| actuator |
An output device that changes the condition of a physical component of a machine or process. For example, some actuators can start and stop a motor or open and close a control valve. |
| analog I/O |
An input/output device or module that uses a variable signal, as opposed to an on or off (binary) signal. Variable signals are often used to communicate conditions in a machine or process, such as temperature, pressure, and rate of flow. |
| analog input device |
An input device that sends a PLC a variable current or voltage signal. |
| analog output device |
An output device that receives a variable current or voltage signal from a PLC. |
| AND |
A Boolean logic function that requires all inputs to be binary 1’s for the output to be a binary 1. |
| automation |
The use of self-regulated equipment, processes, or systems that meet manufacturing requirements with limited human intervention. |
| binary |
A number system that has a base of 2 and uses only two characters, 1 and 0. |
| bit |
A binary digit. The smallest unit of information on a computer. |
| Boolean algebra |
A type of mathematics that deals with the use of logical operators such as OR, AND, and NOT. |
| central processing unit |
CPU. The main device that processes information and carries out instructions in a PLC. Also known as the processor or controller. |
| communication port |
A port on a PLC that allows it to be connected to a specific type of device or computer network. |
| counter |
A type of instruction in a PLC program that counts items or events. |
| CPU |
Central Processing Unit. The main device that processes information and carries out instructions in a PLC. Also known as the processor, or controller. |
| discrete I/O |
An input/output device or module that recognizes only binary on/off signals. A pushbutton is an example of a discrete input device, and a light is an example of a discrete output device. |
| discrete input device |
An input device that recognizes only binary on/off signals. A pushbutton is an example of a discrete input device. |
| discrete output device |
An output device that recognizes only binary on/off signals. A light is an example of a discrete output device. |
| function block diagram |
FBD. A graphical PLC programming language defined in the IEC 61131 standard that uses a box format to represent instructions. |
| hardware |
The physical equipment used in a computer system. PLC hardware includes the CPU, power supply, input/output section, and programming device. |
| hard-wired control |
A method of controlling machines based on the interconnection of control devices instead of through program in a PLC. |
| HMI |
Human machine interface. A device connected to a PLC to allow operation and maintenance personnel to monitor and control a machine or process. |
| human machine interface |
HMI. A device connected to a PLC to allow operation and maintenance personnel to monitor and control a machine or process. |
| I/O points |
PLC connections for input devices such as switches and sensors and output devices, which include a variety of actuators. |
| IEC 61131 |
An international standard that defines important PLC characteristics such as programming languages. Part 3 of this standard identifies the most common PLC programming languages. Two of these languages are Ladder diagram (LAD) programming and function block diagram (FBD) programming. |
| input device |
A type of device, such as a switch or sensor, that sends information to a PLC’s input points. |
| input module |
PLC module that connects to input devices and communicates input device signals to the PLC’s CPU. |
| input point |
A connection point on a PLC for an input device such as a switch or sensor. |
| input/output module |
A module for a PLC that connects to input or output devices and converts the signals from the devices into binary for the CPU. |
| ladder diagram |
A representation of the logic of an electrical circuit or system using common symbols. The individual lines or "rungs" connected between two vertical lines resemble a ladder. |
| ladder logic |
A method for drawing electrical control diagrams. Ladder diagram PLC programming is sometimes referred to as ladder logic. |
| OR |
A Boolean logic function for which the output will be a logic 1 if one or more inputs is a logic 1. |
| output device |
A type of device that responds to signals from a PLC’s output points. |
| output point |
A connection point on a PLC for an output device. |
| programmable logic controller |
PLC. A processor controlled device that uses logic-based software to provide electrical control of machines and processes. |
| programming device |
A device, usually a personal computer, used to enter instructions into the PLC. |
| scan |
One complete cycle of a PLC’s cyclical operation that includes checking inputs, executing its programs, updating the status of its outputs, and performing various other required processes. |
| sensor |
A device that detects a change in a physical condition stimulus and turns it into a signal that can be measured or recorded. Some sensors are used as PLC input devices. |
| software |
The coded instructions or programs that control computer hardware functions and operations. |
| timer |
A type of instruction in a PLC program that times intervals or events. |