PLC Fundamentals
What does the acronym PLC stand for?
Programmable Logic Controller
These modules handle devices with only two states, like pushbuttons.
Discrete I/O.
This process uses light to electrically isolate the PLC from field devices.
Optical Isolation
This is the "brain" of the PLC that executes the program.
Central Processing Unit (CPU).
This memory area is where the technician’s ladder logic is stored.
User Memory.
PLCs were originally designed to replace these hard-wired systems.
Electromechanical Relay Systems.
This type of module handles variable signals like temperature or pressure.
Analog I/O.
In this DC circuit type, the I/O module connects to the positive power side.
Sourcing.
These are the three main steps of a single PLC scan cycle.
Read Inputs, Execute Program, and Update Outputs.
This memory area stores the status of I/O bits and timer values.
Data (or Storage) Memory.
Name the 4 main components of any typical PLC system.
CPU (Processor), I/O Section, Power Supply, and Programming Device.
In this PLC type, the processor and I/O are fixed in one single package.
Fixed I/O.
In this DC circuit type, the I/O module connects to the negative (common) side.
Sinking.
This type of volatile memory is lost when power is removed.
RAM (Random Access Memory).
This non-volatile memory can be erased and rewritten electrically.
EEPROM (or Flash).
This is the ability to change a machine's operation without rewiring.
Flexibility.
This PLC style allows you to add or swap modules in a rack/chassis.
Modular I/O.
This device is used between a PLC and a high-current load for protection.
Interposing Relay.
This type of non-volatile memory must be erased using UV light.
EPROM (Erasable Programmable Read-Only Memory).
This internal timer faults the PLC if a program scan takes too long.
Watchdog Timer.
This man/company is credited with developing the first PLC
Richard Morley / Bedford Associates (Modicon).
This component protects output module circuitry from overcurrent.
Fuse
This physical feature prevents the wrong module from being plugged in.
Module Keying.
This is the total time it takes the PLC to complete one full cycle.
Scan Time.
This term describes how the PLC organizes its internal memory locations.
Memory Map