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Archived: Allen-Bradley PLC Addressing

This document has been archived and is no longer updated by National Instruments

Primary Software: Lookout
Primary Software Version: 6.0
Primary Software Fixed Version: N/A
Secondary Software: N/A

How do I address Allen-Bradley (AB) I and O data members in Lookout?

For an Allen-Bradley Programmable Logic Controllers (AB PLC), when you specify the point 'I:9', this is equivalent to specifying 'I1:9', since the default physical input file is file 1. An AB PLC stores consecutive words (word = 16-bit value) of input values in file 1, beginning with word 0 (I:0) and continuing to word 1 (I:1), etc. Some slots can use multiple input words, and empty slots use no words at all. Allen-Bradley does not release the portion of their protocol documentation which would allow SCADA packages to query an AB PLC to determine how the slots in the PLC are populated. Without this information Lookout can not determine card types and thus does not know how many input/output words are used by each slot. As a result, Lookout can only let you read these words based on their consecutive addresses, as opposed to accessing them by slot number.

You will find that other packages completely eliminate this problem by not allowing you to read input/output files at all. Instead they require that you copy all your I/O data to/from an integer file and read/write the integer file instead.

For example, assume you have an SLC 503 populated with cards in the indicated slots. Here's how you would read the data from your cards using Lookout:

========================= ========= ====================
Thermocouple Input 1 I:0, I:1, I:2, I:3
Combo Analog Input/Output 2 I:4, I:5 and O:0, O:1
16 bit Digital Input Card 4 I:6_0 to I:6_15
16 bit Digital Output Card 5 O:2_0 to O:2_15
16 channel Analog Input Card 6 I:7 to I:22

Notice the sequential progression of slot numbers and the address interlacing between file types. Also be aware that other cards (example - Prosoft card) can also use input and output registers, and that you will need to take this into account in the numbering.

Also be aware that the help documentation entry entitled "Allen-Bradley Register Addressing" in Lookout is incorrect. The numbering should start at I0 and O0 rather than I1 and O1.

Related Links:
KnowledgeBase 24ICL36W: How Does the Lookout Register Addressing for the Allen-Bradley Micrologix 1500 Compare to the Micrologix 1000?
Developer Zone Tutorial: Notes on Allen Bradley PLC Addressing


Report Date: 05/14/1996
Last Updated: 02/03/2014
Document ID: 0LDD15EC

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