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How to address the I/O of Allen Bradley PLC's?
How to address the I/O of Allen Bradley PLC's
By Kokou Avowlanou on 28 December, 2001 - 3:53 pm

I use to programm Modicon Telemecanique and Siemens PLCs. I'm new in those of Allen Bradley. I'd like to know how to address the digital I/O of Allen Bradley PLC's (Logix5000 for instance). I can't see anything about that in the on-line manuals I get on the ab website. Could you help me. Thanks.

By john peters on 2 January, 2002 - 9:03 am

The AB line of controllers has been one of the few that kept a standard that typically doesn't require any software to identify the I/O map.
The PLC-5 and SLC-500 use a rack/slot/register/bit method for addressing the I/O modules.

For example input point 15 in slot 2 of rack 1 would be addressed - I:1:2/15. The same goes for outputs.

Now the legacy problems prop up. In the old days most things were octal so AB allows for different methods for addressing modules in the
rack. So a 16 point input module could be bits 0 -15 or 0-7/10-17...

If you have the logix5000 software then this is not really an issue since it just asks the programmer to put the cards in the rack
graphically and then addresses the modules per the controller card, networking and I/O interfaces. RSLogix5 and RSLogix500 also pretty much
configure the rack for the user. I think the most confusing stuff is the remote I/O interfaces that support all the AB product lines.

Just keep it simple - Rack:Slot/bit.. and the rest should follow. The manuals in do have the documentation for programming all the
different controllers.


By Bob Peterson on 2 January, 2002 - 12:31 pm

Actually, this is not the case.

SLC I/O are addressed by slot number. Example:

I:1/15 would be last (16th) point in slot 1. There is no rack number involved. All addresses are decimal. Slot number can be 1 to 30 (slot 0 is always the CPU). Certain cards have more then 16 bits per card (such as 32 point cards or analog I/O). These are addressed as X:ss.ww/bb, where X is either I or O, ss is the slot number, ww is the word within the slot, and bb is the bit. An example would be I:1.1/15. This could be the last point of a 32 point input card, or a single bit of an analog input. Note that I:1/15 is equivilant to I:1.0/15.

PLC5s use the rack-group-slot addressing scheme. Example:

I:rrg/sb is a point on the card in rack rr, group g, slot s, bit b. All addresses are octal.

I:002/12 is a legal address. this is rack 0, group 2, slot 1, point 2.

Racks can be 0 to 77, groups are 0 to 7,slot is either 0 or 1.

To make this even more confusing, this addressing scheme is based on the old 8 point cards that took two cards to fill a word. Take a 16 slot rack. If you used this style of addressing, it would be one logical rack, with 8 groups, and 2 slots per group. For instance, the first slot would be I:000/00-07, the second slot would be I:000/10-17, if both are 8 point inputs. This addressing method is called 2 slot addressing, meaning it takes 2 physical slots per logical word. Each physical slot is only 1/2 a logical slot. This is typically only useful if using 8 point cards, or alternating 16 point inputs and outputs in the same group, i.e.-input in first physical slot then output, input, ..., etc.

Single slot addressing means each physical slot is 16 bits wide. