PLC Fundamentals

The physical communication bus in modular PLCs that connects the CPU, power supply, and I/O modules.

Main PLC unit containing CPU and minimum I/O, expandable with additional modules.

Startup process initializing PLC hardware and loading program from memory.

All-in-one PLC with fixed I/O configuration, suitable for small to medium applications.

PLC state for downloading programs and setting parameters without execution.

Enclosure housing PLC, power supplies, and electrical components with environmental protection.

The main processor in a PLC that executes program logic, manages memory, and coordinates I/O operations.

PLC memory area for storing program variables, timer/counter values, and intermediate calculations.

PLC characteristic ensuring consistent, predictable response times critical for real-time control applications.

Standard 35mm metal rail for mounting industrial control equipment including compact PLCs and I/O modules.

Network-connected I/O modules placed near field devices to reduce wiring costs and improve flexibility.

Compact PLC integrated into equipment or machinery for dedicated control.

Additional I/O or specialty modules that extend PLC capabilities beyond base unit configuration.

PLC capability to continue operation despite hardware failures through redundancy and error handling.

Instruments and actuators installed in the process area connected to control system.

Low-level software stored in PLC non-volatile memory that controls hardware operations and provides system services.

Ability to replace PLC modules while system is running without disrupting control operations.

Interface cards that connect field devices (sensors/actuators) to the PLC, converting between field signals and processor data.

Memory area storing snapshot of physical input states read during input scan phase.

First phase of scan cycle where PLC reads all physical input states and stores them in input image table.

I/O modules installed in same rack or directly connected to PLC CPU without network communication.

Different storage areas in PLCs including RAM for data, ROM for firmware, and EEPROM/Flash for programs.

A PLC built from separate, rack-mounted modules — CPU, power supply, and I/O cards — that you size and expand per application, unlike a fixed-I/O compact PLC.

Memory that loses its contents when power is removed, typically used for temporary data.

Memory area storing output values calculated by program, written to physical outputs during output scan.

Final phase of scan cycle where PLC updates physical outputs based on output image table values.

Advanced controller combining PLC functionality with PC features and open standards.

An industrial digital computer designed for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures.

Converts AC mains voltage to DC voltages required by PLC modules, typically providing 24VDC for I/O and 5VDC for logic.

Middle phase of scan cycle where PLC processes user program logic using input data to determine outputs.

Non-volatile storage area containing user program, typically stored in Flash or EEPROM.

The mounting chassis that holds PLC modules and provides electrical interconnection through the backplane.

System capability to respond to inputs and execute control logic within strict time constraints.

Duplicate hardware configuration (CPU, power, network) ensuring continued operation if primary component fails.

I/O modules located away from main PLC rack, connected via communication network to extend system reach.

Retentive memory means PLC data memory that preserves its value across power cycles, processor restarts, and program transitions to STOP mode. Battery-backed RAM or non-volatile flash holds the data; non-retentive memory is cleared on power-up.

Normal PLC operating state executing program and controlling outputs.

Operating system environment executing PLC program and managing resources.

Probability that safety system performs required functions under stated conditions.

The PLC scan cycle is the continuous loop a PLC repeats every 1–100 ms: read all inputs → execute the user program top-to-bottom → write all outputs → housekeeping (communications, diagnostics). Inputs only update at scan start; outputs only change at scan end.

The time required for one complete PLC scan cycle, typically 1-100ms depending on program complexity.

PC-based control system running PLC runtime software on standard computer hardware.

Reserved memory for PLC operating system, diagnostics, and system configuration parameters.

Safety feature that monitors PLC operation and triggers fault mode if scan time exceeds preset limits.