9. Glossary
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Mapping
Variable addressing to an action, protocol, etc.
Menu
Set of options available and displayed by a program on video and that can be selected by the user to
activate or perform a certain task.
Master
Equipment connected to a communication network where the commands requests to the other network
devices originate.
Module (hardware)
Basic element of a complete system that has well defined functions. Normally the system is connected by
connectors and can be easily replaced.
Module (software)
Part of an application program capable of performing a specific function. It runs independently or in
conjunction with other modules, exchanging information via parameter passing.
I/O Modules
NET
Module belonging to the inputs and outputs subsystem.
Physical interface of the protocol communication CPU via Ethernet
Node
Any station network with an ability to communicate through an established protocol.
Operands
Elements on which the instructions work. They may represent constants, variables or set of variables.
Protocol
Rules of procedures and standard formats that through control signals, allow the establishment of a data
transmission and error recovery between equipment.
POU
Program Organization Unit. It is a subdivision of the applicative program which can be written in any of
the available languages.
Applicative Program
It’s the program loaded into a PLC, which determines the operation of a machine or process.
Communication
Network
Group of devices (nodes) interconnected by communication channels.
Deterministic
Communication
Network
Communication network where the transmission and reception of information between different nodes is
guaranteed with a maximum known time.
Master-Slave
Communication
Network
Communication network where information transfers are initiated only from a single node (network
master) connected to the data bus. The other network nodes (slaves) respond only when requested.
Multimaster
Communication
Network
Communication network where the information transference is initiated by any node connected to the
data bus.
RX
Acronym used to indicate serial reception.
Redundant System
System that contains Standby doubled elements to perform a certain task, which can tolerate certain
types of failure without damaging the task execution.
SNTP
Simple Network Time Protocol. Protocol for time synchronization via network.
Software
Computer programs, procedures and rules related to the operation of a data processing system.
Subnet
Segment of a communication network that interconnects a group of devices (nodes) in order to isolate the
local traffic or use different protocols or physical layer.
Tag
Name associated with a variable or a logic that allows a brief identification of its contents.
Time-out
Predetermined maximum time that a communication is completed. If exceeded retentive or diagnostic
procedures will be activated.
Hot swapping
Procedure for replacement of system modules without the need for de-energizing the same. Usually used
in exchanges of I/O modules.
TX
Acronym used to indicate serial transmission.
CPU
Abbreviation for central processing unit. Controls the information flow, interprets and executes program
instructions and monitors the devices in the system.
Active CPU
In a redundant system, the Active CPU performs the system control, reading the input points values,
executing the applicative program and driving the output values.
Non-Active CPU
CPU is inactive state (controlling the system) or in stand-by state (overseeing the Active CPU). It cannot
assume control of the system.
Redundant CPU
Corresponds to the other system CPU, e.g. the CPU1 in relation to CPU2 and vice versa.
Stand-by CPU
Remote Terminal Unity
In a redundant system, it refers to the CPU that oversees the Active CPU. It does not execute the system
control, but is ready to take control in case of failure on the Active CPU.
Also named as RTU. It is equipment which controls the applicative program. It is comprised by a CPU, a
power supply and an I/O structure.
Upload
PLC configuration or program reading.
Direct Representation
Variable
The variable which be accessed directly in the memory using the desired address. E.g. QB0%.
Symbolic Variables
IEC Variables created in POUs and GVLs during the applicative development, which are not addressed
directly in the memory.
WD
Watchdog.
Word
Information unit composed by 16 bits.
Execution Time
See Cycle Time.
Cycle Time
It is the time which the CPU takes to perform a given task.
Interval
It defines the period of time in which a particular task will run.
LCD
Acronym for Liquid Cristal Display.