PLC Glossary

Question 1

A character and number combination that identifies a particular and unique area of memory.

Answer 1

Address

Question 2

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.

Answer 2

Analog

Question 3

Binary digit, the smallest particle of data. A bit can have a value of 0 or 1.

Answer 3

Bit

Question 4

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”.

Answer 4

Branch

Question 5

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.

Answer 5

Data Table

Question 6

A group of bits: Typically a PLC’s word is composed of 16 bits.

Answer 6

Data Word

Question 7

Only two possible states, on or off. Discrete I/O are switches, lights, etc.

Answer 7

Discrete

Question 8

To copy a program into the PLC memory from a computer of storage device.

Answer 8

Download

Question 9

A PLC instruction which is logically true whenever the bit that is addressed to examine is a logic 0 (off).

Answer 9

Examine If Off (Examine If Open) XIO -]/[-

Question 10

A PLC instruction which is logically true whenever the bit that is addressed to examine is a logic 1 (on).

Answer 10

Examine If On (Examine If Closed) XIC -] [-

Question 11

Inputs and Outputs, real word connections between field devices and the PLC.

Answer 11

I/O

Question 12

A section of the data table associated with real (hardware) inputs and outputs.

Answer 12

Image table

Question 13

An internal command to the PLC program to read from or write to (modify) a value in the PLC memory.

Answer 13

Instruction

Question 14

The most common method of programming PLC’s. Derived from and used because of the similarity to relay logic schematics.

Answer 14

Ladder Logic

Question 15

Master Control Relay, a relay used to shut off power to the I/O.

Answer 15

MCR (CRM)

Question 16

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.

Answer 16

Offline

Question 17

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.

Answer 17

Online

Question 18

A light activated device used to electrically isolate the higher voltages of the Inputs and Outputs from the logic level voltages of the processor.

Answer 18

Optocoupler (Optical Coupler)

Question 19

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.

Answer 19

Output Instruction -( )-

Question 20

The program that the PLC manufacturers sell which is used to create and monitor the PLC program.

Answer 20

Software

Question 21

To copy a PLC program from the PLC into the computer so it can be modified, documented or monitored.

Answer 21

Upload

Question 22

A program element associated with a hardware address or internal memory bit.

Answer 22

Variable (Tag)