P

parameter initial value attribute
The initial value is an attribute of a sequencing parameter or step tag contains the data stored into the value attribute when a sequence is initialized. A sequence is initialized when it is commanded to RESET or STARTED, depending on the configuration of the sequence.
parameter value attribute
An attribute that is the current value of a sequencing input parameter, sequencing output parameter, step input tag, or step output tag. The Value attribute is assigned data from one of these sources:
  • The initial value when the sequence is initialized.
  • An expression, when the expression is evaluated by the executing sequence program.
  • Assigned a value by the operator or control engineer.
parity bit
An additional non-data bit attached to a binary word. Its purpose is to provide a check of the data integrity by making the sum of the number of ones in a word always even or odd.
PCCC
Programmable Controller Communication Commands. An application-level command set that Allen-Bradley programmable controllers use to communicate across networks.
pending edit
A change to a routine that has been made in the
Logix Designer
 application, but has not yet been communicated to the controller.
In the ladder editor, pending edits are indicated with a lower case letter (e, I, or r) to the left of the rung. Pending edit rungs are not executed.
In the function block diagram editor, the Pending Edits view displays any pending edits.
period
The length of time for a cyclical operation to complete one full cycle. For example, the length of time from one point in a cyclical wave form to the same point in the next cycle of the waveform.
periodic task
A task executed at a fixed interval, ranging from .1 msec to 2,000,000 msec, in increments of 1 msec.
Periodic tasks perform a function according to a specific rate. When the end of this specified time period is reached, the task is triggered. When triggered, the task interrupts any lower priority task (either continuous, periodic, or event), and all programs assigned to the task are executed or scanned once, from top to bottom. After this single scan, an output update is triggered and control is returned to the task that was interrupted, at the point at which it was interrupted.
Tip:
Multiple periodic tasks may exist on a controller, where each task is configured to operate at independent rates.
phantom operand
A type of operand that represents the value of a tag.
Phase failure
An Equipment Phase failure is when the Equipment Phase of an active step within an Equipment Sequence executes a Phase Failure (PFL) instruction. The instruction includes a parameter set by the user to identify the failure.
PID
A structure data type that contains status and control information for a PID instruction.
pitch of the ball screw
The amount of linear motion that results from each turn of the screw. The pitch is usually given in inches or millimeters per revolution.
PLC/SLC
An Allen-Bradley programmable controller.
PLC/SLC tag mapping
Tables that enable legacy devices to communicate to a controller as if it were a PLC-2, 3, or 5 or an SLC processor. Thus, these devices can be used in the new system without update.
polarity
The direction of normally positive feedback or output. The polarity compensates for having wiring or the physical devices mounted backwards from the normal direction of the machine.
port
An opening in an enclosure, housing or block.
position error
The difference between the command position and actual position.
position loop
A feedback control loop in which the controlled parameter is Mechanical Position. The position loop compares position feedback with the position command to modify the velocity output signal to correct for any position error.
position transducer
A feedback element (for example, encoder or resolver) that measures incremental or absolute positions and converts this measurement into a feedback signal convenient for transmission.
post-scan
The function of the controller whereby the logic within a program is examined before disabling it in order to reset instructions and data.
Power rail
The right or left sides of a rung, which are the vertical lines on a graphical rung.
The position of the left power rail is fixed for all rungs. The position of the right power rail is not fixed because the number of instructions may vary from rung to rung. When
Logix Designer
 is online and in Run mode, rungs in the running controller are highlighted in green for programs or tasks that are not inhibited. For inhibited programs or tasks, the rungs are simple blue lines.
Power-up Handler program
A program that can be configured to handle startup execution after an abnormal loss of power in Run mode.
powerup fault
A major recoverable fault generated by the controller if it powers-up in run or test mode and it has a Power-up Handler program configured. The Power-up Handler program must clear the fault or the controller will power up in Faulted mode.
predefined data type
An atomic or structure data type that is defined by the controller.
prescan
The function of the controller whereby the logic within a program is examined prior to execution. This allows for instruction and data initialization.
primary control chassis
A control chassis that is designated as the one to perform control functions. The primary control chassis may or may not have a secondary control chassis.
primary/secondary DNS server address
This configuration option allows a server to accept a text-based address and perform a database lookup to convert that address to an IP address that can be used by the network. If a message uses a host name address, it must be converted somehow to an IP address before it can be sent to the destination controller. When the controller has a DNS server address configured, then the DNS server is used to take the host name address and convert it to the IP address of the destination module and the message can be sent. The two fields are set to 0.0.0.0 by default.
