Basics of Siemens PLCs 200

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.