Allen Bradley PLC Data Address Format

Introduction

There are two distinct ranges of Allen Bradley PLCs:

  1. SLC-500, MicroLogix, PLC-5 and earlier models. These models are configured in ladder logic and use a file based addressing.
  2. ControlLogix and CompactLogix. These models implement the IEC-61131-3 standard and data is addressed by variable name.

This article describes the file based addressing used by SLC-500, MicroLogix, PLC-5 and earlier models.

The Allen Bradley PLC Data Address Format is a string that defines the location of data in a Allen Bradley PLC. The format of the string is:

Xf:e.w/b

Where:

File Type

Data in Allen Bradley PLCs is stored in files. For example there are separate files for Inputs, Outputs, Status Bits, Timers, Counter, Integers and so on. Some file types are fixed, for example there can only be one file each for Inputs, Outputs and Status Bits. Other files are more flexible, for example there can be multiple Integer files.

File TypeNotes
O Outputs If specified the File Number must be 0.
The Element Number specifies the I/O slot.
The Word Offset specifies the word offset within the I/O slot.
The Bit Offset specifies the bit number (0-15).
I Inputs If specified the File Number must be 1.
The Element Number specifies the I/O slot.
The Word Offset specifies the word offset within the I/O slot.
The Bit Offset specifies the bit number (0-15).
S Status If specified the File Number must be 2.
The Element Number specifies the status word..
The Bit Offset specifies the bit number (0-15).
B Bit The File Number specifies the which bit file to use (3, 9-255).
If not specified the File Number is 3.
The Element Number specifies the word.
The Bit Offset specifies the bit number (0-15).
T Timer The File Number specifies the which timer file to use (4, 9-255).
If not specified the File Number is 4.
The Element Number specifies the timer index.
The Word Offset specifies the word offset within the timer (0-2).
The Bit Offset specifies the bit number (0-15).
C Counter The File Number specifies the which counter file to use (5, 9-255).
If not specified the File Number is 5.
The Element Number specifies the counter index.
The Word Offset specifies the word offset within the counter (0-2).
The Bit Offset specifies the bit number (0-15).
R Control The File Number specifies the which control file to use (6, 9-255).
If not specified the File Number is 6.
The Element Number specifies the control index.
The Word Offset specifies the word offset within the control (0-2).
The Bit Offset specifies the bit number (0-15).
I Integer The File Number specifies the which integer file to use (7, 9-255).
If not specified the File Number is 7.
The Element Number specifies the word.
The Bit Offset specifies the bit number (0-15).
F Float The File Number specifies the which float file to use (8-255).
If not specified the File Number defaults to 8.
The Element Number specifies the float index.
ST String The File Number specifies the which string file to use (9-255).
The Element Number specifies the string index.
The Word Offset specifies the word offset within the string (0-41).

File Number

In an Allen Bradley PLC data is organized into files. The File Number is optional and, if included, specifies which file to access. If the File Number is not included the file number is implied by the File Type. All PLCs have a predefined set of files as follows:

FileType
0Outputs
1Inputs
2Status
3Bit
4Timer
5Counter
6Control
7Integer

Notes:

  1. On a MicroLogix 1000 PLC no additional files can be defined.
  2. On a MicroLogix 1200 PLC, SLC 5/01 CPU, or SLC 5/02 CPU file 8 is reserved. Additional file numbers 9 to 255 are user defined.
  3. On a other PLC types file 8 is a Float file. Additional file numbers 9 to 255 are user defined.

Element Number

The Element Number specifies which element of the file to access.

For Input and Output files, the Element Number specifies which I/O slot to access. An I/O slot may consist of 1 word, or multiple words depending on the type of I/O configured.

For other file types, the Element Number specifies which element to access.

Word Offset

The Word Offset field is optional, and if specified, defines which word of a multi-word element to access.

Bit Offset

The Bit Offset field is optional, and if specified, defines which bit of the word to access.

Examples

Some examples of Allen Bradley Data Addresses:

AddressDescription
O:1, O1:0, O:1.0The first word of output slot 1.
O:1/0, O1:0/0, O:1.0/0The first bit of the first word of output slot 1.
I:5.1, I1:5.1The 2nd word of input slot 5
N:20, N7:20Word 20 of the default integer file
N:20/15, N7:20/15Bit 15 of word 20 of the default integer file
N10:15Word 15 of user defined integer file 10

Where Used

The Allen Bradley PLC Data Address Format is used in these contexts:

Data Address Editor

The Data Address Editor is accessed by clicking the Browse button Browse button found at the right hand side of the DataAddress field:

Allen Bradley Data Address Editor

Further Information

Allen Bradley Register Block Tag

To learn about Allen Bradley Register Block Tags.

Allen Bradley Driver

For an overview of the Allen Bradley Driver.

Glossary

For definitions of the terms used in Fernhill SCADA.