Each octet of the IP address must be in the range of 0…255. The first octet cannot be 127 or greater than 223.
priority
The order of task execution. If two tasks are triggered at the same time, the task with the higher priority will execute first. Priorities range from 1 to 15 with 1 being the highest priority. If two tasks with the same priority are triggered at the same time, the controller switches between them every millisecond. A continuous task runs at a fixed priority lower than all other tasks in the controller.
Tip:
If you are using the SoftLogix 5860 controller, there are only 3 priority levels available for periodic tasks.
process variable (PV)
A variable representing the current states or value of the process being controlled in a feedback control loop.
produce
To make a controller tag value available to a remote controller.
produced tag
A tag that a controller makes available for use by other controllers. Produced tags are always at controller scope.
producer/consumer model
A communication model in which data is identified by its content rather than by its source or destination. Devices that need the data (consumers) recognize the data they need and consume it. Therefore, data only needs to be sent out on the network in a single message, no matter how large the number of nodes to which it needs to go.
program
A program executes a set of instructions in a project.
program fault
A controller fault that occurs as the result of instruction execution. For example, an operand that is out of range will cause a program fault (as will division by zero) or an overflow. Program faults can trigger the execution of a fault routine if one is configured for the program.
program file
A PLC-5/SLC term. (See routine.)
Program mode
The controller mode during which logic is not executing, outputs are not controlled, and editing operations are available. If the keyswitch is in Remote, the controller is in Remote Program mode.
program parameter
A variable that is exposed for external access by a program and that is designated as an Input, Output, InOut, or Public parameter.
program scan
A part of the controller’s operating cycle. During the scan, the ladder program is executed; the output tag data is updated based on the program and the input data file.
program scope
The data accessible only within the current program. Each program contains a collection of tags that can only be referenced by the routines and aliases in that program.
program tag
A tag at program scope.
programmable controller
A solid-state control system with a user-programmable memory for storage of instructions to implement specific functions such as: I/O control, logic, timing, counting, report generation, communication, arithmetic and data file manipulation. A controller consists of a central processor, input/output interface and memory. A controller is designed as an industrial control system.
project communications path
A communications path is stored in each project. When you create a project, the path is set using the current communications path; you can change this path from the>Who Active dialog.
NOTE:
If the path is not set correctly, you can simply open the project file and select any of the communications commands (i.e., Go Online, Upload, or Download), without having to select the controller in the Who Active dialog each time. Since communications paths are generally specific to a workstation, this method is effective only if the project file is stored on your workstation.
If the file is on a network server, the path stored in the project file will probably be incorrect for all but one workstation, so each time you wish to go online from a different workstation, you'll have to select the controller from within the Who Active dialog.
project component
The building blocks of a project. You create, delete, organize, configure and edit components using the facilities provided by
Logix Designer
. Project components include:
  • Tasks
  • Programs
  • Routines
  • Modules
  • Data Types
  • Tags
  • Axes
  • Motion Groups
  • Messages
project documentation
Project documentation that supports multiple translations includes these variables:
  • Component descriptions in tags, routines, programs, equipment phases, user-defined data types, and Add-On Instructions
  • Engineering units and state identifiers added to tags, user-defined data types, or Add-On Instructions
  • Trends
  • Controllers
  • Alarm Messages (in configuration of ALARM_ANALOG and ALARM_DIGITAL tags)
  • Tasks
  • Property descriptions for module in the Controller Organizer
  • Rung comments, SFC text boxes, and FBD text boxes
project file
The file on your hard drive that the
Logix Designer
 application uses to store a controller's programming and configuration information. The file extension for this file must be .ACD; if you change the extension using a utility like Windows Explorer, the application is not able to open the file.
proportional control
See proportional, integral, derivative control (PID).
proportional, integral, derivative control (PID)
An intelligent I/O module or program instruction providing automatic closed-loop operation of process control loops. For each loop, this module or instruction can perform proportional control and, optionally, integral control, derivative control, or both. The results are as follows:
  • Proportional control causes an input signal to change as a direct ratio of the error signal variation.
  • Integral control causes an output signal to change as a function of the integral of the error signal over time duration.
  • Derivative control causes an output signal to change as a function of the rate of change of the error signal.
protected routine
A routine that cannot be viewed, printed, or imported/exported. Protected routines are displayed as disabled in the Controller Organizer.
protocol
The packaging of information transmitted across a network.
public parameter
A parameter that operates like a local program scoped tag, but other parameters and controller scope tags can communicate with it through connections and bindings. A Public parameter is meant to be an encapsulated version of a controller scope tag, and typically it is used for large data structures that need to be shared among programs.
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