This document covers the conversion of decimal numbers to binary coded decimal (BCD) and the use of subroutines in programming within PLC systems. It details BCD conversion instructions, subroutine creation, and program control instructions relevant to the SLC500 and ControlLogix systems. Additionally, it highlights the importance of understanding the force I/O function before applying it to ensure safety and proper operation.

1. Conversion, Subroutines and
Forcing I/O
Chapter 07 and supplemental material
Chapter 08 Sections 8-1, 8-2, 8-3-3 through
8-3-5
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2. Binary Coded Decimal (BCD) Review
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3. BCD Conversion Review
The BCD representation of a decimal number is obtained
by replacing each decimal digit by its equivalent four-bit
binary number.
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4. BCD Push Switch (Thumbwheel)
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5. Push Switch (Thumbwheel) Truth Table
Dial Bit Weight
Position 8 4 2 1
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1
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6. PLC Input Module Connections
(BCD Input Device)
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7. PLC Output Module Connections
(BCD Output Device)
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8. Allen Bradley BCD Conversion Instructions
Convert from BCD (FRD) Convert to BCD (TOD)
Conditional or Unconditional Conditional or Unconditional
FRD TOD
From BCD 0001 To BCD
Source I:1.0 Source N22:3
2437h< 1234<
Dest N7:4 Dest O:5.0
2437< 1234h<
FRD TOD
From BCD 1 To BCD
Source PushSw Right Source DspyOutValue
16#0000 0
Dest PushSw Value Dest DisplayOutput
0 16#0000
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9. Convert To BCD (TOD) Instruction – SLC500
When the rung is true, converts the binary value of a
decimal number to its equivalent BCD value.
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10. Convert To BCD (TOD) Instruction – ControlLogix
When the rung is true, converts the binary value of a
decimal number to its equivalent BCD value.
The decimal value in the source is converted to BCD binary and then
the binary is displayed as the decimal result in the destinatio n.
9760 Decimal_So urce 0 0 0 0 9 7 6 0
0000 0000 0000 0000 1001 0111 0110 0000
0000 0000 0000 0000 1001 0111 0110 0000 is then converted to decimal
and displayed as a Decimal number in the Dest.
38752 BCD_Dest 0000 0000 0000 0000 1001 0111 0110 0000
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11. Convert From BCD (FRD)
When the rung is true, converts the BCD value in the
Source to its equivalent decimal value.
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12. Convert To BCD (FRD) Instruction – ControlLogix
When the rung is true, converts the BCD value in the
Source to its equivalent Decimal value.
The value in the source is c onverted to binary and then
the binary is c onverted to a BCD number
38752 BCD_Sourc e 0000 0000 0000 0000 1001 0111 0110 0000
0 0 0 0 9 7 6 0
9760 is then c onverted to binary and displayed as Dec imal in the Dest.
9760 Dec imal_De st 0000 0000 0000 0000 0010 0110 0010 0000
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13. FRD Source is a non-BCD Value SLC500
When the Source of an FRD is a non-BCD value the error:
Minor error at end of scan will occur and the processor will
fault.
This is a recoverable error and the processor fault can be
avoided by unlatching the Overflow Trap Bit S:5/0.
I:1 B3:0 FRD
0000 OSR From BCD
4 0 Source I:1.0
A106h<
Dest N7:4
32767<
Processor
Mode Overflow
Bit 0 Trap
S:1 S:5
0001 U
0 0
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14. ControlLogix BCD Instructions
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15. Subroutines
Chapter 08
Sections 8-3-3 to 8-3-5
Supplement Document
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16. Subroutines
Subroutine(s) is a group of program code that performs a specific
task. The subroutine(s) are part of a program (in our case the
MainProgram) and are not part of the MainRoutine.
A subroutine can be invoked, (called), from anyplace in the
MainRoutine or from another subroutine. When a subroutine is
called from within another subroutine it is referred to as nesting
subroutines.
Most, if not all, computer programs and PLC/PAC programs
contain subroutines.
Subroutines are used to make what would be an enormous
program more manageable by breaking up the code into smaller
tasks. Specific functions within a task should be placed in
subroutines.
Organizing a program by using subroutines makes the code
easier to read, understand and maintain.
Subroutines can also be reused in other programs that require
the same task be performed.
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17. Subroutines
SLC500 ControlLogix
The SLC500 uses The ControlLogix uses
separate ladder files to separate routines to store
store and execute and execute subroutines.
subroutines. There are a The number of routines is
total of 253 subroutine limited to 32 per program.
ladder files; #3 through
#255.
The MainRoutine is the
main ladder routine and is
Ladder file #2 (LAD2) is the one we have been
the main ladder file and is using in all labs to this
the one we have been date.
using in all labs to this
date.
Scan starts at rung 0 in the
MainRoutine and ends at
Scan starts at rung 0 LAD2 the End statement in the
and ends at the End MainRoutine
statement in LAD2.
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18. Creating a ControlLogix Subroutine
1. To create a new subroutine right
click on MainProgram and select
New Routine… from the pop-up 2. New Routine dialog box will open.
menu Type in the name of the subroutine
and from the Type: dropdown,
select the type of program code.
3. The new
subroutine will appear
listed under the
program that it was
created in. Double
click the subroutine to
open the ladder
editor.
