Chapter 03 Introduction to PLC Programming Slide set 04

1. Chapter 03 Sect. 3-5-4,
3-6-4, 3-7-2
Tags and Memory Structure - ControlLogix

2. Tag Based Memory
 Tag based memory structure
 Tag based memory structures are used in most
PLC/PAC platforms produced in the last 10-
years.
 A tag is a friendly name for a memory address. In
languages such as C++, VB.NET, Java and many
others, the term ‘variable’ is used. The control
industry calls these ‘variable’s’, tags.
 As an example: a switch used to select the Manual
Control function of a machine could have a tag named
‘manualControl’. This tag name would point to a
memory location and in this case would be assigned
the data type of BOOL.

3. Tag Naming - ControlLogix
 Tag names should describe the function or
purpose of the tag. The name can be
anything you want as long as the name
follows these rules:
 Tag names can contain numbers, letters and a
single underscore and are not case sensitive.
 The maximum name length is restricted to 40-
characters.
 Tag names must begin with a letter or a single
underscore. They can not end with an underscore
or begin with a number.
 Mixed case is used for ease of reading such as:
Conveyor_2 or Conveyor2 and not
CONVEYOR_2 OR CONVEYOR2.

4. Invalid Tag Names
 The following tag names are invalid:
 Conveyor2_motor_
 This tag is invalid because a tag name cannot end
with an underscore.
 2Conveyor_motor
 This tag is invalid because a tag name cannot start
with a number.

5. Viewing/Sorting Tags
When viewing tags in the
RSLogix5000/Studio5000 software the tags
will be displayed in alphabetical order. Use this
to your advantage when naming tags. Using
the same word to start tag names from the
same process areas will keep them grouped
together. As an example:
Conveyor2_endOfConveyor
Conveyor2_inputSensor
Conveyor2_motor
Would keep all the tags associated with
Conveyor2 grouped together.

6. Tag Data Types
 When a tag is created it must be assigned a
data type. The data type is assigned based on
the type of data that will be stored in the tag.
 There are many different data types. The five
basic data types are:
 Bool Bit level data (0 or 1)
 Sint 8-bit integer data (Single Integer)
 Int 16-bit integer data (Integer [same as N7])
 Dint 32-bit integer data (Double Integer)
 Real 32-bit floating point data (Floating Point)

7. Basic Data Types
Data Type Bits
31 16 15 8 7 1 0
Bool Not used Not used Not used 0 or 1
Sint Not used Not used -128 to 127
Int Not used -32,768 to 32,767
Dint -2,147,483,648 to 2,147,483,647
Real
-3.40282347E38 to -1.17549435E-38 (negative values)
0
1.17549435E-38 to 3.40282347E38 (positive values)
All words in ControlLogix processors are 32-bits

8. Project Structure
 Before continuing with tags, data types
and structures, it is important to
understand the structure of a ControlLogix
project.
 ControlLogix projects consist of:
 Tasks
 Programs
 Routines
 The next slide shows a graphical
representation of this structure.

9. Project Structure
Every project, shown
in the light blue box
labeled Project, is
required to have at
least one:
• Task
• Program
• Routine (Main)
The white blocks in
the diagram are
required for every
project.

10. Task
 There are three types of tasks:
 Continuous
 Periodic
 Event
 Every project must have one continuous task and
it can be the only continuous task in the project.
 A project can have up to 32-tasks.
 A task is associated with a program and has two
functions:
 It stores information necessary to schedule the
programs execution.
 It sets and determines the execution priority for one or
more programs.

11. Tasks Types
 Continuous
 A continuous task is represented by the white task
box in the previous project structure graphic.
Continuous tasks execute non-stop. Every project
must have, and can only have one continuous task.
 Periodic
 The other tasks shown in the previous project
structure graphic are periodic tasks. Periodic tasks
are not required in a project. Periodic tasks interrupt
the continuous task and executes for a prescribed
length of time and at specific time intervals. When the
time expires, the task executes one last time. Periodic
rates can be from 1mS to 2000 seconds. The default
period is 10mS.

12. Tasks Types
 Event
 A event task executes based upon an event
occurring. The event could be as simple as a
contact in the field closing or as complex as a
certain string of data arriving in a particular
tag. Any or all of these events could trigger a
task to execute.

13. Programs
 Each task requires at least one program.
 A task can have up to 32-programs.
 Only one program can execute at-a-time.

14. Routine
 Routines are where the executable code
resides.
 Routines can be written in:
 Ladder Logic (This is what we will be using)
 Sequential Function Chart (SFC)
 Function Block Diagram (FBD)
 Structured Text (ST)
 There are two type of routines:
 Main routine
 Subroutine

15. Routines
 Main routine
 When a program executes the main routine executes first.
 The main routine is used to call (execute) other routines in the
program (subroutines).
 Subroutine
 Any and all additional routines in a program are called
subroutines. Subroutines are used to keep programs organized.
 They contain executable code just like the main routine and can
be in any of the 4-languages mentioned earlier.
 Subroutines will only execute when they are called from the main
routine or from another subroutine.
 Example:
 An assembly area might use a rotary indexer that has 10-different
assembly stations.
 There could be 10-subroutines with each subroutine containing the
program code that controls one of the assembly stations.
 These subroutines are called from the main routine or from other
subroutines.

