PLC Glossary Flashcards
A character and number combination that identifies a particular and unique area of memory.
Address
A mechanism in which data is represented by continuously variable physical quantities. Analog I/O converts PLC data into a continuous signal and vice versa.
Analog
Binary digit, the smallest particle of data. A bit can have a value of 0 or 1.
Bit
A possible alternate path for the flow of “control power” in ladder logic. Instructions that are placed on different branches with a common end point are said to be logically “OR’d”.
Branch
An area of memory which stores the status of individual bits or group of bits. A data table is a part of a PLC memory map.
Data Table
A group of bits: Typically a PLC’s word is composed of 16 bits.
Data Word
Only two possible states, on or off. Discrete I/O are switches, lights, etc.
Discrete
To copy a program into the PLC memory from a computer of storage device.
Download
A PLC instruction which is logically true whenever the bit that is addressed to examine is a logic 0 (off).
Examine If Off (Examine If Open) XIO -]/[-
A PLC instruction which is logically true whenever the bit that is addressed to examine is a logic 1 (on).
Examine If On (Examine If Closed) XIC -] [-
Inputs and Outputs, real word connections between field devices and the PLC.
I/O
A section of the data table associated with real (hardware) inputs and outputs.
Image table
An internal command to the PLC program to read from or write to (modify) a value in the PLC memory.
Instruction
The most common method of programming PLC’s. Derived from and used because of the similarity to relay logic schematics.
Ladder Logic
Master Control Relay, a relay used to shut off power to the I/O.
MCR (CRM)
Not actively monitoring the state of the program and I/O. A computer that is displaying a program that is not communicating with the PLC is said to be offline.
Offline
Actively monitoring the state of the program and I/O. A computer that is Online is updating the PLC program and status in real time.
Online
A light activated device used to electrically isolate the higher voltages of the Inputs and Outputs from the logic level voltages of the processor.
Optocoupler (Optical Coupler)
When on (energized) sets the data table bit it’s associated with to a 1. When off (de-energized) it sets the data table bit to a 0.
Output Instruction -( )-
The program that the PLC manufacturers sell which is used to create and monitor the PLC program.
Software
To copy a PLC program from the PLC into the computer so it can be modified, documented or monitored.
Upload
A program element associated with a hardware address or internal memory bit.
Variable (Tag)
