signals and CPU memory. Communication modules connect the CPU to HMIs, other controllers and engineering workstations.
When troubleshooting, always identify the boundary you are working on: field device, field wiring, terminal or marshalling, I/O module, CPU memory, or application logic. Most wasted commissioning time comes from testing at the wrong boundary.
Power supply, CPU, I/O and communications; the CPU executes cyclically; identify the correct troubleshooting boundary before testing.