- Describe PLC architecture and the scan cycle sequence
- Identify ladder logic instructions including XIC, XIO, OTE, and OTL
- Explain timer and counter functions including retentive types
- Compare DCS and SCADA architectures and describe hot backup
Leçon 1
PLC Architecture & Scan Cycle
PLC Components
A Programmable Logic Controller (PLC) consists of a processor (CPU), power supply, input modules, output modules, and a communication interface. The PLC executes a stored program to read inputs, make logical decisions, and control outputs.
The Scan Cycle
The PLC scan cycle is the fundamental operating sequence. The purpose of the scan cycle is to read inputs, execute logic, and update outputs. This happens continuously in a repeating loop.
Input Scan
Read all input modules and store values in input image table
→
Program Execution
Execute ladder logic from first to last rung
→
Output Update
Write output image table values to physical outputs
→
Housekeeping
Communications, diagnostics, then repeat
1-10 ms
Typical Scan Time
3 Phases
Input - Execute - Output
Key Takeaway
The PLC scan cycle reads inputs, executes the program logic, and updates outputs in a continuous repeating sequence. Scan times are typically 1-10 milliseconds.