AC

Alternating current. Current that reverses direction at regularly recurring intervals of time. PLCs may require DC or AC power, depending on the application.

analog I/O devices

An input/output device that recognizes a variable quantity, as opposed to a binary signal. Examples of variable quantities include temperature, pressure, and rate of flow.

AS-i

AS-Interface. A communication network that allows actuators and sensors to communicate with controllers.

AS-Interface

AS-i. A communication network that allows actuators and sensors to communicate with controllers.

backplane

A part of a PLC rack that provides communication paths for installed modules.

byte

Eight consecutive bits. A bit is a single 1 or 0.

Central Processing Unit

CPU. The main device that processes information and carries out instructions in a PLC. Also known as the CPU, processor, or controller.

closed-loop control

A type of control system that automatically changes the output based on a feedback signal. Special purpose PLC modules are often used for closed-loop control.

communication modules

Modules that connect the PLC to a network or other type of communication link.

control algorithm

The set of rules or calculations that a control system uses.

CPU

Central Processing Unit. The main device that processes information and carries out instructions in a PLC. Also known as the CPU, processor, or controller.

DC

Direct current. Current formed when electrons flow in one continuous direction. PLCs may require DC or AC power, depending on the application.

DIN rail

A metal track or rail to which PLCs and other control devices can be easily attached or removed.

discrete input devices

An input device that recognizes only binary on/off signals. A pushbutton is an example of a discrete input device.

discrete input module

An input module that recognizes only binary on/off signals.

discrete output devices

An output device that recognizes only binary on/off signals. A light is an example of a discrete output device.

discrete output module

An output module that recognizes only binary on/off signals.

EEPROM

Electrically-erasable programmable read only memory. A non-volatile memory type that allows stored information to be easily changed. SIMATIC memory cards are an example of this type of memory.

electrically-erasable programmable read only memory

EEPROM. A non-volatile memory type that allows stored information to be easily changed. SIMATIC memory cards are an example of this type of memory.

electrostatic discharge

A discharge of static electricity that can potentially cause injury or damage components.

embedded controllers

A type of PLC which consists of a combination of hardware and software preconfigured for various tasks.

EPROM

Erasable Programmable Read Only Memory. A secure memory type in which programs are protected from unauthorized changes. EPROM can be easily read from, but, once programmed, cannot be changed without special equipment.

erasable programmable read only memory

EPROM. A secure memory type in which programs are protected from unauthorized changes. EPROM can be easily read from, but, once programmed, cannot be changed without special equipment.

ethernet

A common local area network (LAN) that is deployed in a factory in the form of Industrial Ethernet. The most widely used form of Industrial Ethernet is PROFINET.

field busses

Industrial networks used for distributed control. Field busses were developed by manufacturers to address the need for common approaches to PLC communications.

firmware

Software stored in EPROM memory.

flash memory

A common type of electrically-erasable programmable read only memory or EEPROM.

giga-

A prefix meaning one billion.

gigabyte

1024 x 1024 x 1024 or approximately 10⁹ bytes.

hardware

The physical equipment used in a computer system. PLC hardware includes the CPU, power supply, input/output section, and programming device.

I/O modules

Input/output modules. PLC modules used to connect input and output devices.

I/O points

Input/output points. The places on input/output modules or a CPU that connect to the input/output devices. Each point, sometimes referred to as a channel, connects to a single input or output device.

information technology

IT. A blanket term for computer systems and services.

input output modules

I/O modules. The part of a PLC where input devices are connected.

input output points

I/O points. The places on input/output modules or a CPU that connect to the input/output devices. Each point, sometimes referred to as a channel, connects to a single input or output device.

integrated circuit

A complete electronic circuit within a single component. Its name resulted from the integration of transistors, resistors, and capacitors, all on a single chip.

IT

Information technology. A blanket term for computer systems and services.

kilo-

A prefix meaning one thousand.

kilobyte

1024 bytes or approximately 10³ bytes.

LANs

Local area network. A LAN is a communication system designed for private use in a limited area. LANs are used in office areas as well as in manufacturing environments; however, LANs used in industrial applications must be able to operate reliably in conditions that are unsuitable for office-grade equipment.

LED

Light Emitting Diode. A semiconductor device that emits light and is often used as an indicator on electronic equipment.

