Memory Layout and SLC500
System Addresses
Processor Memory Division
An SLC 500 processor's memory is divided
into two storage areas. Like two drawers in
a filing cabinet, one area is for data files and
the other for program files. Processor
memory division and file capacity are
shown in the following graphic
Processor Memory Organization
Program Files
• Program files contain processor
information, the main ladder program,
and other ladder files.
• An SLC 500 processor can contain up to
256 program files.
• Program files are located in the Program
Files folder of the RSLogix 500 project tree.
Program Files
Program Files
• Program files are assigned as follows:
• File 0 always contains system information.
• File 1 is reserved.
• File 2 contains the main ladder file.
• File 3-255 contains other ladder files
(subroutines).
Data Files
• Data files contain the status information
associated with external I/O and all other
instructions used in the main and
subroutine ladder program files.
• Data files are located in the Data Files
folder of the RSLogix 500 project tree.
Data Files
Data Files
Data file organization
for SLC-500 controller.
The data file portion of memory
stores input and output status,
processor status, the status of
various bits and numerical data.
•
Input Table File Operation
Input module
Switch Open
Binary 0 stored
0
•
Input Table File Operation
Input module
Processor continually
reads current input
status and updates
input image table file
Switch Closed
Binary 1 stored
1
•
Output Table File Operation
•
Output module
Output OFF
Processor continually
activates or deactivates
output status according
to output image table
•\file status
•
0
Status 0
•
Output Table File Operation
Output module
Processor continually
activates or deactivates
output status according
to output image table
file status
Output ON
1
Status 1
SLC 500 Software and Hardware
Addressing
SLC 500 Processor Data Storage
Units
The SLC 500 processor stores data in the following units of
memory:
• Bit: A digit in the binary radix (0 or 1). A bit may represent
the state, on or off, of a discrete I/O device.
• Word: A sequence of 16 bits that is treated as a unit. For
example, the 16 bits representing the 16 points of an I/O
module comprise one word.
• Element: A word or group of words that work together as a
unit.
• Sub-element: Individual words within an element.
SLC 500 Hardware Address
Characteristics
• The address for a real-world device (input or output) is
•
•
•
•
•
directly determined by the module slot number and
terminal to which the hardware device is wired.
Slot numbers are assigned from left to right, beginning
with 0. The SLC 500 processor is in slot 0.
A hardware address contains the following information:
The module type, either an input (I) or an output (O)
module
The slot number (numbered in decimal from 1 to 30)
The terminal number (numbered in decimal from 0 to
15)
SLC 500 Hardware Address
Characteristics
SLC 500 Output addressing
O:5/0 means that it is a physical output.
O:5/0 means that it uses Slot 5 (the 6th physical
slot) in the rack.
O:5/0 means that it is the first output on the card.
SLC 500 Input addressing
I:4/0 means that it is a physical input.
I:4/0 means that it uses Slot 4 (the 5th slot
in the rack).
I:4/0 means that it is the first input on the
card.
Internal Coil Addressing
B3:0/0 means that it references an internal
Binary file
B3:0/0 means that it uses the first word in
the table
B3:0/0 means that it is the first bit in the
word.
An example of an SLC 500 I/O address is shown in the following
graphic
Internal Coil Addressing
SLC 500 Input addressing
SLC 500 Output addressing
SLC 500 Input addressing
SLC 500 Output addressing
•
Program Scan
During each operating cycle, the processor reads all
inputs, takes these values, and energizes or de-energizes
the outputs according to the user program. This
process is known as a scan.
I/O scan – records status data of input
devices. Energizes output devices that
have their associated status bits set to
ON (1)
Program scan – instructions are
executed sequentially
Because the inputs can change at any time, the PLC must
carry on this process continuously.
•
•
•Scan Process
The scan time indicates how fast the controller can react
to changes in inputs. Scan times vary with computer
model and program content, and length. If a controller
has to react to an input signal that changes states twice
during the scan time, it is is possible that the PLC will
never be able to detect this change.