16. Tag Types and Scope
 Tag types:
 Program tags
 The program tag area is represented by the violet box in the
memory structure graphic shown on a previous slide.
 Tags used in a program are stored in that program and have a
scope of that program.
 Scope
 The scope of a tag is where and when that tag and its data is
available. Some languages call scope, lifetime.
 The data in a program scope tag is only available when that
program is running. The tags and it’s data are not available to
any other program. When the program stops executing and
transfers control to another program, the tags in the program,
along with their data, are no longer available.
 Program level tags assist in keeping data organized and assist
in security by providing data hiding.
 For this course we will only be using one program, with one
main routine and multiple subroutines and most of the tags
created will be at that programs scope.

17. Tag Types and Scope
 Tag types:
 Controller tag
 Controller tags, shown in the white box near the
top of the memory structure graphic is an area of
memory that saves global tags.
 Global tags are tags that are available to the entire
project; all tasks, programs and routines.
 I/O data and system-shared data are examples of
global tags.
 Global tags should only be used when it is
absolutely necessary.

18. RSLogix 5000 Project

19. Tag Based Addressing
 There are two types of tags:
 Base tag
 Alias For tag
 Base Tag
 All instructions have and need a base tag. The
base tag is a tag name that represents the
process function and is assigned a data type
appropriate for that function.
 Example:
 A mixing vessel on the north side of the plant might
have a base tag named: northVesselMixer and if this
tag is being used to turn on/off the mixer motor it would
be assigned the data type of Bool (0 or 1).

20. Tag Based Addressing
 Alias For tag:
 An Alias For tag is another name for a Base
Tag.
 At this point in the course we will use Alias
For tags to assign an I/O point to a Base tag.
 Example:
 If the mixer motor controller is wired to an output
module in slot 2, output 14, the I/O point would be
assigned using an Alias For tag.
 Therefore; the base tag: northVesselMixer would
be assigned an Alias For tag of (that looks like):
Local:2:O.Data.14

21. Tag Based Addressing
Graphical representation of how a Base and Alias tag are used to point to
memory locations. This example is not related to the previous mix motor.

22. I/O Tag Format
 The form for a physical address in the
ControlLogix processor is:
Location:Slot:Type.Member.Submember.Bit
 Location: specifies the network location for
the data.
 Local is used if the I/O module is in the same
rack as the processor module.
 An adapter name is used to identify a remote
communication adapter such as a DeviceNET
remote I/O block. (Not used in this course)

23. I/O Tag Format
 Slot
 Designates the slot number of the I/O module.
 Type
 Type specifies one of four types of data:
 I for inputs
 O for outputs
 C for configuration (Not covered in this course)
 S for status (Not covered in this course)

24. I/O Tag Format
 Member
 Member specifies the type of data that the
module can store.
 Digital (discrete) I/O modules use a DATA
member.
 Analog I/O modules use a Channel Member
(CH#). (EMEC245)

25. I/O Tag Format
 Submember (optional)
 A submember is specific data related to a
member. This will be talked about when needed.
 Bit (optional)
 The bit specifies a bit number for internal
instructions or an input/output point for I/O
modules.
 Delimiters
 Only two delimiters are used; Colon (:) and period
(Dot) (.)
 If an address is a control-type tag, a (C) will
automatically be placed at the end of the address
to indicate that the tag is a controller scoped tag.

26. I/O Tag Format
 Therefore:
 If a base tag is assigned to a Start Push Button on
process 1, that is wired to the input module in slot 7,
IN8, and if this input module is in the same rack as
the controller, the base tag might be:
Process1_StartPB
and then it would be assigned an Alias For of:
Local:7:I.Data.8(C)
There is no submember for this Alias For.
 Here is the good thing. You don’t have to remember
the address structure for I/O because it can be
selected from a pop-up dialog box in the
RSLogix5000/Studio5000 program. However, it’s still
a good idea to understand the structure.

27. Processor Scan
 The processor evaluates the program logic
similar to the way we would evaluate
electromechanical ladder logic:
 Top to bottom, left to right, one rung at-a-time.
 This process is called Processor Scan or
simply Scan.
Read Input
Device Status
Solve the control program
logic based upon the
status of the input devices
Update the
status of
the output
devices
Housekeeping
and
communications
Scan Cycle
Read input
status
Solve the control
program and turn
internal coils on/off
Update the
outputs
Housekeeping
and
Communications

28. Input Interaction
 Limit switch is open, or off.
 It is wired to IN7 of the input
module. If this input module is
in slot-1 of the PLC rack, the
switch status will be stored in
bit-7 of the word associated
with slot-1.

29. Input Interaction
 Limit switch is closed, or on.
 It is wired to IN7 of the input
module. If this input module is
in slot-1 of the PLC rack, the
switch status will be stored in
bit-7 of the word associated
with slot-1.

30. Output Interaction
 The logic of the control
program evaluated bit-5 of the
output module in slot-2 as
false, or off.
 The output device wired to
OUT5 of the output module in
slot-2 will turn off.

31. Output Interaction
 The logic of the control
program evaluated bit-5 of the
output module in slot-2 as
true, or on.
 The output device wired to
OUT5 of the output module in
slot-2 will turn on.

32. Basic Bit Level (BOOL)
Instructions
Normally Open
Examine if Closed
XIC
Normally Closed
Examine if Open
XIO
Output Coil
Output Energize
OTE

33. Intro RSLogix 5000 Lab
 The next lab can be an instructor led or a
self-paced introduction to RSLogix 5000
and will cover:
 Starting a new project
 Configuring the hardware
 Configuring communications
 Entering ladder logic
 Creating tags and data types
 Downloading the project and running the
program.