LED status indicators

LED lights on a device or module or that indicate either the status of the device or module or its channels.

light emitting diode

LED. A semiconductor device that emits light and is often used as an indicator on electronic equipment.

load current power supply

Load power supply. Provides power to input and output devices and related circuits.

load memory

Non-volatile storage for the user program, program data, and configuration information. When a project is first loaded into the CPU, it is stored in the load memory.

load power supply

Load current power supply. Provides power to input and output devices and related circuits.

local area networks

LAN. A LAN is a communication system designed for private use in a limited area. LANs are used in office areas as well as in manufacturing environments; however, LANs used in industrial applications must be able to operate reliably in conditions that are unsuitable for office-grade equipment.

mega-

A prefix meaning one million.

megabyte

1024 x 1024 or approximately 10⁶ bytes.

memory map

A table or listing showing how a computer’s memory is organized.

microprocessor

An integrated circuit that executes computer software. PLC central processing units and many other modules include one or more microprocessors.

modular controllers

A type of controller, such as a PLC, that is assembled from modules allowing the controller to have a structure that is meets the needs of an application.

Modular SIMATIC controllers

The modular PLC controllers from Siemens SIMATIC PLC line. Modular controllers are the most commonly used PLCs.

module address

A module’s address is part of a module’s configuration and is needed to allow communication between modules.

network ports

Ports on a PLC to connect the PLC to a network or other devices.

non-volatile

Indicates that a type of memory retains its data even in the event of a power loss.

open standards

Standards that allow any company to design a device, process, or protocol. Open standards in PLCs have allowed numerous companies to develop devices that communicate with each other.

operating modes

The various modes of a PLC CPU available for PLC operation. For example, the PLC must be in the RUN mode to execute the user program.

PC-based controllers

A type of controller, such as a PLC, that uses a PC as the central controller with some specialized software. Siemens PC-based controllers use the SIMATIC WinAC software.

PI

Proportional-integral control. One of the most common types of process control. Proportional integral control adjusts system outputs by reacting to two values, one that is proportional to an error, one that represents the error over time.

PID

Proportional-integral-derivative control. One of the most common types of process control. Proportional, integral, derivative control adjusts system outputs by reacting to three values, one that is proportional to an error, one that represents the error over time, and one that represents the rate of change of the error.

PLC

Programmable logic controller. A type of industrial computer used to control machines and processes.

positioning control

A type of control system designed for moving objects to a precise position.

power budget

The amount of power available from a power supply.

PROFIBUS DP

An open standard of PROFIBUS & PROFINET International that allows control-level devices in factory automation to communicate with each other.

PROFIBUS PA

An open standard of PROFIBUS & PROFINET International that allows a variety of control-level devices to communicate in automated processes.

PROFINET

An open standard of PROFIBUS & PROFINET International that allows both control and IT communication. PROFINET is the most widely used form of Industrial Ethernet.

programmable logic controller

PLC. A type of industrial computer used to control machines and processes.

proportional-integral control

PI. One of the most common types of process control. Proportional integral control adjusts system outputs by reacting to two values, one that is proportional to an error, one that represents the error over time.

proportional-integral-derivative control

PID. One of the most common types of process control. Proportional, integral, derivative control adjusts system outputs by reacting to three values, one that is proportional to an error, one that represents the error over time, and one that represents the rate of change of the error.

proprietary network

A network designed by a manufacturer for use by that manufacturer’s products.

RAM

Random Access Memory. A temporary storage area in which data can be immediately stored and accessed. RAM is also called read/write memory.

random access memory

RAM. A temporary storage area in which data can be immediately stored and accessed. RAM is also called read/write memory.

read only memory

ROM. Stores programs and data that can be read from but not changed. ROM is used for programs and data that define the operation of a system and don’t need to change.

retentive memory

Non-volatile storage for a limited quantity of work memory values that can remain stored during power loss.

ROM

Read Only Memory. Stores programs and data that can be read from but not changed. ROM is used for programs and data that define the operation of a system and don’t need to change.

RUN mode

The operating mode in which the CPU executes the user program repeatedly.

software

The coded instructions or programs that control computer hardware functions and operations.

solid-state memory

Memory that functions by means of purely electronic components without the use of moving parts.

special purpose modules

Modules that allow PLCs to perform specific applications. For example, some special purpose modules allow PLCs to perform closed-loop control applications.

STARTUP mode

The operating mode in which the CPU executes any startup logic, if present.

STOP mode

The operating mode in which the CPU is not executing a program. A new program can be loaded into the CPU during STOP mode.

system power supply modules

PLC components that convert external power to the filtered, low-voltage DC for modules that do not have an internal power supply.

volatile

Indicates that a type of memory loses its stored data in the event of a power loss.

work memory

Volatile storage for some elements of the user program. The CPU copies these elements from load memory into work memory during program execution.