Chapter 03 Sect. 3-4, 3-5, 3-5-1, 3-5-3, 3-6, 3-6-2, 3-6-3, 3-7, 3-7-1, 3-9, 3-10PLC/PAC Memory Structure and Addressing

Memory Structure & Organization

As mentioned previously, there are three basic types of memory structures and organizations: Address-based memory Tag-based memory Combination of Address/Tag-based memory

Rack-Based Memory

Representation of most rack/slot address-based PLCs.

Represented by a two drawer file cabinet.

One drawer for data files

One drawer for program files

James A. Rehg, Glenn J. Sartori, Programmable Logic Controllers, 2nd Edition, Pg. 85

Program Memory

The user program accounts for most of the memory in a PLC system.

Program files contain the logic that controls the machine operation. This logic consists of instructions that are programmed in ladder logic format.

James A. Rehg, Glenn J. Sartori, Programmable Logic Controllers, 2nd Edition, Pg. 85

Memory Addressing

A memory address is a location within the PLC memory where information is stored.

The Allen Bradley SLC500 series PLC uses an alpha numeric sequence separated by delimiters that specifies the:

Type of data (bit level or word level) Slot location of an I/O module in a rack when

addressing field I/O Word number in memory, (Word is also referred to as

an element) Input or output number for field I/O and bit number for

internal coils and contacts.

AB SLC500 Slot Locations

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6

AB SLC500 I/O Address Format

I1:4.0/4

File typeFile number

File delimiter

Slot or element number

Word delimiter

Word number

Bit delimiter

Bit or terminalnumber

States: Input module in slot 4 IN4

O0:5.0/13 – Output module, slot 5, OUT13

AB SLC500 I/O Address Format

I1:4.0/4

File typeRequired

File numberOnly required when a

user defined file is being specified.

File delimiterRequired

Slot or element numberRequired

Word delimiterOnly required when

specifying a word number.

Word numberOnly required if pointing to a word other than word zero.

Defaults to word zero.

Bit delimiterOnly required when

addressing to “bit level”.

Bit or IN/OUT numberOnly required when addressing to “bit

level”.

This address can be rewritten as: I:4/4

SLC500 Default File Types

File Type Identifier File NumberOutput O 0

Input I 1

Status S 2

Bit B 3

Timer T 4

Counter C 5

Control R 6

Integer N 7

Floating Point * F 8

* Available in SLC-5/03 OS301, OS302 & SLC-5/04 OS400, OS401 & SLC-5/05 processors

SLC500 User Defined File Types

File Type Identifier File NumberBit B 91 - 255

Timer T 91 - 255

Counter C 91 - 255

Control R 91 - 255

Integer N 91 - 255

Floating Point * F 91 - 255

String * St 91 – 255

ASCII * A 91 - 255

Special Note1 Computer Interface File (CIF)1 9

• Available in SLC-5/03 OS301, OS302 & SLC-5/04 OS400, OS401 & SLC-5/05 processors

1 The only file that data can be xmitted/received to/from non-IP devices

Data Files in the Project Window

RSLogix 500 Default Files LogixPro Default Files

Input/Output Elements

I/O modules are considered to be one element (word) files. In the SLC500 series this means that a 16-point module uses one 16-bit word and in the ControlLogix series it uses 16-bits of one 32-bit word. An SLC500, 32-point module will use two 16-bit words (32-bits) and a ControlLogix will use one, 32-bit word.

0 0 0 0 0 0 0 00 0 0 0 0 0 0 0

15 10 9 8 7 6 5 4 3 2 1 014 13 12 11

0 0 0 0 0 0 0 00 0 0 0 0 0 0 00 0 0 0 0 0 0 00 0 0 0 0 0 0 0

0

1

2

Bit numbers (16-bit word(s))

Element or word number

Addressing Input/Output Elements (Words (16-bit PLC))

I/O addressing for modules that have 8-points, 16-points & 32-points. To address bit-9 of the input module in slot-1; I1:1.0/9 or I:1/9 To address bit-5 of the input module in slot-3; I1:3.0/5 or I:3/5 To address bit-14 of the input module in slot-6; I1:6.0/14 or I:6/14 To address bit-6 in the second word of the module in slot-6; I1:6.1/6 or I:6.1/6 or I:6/22

16-point module in slot-1Requires 1-word

16-point module in slot-4Requires 1-word

8-point module in slot-3Requires 1-word

only uses the lower byte

32-point module in slot-6Requires 2-word

Word-0 for the first 16-bitsWord-1 for the last 16-bits

Screen shot of an input data table from RSLogix 500

Bit File, File #3

File #3 is the bit file and is designated with the letter ‘B’.