Scan time may be a
concern
in high speed operations
•
Scan Process
Read inputs
The scan is a
a continuous
and sequential
process
Adjusts
outputs
Run
program
•
Data Flow Overview
Input
modules
Input
data
Input
image
table file
Output
image
table file
Examine data
Output
data
Output
modules
Return results
Program
Check/compare/examine
specific conditions
•Lecture – PLC Programming Basics MME 486 – Fall 2006 16 of 62
Take some
action
Scan Process
Output
Module
Input
Module
Input
device
Input
file
Output
file
I:3/6
O:4/7
Output
device
O:4/7
I:3/6
•Program
When the input is
closed, the input
module senses a
voltage and an ON
condition (1) is
entered into the
input table bit I:3/6
I:3/6 O:4/7
During the program scan the
processor sets instructions
I:3/6 and O:4/7 to ON (1)
The processor turns
light output O:4/7 ON
during the next I/O
scan
•
Scan Patterns
Horizontal Scanning
Order
The processor examines
input and output
instructions from the
first command, top left
in the program,
horizontally, rung by
rung.
End of ladder
In addition to the program itself, the scan time is also dependent on
the clock frequency of the processor!
•
PLC Programming Languages
The term PLC programming language refers to the method
by which the user communicates information to the PLC.
The three most common
language structures are:
ladder diagram language,
Boolean language, and
functional chart.
Ladder diagram
language
Functional
chart
Boolean language
•
Comparing Programming Language
PB1
CR1
CR2
SOL
Relay Schematic
LS1
PB1
CR1
CR2
SOL
Equivalent ladder
diagram language
LS1
Equivalent Boolean language
•
Relay-Type Instructions
The ladder diagram language is basically a
symbolic set of instructions used to create the
controller program.
These ladder instructions symbols are
arranged to obtain the desired control logic.
•
Examine If Closed (XIC) Instruction
Symbol Analogous to the normally open relay
contact. For this instruction we ask
the processor to EXAMINE IF (the
contact is) CLOSED or ON (XIC)
•
Typically represents any input. Can be a switch
or pushbutton, a contact from a connected output,
or a contact from an internal output.
Has a bit-level address which is examined for an
ON condition.
The status bit will be either 1 (ON) or 0 (OFF).
•
•
Examine If Open (XIO) Instruction
Symbol Analogous to the normally closed
relay contact. For this instruction we
ask the processor to EXAMINE IF (the
contact is) OPEN (XIO).
•
Typically represents any input. Can be a switch or
pushbutton, a contact from a connected output,
or a contact from an internal output.
Has a bit-level address which is examined for an OFF
condition.
The status bit will be either 1 (ON) or 0 (OFF).
•
•
Output Energize (OTE) Instruction
Symbol
Analogous to the relay coil. The
processor makes this instruction true
(analogous to energizing a coil) when
there is path of true XIC and XIO
instructions in the rung.
Typically represents any output that is controlled by
some combination of input logic. Can be a connected
device or an internal output (internal relay).
If any left-to-right path of input conditions is TRUE,
the output is energized (turned ON).
•
Allen-Bradley SLC-500 Controller Addressing
Output image table
file 0
Address
output
terminal
O0:4/6
O:0:4/6
it address
Address
input
terminal
I1:3/12
Input image table
file 1
I:3/12
Bit address
Closed input
I1:3
O:4
User-programmed rung
12
6
Energized
output
•
PLC Matrix Limitation Diagram
Max series
contacts
Max parallel
lines
No. outputs
per rung and
location of the
output in the rung
There may be limitations to the number of series contacts
instructions, number of parallel lines, and the number of outputs and
their location on the rung.
•
•
Extending the Number of Series Contacts Using
an Internal Control Relay
Internal
relay coil
Rung 1
Rung 2
Internal
relay
contact
Discrete output (requires
one physical connection
on the output module)
Nested Input and Output Branches
Input and output branches can be nested to avoid
redundant instructions and to speed up the processor
scan time.
A nested branch starts or ends within another branch.
•
A
Nested Contact Program
B
C
Y
D
Nested
contact
On some PLC models,
the programming of a
nested branch circuit
cannot be done directly.
E
A
B
C
D
E
Y
C
Contact
instruction
C repeated
Reprogrammed to
obtain the required
logic.
Nested Input and Output Branches
Input and output branches can be nested to avoid
redundant instructions and to speed up the processor
scan time.
A nested branch starts or ends within another branch.
•
A
Nested Contact Program
B
C
Y
D
Nested
contact
On some PLC models,
the programming of a
nested branch circuit
cannot be done directly.
E
A
B
C
D
E
Y
C
Contact
instruction
C repeated
Reprogrammed to
obtain the required
logic.