Programmable Logic Controllers (PLC) - Lecture Flashcards

Vocabulary practice flashcards covering PLC architecture, history, logic gates, signal types, memory structures, addressing, and programming languages based on the lecture slides.

Programmable Logic Controller (PLC)

An industrial computer designed to automate and control machines and manufacturing processes by reading input signals, executing internal logic, and driving output devices.

Electromechanical Relays

Electrically controlled switches used prior to PLCs to build control systems, which required large, heavy, complex, and rigid wiring panels.

Dick Morley

The engineer considered the father of Programmable Logic Controller (PLC) technology who invented the first functional PLC for General Motors in the late 1960s.

Modicon 084

The first functional model of a Programmable Logic Controller created to replace electromechanical relay systems in manufacturing assembly lines.

Central Processing Unit (CPU)

The brain of the PLC that reads input signals, executes the stored user program logic, and sends actuation commands to the outputs.

Program Memory

Non-volatile memory (such as EEPROM, Flash, or programmable ROM) in a PLC that retains the user's main program logic even when power is turned off.

Volatile Data Memory (RAM)

Temporary memory in a PLC used during real-time program execution to store variable values, sensor signals, counter states, and timer states, which clears on power loss unless configured as retentive.

Memory Cards (External Memory)

Removable media (such as SD or MMC cards) used in PLCs for data backup, transferring programs between machines, or storing recipes, logs, and CSV files.

Digital Inputs

Discrete binary PLC input signals that have only two states (ON or OFF), provided by field devices like emergency stop buttons, limit switches, selector switches, and proximity sensors.

Digital Outputs

Discrete binary ON/OFF control signals sent by the PLC to operate field actuators like contactor coils, indicator lights, alarms, solenoid valves, or signal towers.

Analog Inputs

Continuous signals representing physical real-world measurements (such as temperature, pressure, or level) sent to a PLC, typically measured in $0\text{--}10\text{ V}$ or $4\text{--}20\text{ mA}$.

Analog Outputs

Continuous electrical signals generated by a PLC to control variable-speed or proportional equipment, such as variable frequency drives, proportional valves, or DC motors.

PLC Addressing

The unique syntax and identifier used by a PLC program to locate and read or write to specific physical inputs, outputs, memory bits, timers, or counters (e.g., %I, %Q, %M, %T, %C).

Bit

The smallest unit of digital information, capable of representing only two states: 0 (OFF / False / absence of signal) or 1 (ON / True / presence of signal).

Byte and Word

Data grouping units in PLC memory where 8 bits form 1 Byte (providing $2^8 = 256$ possible states) and 16 bits form 1 Word.

AND Logic Gate

A digital logic element that activates its output (1) only when all of its inputs are active (1), defined by the boolean equation $X = A \times B$.

OR Logic Gate

A digital logic element that activates its output (1) if at least one of its inputs is active (1), defined by the boolean equation $X = A + B$.

NOT Logic Gate

A logic inverter that flips the input signal, yielding an output of 1 when the input is 0 and an output of 0 when the input is 1 ($X = \overline{A}$).

NAND Logic Gate

An inverted AND logic gate that outputs 0 only when all inputs are 1, and outputs 1 in all other conditions ($X = \overline{A \times B}$).

NOR Logic Gate

An inverted OR logic gate that outputs 1 only when all of its inputs are 0 ($X = \overline{A + B}$).

XOR Logic Gate

An Exclusive-OR logic gate that produces an output of 1 only when its input values are different ($X = A \oplus B$).

XNOR Logic Gate

An Exclusive-NOR logic gate that produces an output of 1 when its input values are identical ($X = \overline{A \oplus B}$).

Analog Signal

A continuous electrical signal that smoothly varies within a given range to represent measurable physical variables such as temperature, tank level, or pressure.

Voltage Analog Signal (0--10V)

An analog signal standard commonly used over short distances inside electrical control panels, which is simple to measure but vulnerable to voltage drops over long cables.

Current Analog Signal (4--20mA)

A noise-immune analog signal standard preferred for long distances, where a current reading of $0\text{ mA}$ immediately alerts the PLC to a fault or broken wire.

Analog-to-Digital Converter (A/D)

An internal PLC hardware module that translates continuous analog voltage or current values (e.g., $7.8\text{ V}$) into discrete digital numbers for CPU execution.

Ladder Diagram (LD)

A graphical PLC programming language styled like electrical relay control schematics, featuring rungs with contacts and output coils.

Structured Text (ST)

A high-level, text-based PLC programming language similar to C or Pascal, using programming logic statements such as IF, WHILE, and FOR.

Function Block Diagram (FBD)

A graphical PLC programming language that connects functional logic blocks (like timers or comparators) with signal lines, widely used in continuous industrial process control.

Normally Open (NO) Contact

A Ladder logic input element that remains open (0) by default and closes to pass power when its associated input or memory bit turns ON (1).

Normally Closed (NC) Contact

A Ladder logic input element that remains closed (1) by default and opens to interrupt power flow when its associated input or memory bit turns ON (1).