One application for the bit file is for internal control relays where each bit can be a relay coil.

In the SLC500 the bit file contains 256, 16-bit words for a total of 4,096 bits.

Each bit, when used as a coil, can have an unlimited number of normally open and/or normally closed contacts programmed to it.

Addressing the Bit File

The bit file is addressed using the same address structure as the Input/Output file.

The differences are: The letter used for the file designator will now be

the letter ‘B’ There is no slot number. What was the slot

number, now becomes the word number within the file.

The following address references word 0, bit 0, of the default bit file.

B3:0/0 or B:0/0

Addressing the Bit File

Addressing Bit 12 in word 255 of the default bit file would be:

B3:255/12 or B:255/12 Addressing Word 42, bit 7, of the user

defined bit file number 12 would be:

B12:42/7In this example the file number is required because it is a user defined file.

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.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Com

2

4

5

6

3

1

0

7

L1

02040506 03 0107 0010121314 11 0915 08

Word I:1.0

LimitSwitchInput

AddressI1:1.0/7

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.

0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0

Com

2

4

5

6

3

1

0

7

L1

02040506 03 0107 0010121314 11 0915 08

Word I:1.0

LimitSwitchInput

AddressI1:1.0/7

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.

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Com

2

4

5

6

3

1

0

7

L1

02040506 03 0107 0010121314 11 0915 08

Word O:2.0

OutputAddressO0:2.0/5

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.

0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0

Com

2

4

5

6

3

1

0

7

L1

02040506 03 0107 0010121314 11 0915 08

Word O:2.0

OutputAddressO0:2.0/5

SLC500 Basic Bit Level Instructions

Normally OpenExamine if Closed

XIC

Normally ClosedExamine if Open

XIO

Output CoilOutput Energize

OTE

Rung Comment

InstructionDescription

Address Symbol

AddressShown split withbit number at the

base of theinstruction

Bit number

Rung number

Default color scheme shown

ControlLogix Basic Bit Level Instructions

Normally OpenExamine if Closed

XIC

Normally ClosedExamine if Open

XIO

Output CoilOutput Energize

OTE

Rung Comment

InstructionDescription

Tag

Alias to theI/O module

Rung number

Modified color scheme shown

LogixPro Basic Bit Level Instructions

Normally OpenExamine if Closed

XIC

Normally ClosedExamine if Open

XIO

Output CoilOutput Energize

OTE

Rung CommentRestricted to

one line

Address SymbolThere is noinstructiondescription

AddressShown in-line.

RSLogix 500 canbe configured to

display the addressin-line.

Rung number

Default color scheme shown

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 InputDevice 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

Other One Element Files (16-bit PLC)

The input and output files are not the only 1-element (1-word) files. The following files are also 1-element (1-word) files:

Status (file-2) Number of words in the file depends upon the processor type

Bit (file-3) 256, 16-bit words are available in each bit file Integer (file-7) 256, 16-bit words are available in each integer file

0 0 0 0 0 0 0 00 0 0 0 0 0 0 0

15 10 9 8 7 6 5 4 3 2 1 014 13 12 11

0 0 0 0 0 0 0 00 0 0 0 0 0 0 0

0 0 0 0 0 0 0 00 0 0 0 0 0 0 0

0

1

2

Bit numbers

Element or word number

Three or more Element Files (16-bit PLC)

Some file types in the Allen Bradley SLC500 series PLC require more than 1-word per instruction.

Timers, Counters and Control instructions are such files. Each timer, counter or Control instruction requires 3-words.

0 0 0 0 0 0 0 00 0 0 0 0 0 0 0

15 10 9 8 7 6 5 4 3 2 1 014 13 12 11

0

1

2

Element or word number. Word-0 will contain instruction status bits. Values stored in words-1 & 2 will depend upon the file type being addressed.

Bit numbers

Software Introduction Labs

Homework Lab – Introduction to the LogixPro Simulator Install and update LogixPro. Gain an understanding of using LogixPro and

submit the electronic version of the file. (.rsl file)