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19. Program Control Instructions
 There are many program control instructions. Below is a list of them and the
platforms they are available on:
Instruction CL 500 Sim Instruction CL 500 Sim
UID – User Interrupt
JMP – Jump to Label ● ● ● ●
Disable
UIE – User Interrupt
LBL – Label ● ● ● ●
Enable
SFR – Reset Sequential
JXR – Jump to External Routine ● ●
Chart
SFP – Pause Sequential
JSR – Jump to Subroutine ● ● ● ●
Chart
EVENT – Trigger Event
RET – Return from Subroutine ● ● ● ●
Task
SBR – Subroutine Label ● ● ● EOT – End of Transition ●
TND – Temporary End ● ● ● AFI – Always False ●
MCR – Master Control Reset ● ● ● NOP – No Operation ●
SUS - Suspend
 This course will only cover the instructions shown in Red. The instruction in
Green can be used, but they will not be discussed in class.
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20. ControlLogix Program Control Instructions
JSR – Jump to Subroutine (an output instruction)
The JSR is an output instruction that is used to “call” a
subroutine.
The instruction rung can be conditional or unconditional.
JSR instructions can have several parameters. The one
shown here has 3-parameters. Subroutine name to
be “called” (required)
Parameter to pass to
the subroutine
(optional)
Parameter to accept a
value returning from
the subroutine
(optional)
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21. ControlLogix Program Control Instructions
Entering JSR parameters:
 Routine Name (Required) –
Double-click this parameter field,
click the dropdown arrow and
select the subroutine name from
the dropdown list. The subroutine
name will only be in the list if the
subroutine has been created.
 Input Par and Return Par
(Optional) – These parameters
will not be used in this course.
When a parameter is not used it
must be removed. Right click on
the parameter field and select
Remove Instruction Parameter
from the context specific menu.
Do this for each parameter that is
not required. The picture on this
slide illustrates removing a
parameter field.
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22. ControlLogix Program Control Instructions
JSR operation
When the rung containing a JSR instruction is true, the
processor scan jumps to the subroutine referenced in the
Routine Name parameter and begins program execution at
the first rung in that subroutine
A jump cannot be made into the middle of a subroutine.
Execution will always start at the first instruction on the first
rung in that ladder routine.
This rung will unconditionally jump to the
subroutine named Routine04
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23. ControlLogix Program Control Instructions
SBR – Subroutine Label
The SBR in an input instruction that is always true and
marks the beginning of a subroutine. It must be the first
instruction on the first rung of the subroutine.
The input parameter field(s) is used to reference tags
whose data is to be used (passed) to the subroutine. (This
course will not be using these parameters).
SBR instructions can have several parameters. The one
shown here has one parameters.
Parameter that passes
data to the subroutine
(optional)
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24. ControlLogix Program Control Instructions
For this course the Input Par parameter will need to be
removed from the SBR instruction. To remove the
parameter right-click on the parameter and select Remove
Instruction Parameter from the context specific menu.
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25. ControlLogix Program Control Instructions
RET – Return from Subroutine
The RET instruction is an output instruction that is used to
stop executing the subroutine and return to the ladder file
that originally “called” the subroutine.
The instruction can be conditional or unconditional and can
contain several parameters. The RET instruction shown
here has one parameter. (This course will not use these
parameters).
Parameter that passes
data back to the
ladder file that
originally “called” the
subroutine. (optional)
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26. ControlLogix Program Control Instructions
For this course the Input Par parameter will need to be
removed from the RET instruction. To remove the
parameter right-click on the parameter and select Remove
Instruction Parameter from the context specific menu.
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27. ControlLogix Program Control Instructions
Several conditional RET instructions can be present in a
subroutine. As an example:
 If an RET instruction becomes true that is on rung 0006 in a
subroutine containing 45-rungs, the program scan will return to the
ladder file that originally called the subroutine and rungs 0007
through 0045 will not execute.
 If the RET instruction on rung 0006 becomes false another RET
instruction becomes true that is on rung 0010 in the same
subroutine, the program scan will return to the ladder file that
originally called the subroutine and rungs 0011 through 0045 will not
executed.
If the entire subroutine ladder file is to be always scanned,
placing a RET instruction on the last rung of the program is
optional. If an RET instruction is not found in the subroutine
ladder file, the END statement performs the return.
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28. Program Execution using Subroutines
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29. Forcing I/O
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30. Using the Force I/O Function
The force function will only work on field I/O devices,
therefore the input and output data files in the SLC500 and
the Controller tags in the ControlLogix.
Before applying a force to any input or any output device,
an understanding of the potential effect that force(s) will
have on the machine or process operation and to the safety
of personal is essential.
DO NOT INSTALL FORCES WITHOUT FIRST
UNDERSTANDING WHAT AFFECT IT WILL HAVE ON
THE OPERATION OF THE MACHINE OR PROCESS
Most programming software provide some visible means of
alerting the user that a force is in affect or installed.
Most processor modules have an LED indicator that will be
lit if there are any forces installed.
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31. Installing and Enabling Forces
This is an instructor led, interactive lab.
If there is no program running in the PLC at your
workstation, open a program that uses I/O field devices,
download the program to the PLC, then place the PLC in
RUN mode.
Your instructor will also switch to RSLogix and attach to
someone’s workstation to lead you through and
demonstrate the force functions.
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