| Analog
| Any type of input or output that has more than two states; on and off (see Digital). An analog signal can vary in magnitude from “off” to a high-end value or between two non-zero values. An example of an analog device would be a level sensor that returns a voltage somewhere between 0 and 10 V that can vary over time. |
| Bit
| A single digit that only has two possible values – 0 or 1. Multiple bits can be combined to form bytes or words. |
| Boolean Math
| A general term used to describe several different types of comparative logic functions. Specific Boolean Math functions include, but are not limited to, AND, OR, XOR, etc. |
| Central Processing Unit (CPU)
| The main processor of information in your computer. This single chip performs all of the logic and math operations of the PLC. |
| Digital
| Any type of input or output signal that has exactly two states, on and off. An example of a digital device would be a pushbutton, which can either be pressed (ON) or released (OFF). |
| Expander
| A module connected to block I/O via a cable connection that increases the number of I/O controlled by a CPU. Expanders do not contain a CPU and therefor are often called “dumb I/O blocks.” |
| I/O
| Inputs and Outputs |
| Ladder Diagram
| The result of ladder programming used to control a PLC. The ladder language is modeled after relay wiring schematics. The fundamental theories behind ladder are consistent among all manufacturers. However, each PLC manufacturer generally has a proprietary ladder software package. |
| Logic
| A series of directives or boundaries created to allow a process to be controlled. Logic can be programmed via hard wiring (as is the case with relay logic) or via a PC (as is the case with a PLC). |
| Network
| Several devices connected together, through electrical means, for data acquisition and/or control. |
| Non-retentive
| All values are resent to zero after powering down the unit. |
| Off-Delay Timer
| Will turn an output OFF after X amount of seconds has passed. |
| On-Delay Timer
| Will turn an output ON after X amount of seconds has passed. |
| Operator Interface (O/I)
| A device that allows the operator of a machine to monitor and control devices attached to a PLC. |
| Register
| A storage area, within the PLC, for information. Registers can have a one or two (or more) word capacity. |
| Relays
| A type of switch that can control AC or DC loads. |
| Relay Circuits
| Devices often used in control. Can be opened and closed electronically to complete logic circuits. |
| Retentive
| Will store data in memory so that it remains intact after powering down the unit. |
| Sensor
| A sensing element. The basic element that usually changes some physical parameter to an electrical signal. |
| Solenoid
| A type of output device and a specific type of coil. Both coils and solenoids utilize voltage to convert electrical energy to mechanical energy via magnetic fields. A solenoid is an actual physical device, where as a coil is a generic description for any type of electrical output. |
| Starter
| A control device usually consisting of a contact and overload. With DeviceNet, it will also contain a communication module used for starting and stopping loads. |
| Transistors
| A solid-state, electronic switch. It is fast, switches a small current, has a long lifetime, and works with DC only. |
| Triacs
| Or silicon controlled rectifiers (SRCs) act as a mediator between the PLC and the AC output device. The triac or SCR functions as a switch that responds to the commands of the PLC logic. |
