Programmable Logic
Controllers
PLC Addressing and Basic
Instructions
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-1
Basic addressing
• For the Allen-Bradley PLCs and the simulator
used, the input and output image areas (in
addition to other data) are held in “data files”
• Each “data file” holds information such as
–
–
–
–
–
–
Discrete input values
Discrete output values
Values associated with timers
Values associated with counters
Status information
Etc.
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-2
3-1
PLC data files
Simulator data files
AB PLC data files
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-3
I/O memory mapping
• Basic AB PLC addressing
Data file type
I – input
O – output
S – status
B – binary
T- timer
C-counter
Slot number (not used with the simulator)
X1:x.0/0
Data file number
(can be omitted for the
basic data files)
Bit number
Word number: selects
a specific word in
a data file
I/O located on the controller (embedded I/O) is slot 0.
I/O added to the controller (expansion I/O) begins with slot 1.
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-4
3-2
Example Addressing
CPU
Module
(slot 0)
Digital
Input
Module
(slot 1)
16 inputs
Digital
Output
Module
(slot 2)
8 outputs
Would occupy
addresses
I1:1.0/0
through
I1:1.0/15
Would occupy
addresses
O0:2.0/0
through
O0:2.0/7
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-5
Basic instructions
•
•
•
•
•
•
Examine If Closed (XIC)
Examine If Open (XIO)
Output Energize (OTE)
Output Latch (OTL)
Output Unlatch (OTU)
One-shot Rising (OSR)
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-6
3-3
Examine if closed (XIC)
• Alternate name: Normally open contact
• Use the XIC instruction in your ladder program to
determine if a bit is on.
– When the instruction is executed, if the bit addressed is on
(1), then the instruction is evaluated as TRUE.
– When the instruction is executed, if the bit addressed is off
(0), then the instruction is evaluated as FALSE.
Instruction symbol
Normally open switch is open
Normally open switch is closed
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-7
Examine if open (XIO)
• Alternate name: Normally closed contact
• Use the XIO instruction in your ladder program to
determine if a bit is off.
– When the instruction is executed, if the bit addressed is on
(1), then the instruction is evaluated as FALSE.
– When the instruction is executed, if the bit addressed is off
(0), then the instruction is evaluated as TRUE.
Instruction symbol
Normally open switch is open
An XIO instruction inverts the
state of the bit it references.
Electrical & Computer Engineering
Normally open switch is closed
Dr. D. J. Jackson Lecture 3-8
3-4
Output energize (OTE)
• Alternate name: coil
• This instruction is usually used in conjunction with
XIC or XIO or any other input instruction.
• If the logic preceding the OTE instruction is true (1),
the OTE instruction will be energized
Instruction symbol
An OTE instruction can
only be the last
instruction on a rung.
Electrical & Computer Engineering
Not energized
Energized
Dr. D. J. Jackson Lecture 3-9
Output latch (OTL)
• The OTL instruction is used only to turn
a bit on and latch it on
Initially not
energized
Instruction symbol
Energized
L
Latched
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-10
3-5
Output unlatch (OTU)
• The OTU instruction is used only to turn
a bit off and latch it off
• OTL and OTU are generally used in pairs
Instruction symbol
U
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-11
One-shot rising (OSR)
• When the rung conditions preceding the OSR
go from false-to-true, the OSR instruction
goes true also but for only one scan
Instruction symbol
OSR
I:1/0
B3:0/0
One PLC scan
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-12
3-6
Branches
• Branches can be inserted in ladder logic programs to
create parallel combinations of various ladder logic
elements
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-13
Multiple Series/Parallel Branches
• Multiple series/parallel branches can be utilized with
multiple inputs and also affecting multiple outputs in
parallel
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-14
3-7
PLC simulator usage
• Ladder logic elements (instructions) can be dragged
and dropped onto the ladder window to create a
ladder logic program
• The ladder logic program is executed by going
online, ‘downloading’ the PLC program, and
switching to run mode
OFFLINE/ONLINE control
Download
Electrical & Computer Engineering
PLC run mode
Dr. D. J. Jackson Lecture 3-15
ProSim simulations
• The Logixpro simulator provides a set of built-in simulations
• The I/O simulation initially will be of most use since it provides
access to simulated discrete inputs (switches) and outputs
(lamps)
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-16
3-8
Other Prosim simulations
• Available simulators
–
–
–
–
–
–
–
Door simulator
Silo simulator
Traffic simulator
Batch simulator
Dual Compressor simulator
Bottle line simulator
Four floor elevator simulator
• Most projects will involve designing ladder
logic programs to control these simulations
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-17
Batch simulator showing example I/O
Electrical & Computer Engineering
Dr. D. J. Jackson Lecture 3-18
3-9