PLC Fundamentals
Core concepts and hardware components of programmable logic controllers
B
3 terms
Backplane
The physical communication bus in modular PLCs that connects the CPU, power supply, and I/O modules.
Base Unit
Main PLC unit containing CPU and minimum I/O, expandable with additional modules.
Boot Sequence
Startup process initializing PLC hardware and loading program from memory.
C
4 terms
Compact PLC
All-in-one PLC with fixed I/O configuration, suitable for small to medium applications.
Configuration Mode
PLC state for downloading programs and setting parameters without execution.
Control Cabinet
Enclosure housing PLC, power supplies, and electrical components with environmental protection.
CPU (Central Processing Unit)
CPU
The main processor in a PLC that executes program logic, manages memory, and coordinates I/O operations.
D
4 terms
Data Table
PLC memory area for storing program variables, timer/counter values, and intermediate calculations.
Deterministic Control
PLC characteristic ensuring consistent, predictable response times critical for real-time control applications.
DIN Rail
Standard 35mm metal rail for mounting industrial control equipment including compact PLCs and I/O modules.
Distributed I/O
Network-connected I/O modules placed near field devices to reduce wiring costs and improve flexibility.
E
2 terms
F
3 terms
Fault Tolerance
PLC capability to continue operation despite hardware failures through redundancy and error handling.
Field Device
Instruments and actuators installed in the process area connected to control system.
Firmware
Low-level software stored in PLC non-volatile memory that controls hardware operations and provides system services.
I
3 terms
I/O Module
Interface cards that connect field devices (sensors/actuators) to the PLC, converting between field signals and processor data.
Input Image Table
Memory area storing snapshot of physical input states read during input scan phase.
Input Scan
First phase of scan cycle where PLC reads all physical input states and stores them in input image table.
M
2 terms
O
2 terms
P
5 terms
PAC (Programmable Automation Controller)
Advanced controller combining PLC functionality with PC features and open standards.
PLC (Programmable Logic Controller)
PLC
An industrial digital computer designed for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures.
Power Supply
Converts AC mains voltage to DC voltages required by PLC modules, typically providing 24VDC for I/O and 5VDC for logic.
Program Execution
Middle phase of scan cycle where PLC processes user program logic using input data to determine outputs.
Program Memory
Non-volatile storage area containing user program, typically stored in Flash or EEPROM.
R
7 terms
Rack
The mounting chassis that holds PLC modules and provides electrical interconnection through the backplane.
Real-Time Control
System capability to respond to inputs and execute control logic within strict time constraints.
Redundancy
Duplicate hardware configuration (CPU, power, network) ensuring continued operation if primary component fails.
Remote I/O
I/O modules located away from main PLC rack, connected via communication network to extend system reach.
Retentive Memory
Battery-backed or non-volatile memory that preserves data values during power loss.
Run Mode
Normal PLC operating state executing program and controlling outputs.
Runtime
Operating system environment executing PLC program and managing resources.
S
5 terms
Safety Integrity
Probability that safety system performs required functions under stated conditions.
Scan Cycle
The continuous loop where a PLC reads inputs, executes program logic, updates outputs, and performs housekeeping tasks, typically completing in milliseconds.
Scan Time
The time required for one complete PLC scan cycle, typically 1-100ms depending on program complexity.
Soft PLC
PC-based control system running PLC runtime software on standard computer hardware.
System Memory
Reserved memory for PLC operating system, diagnostics, and system configuration parameters.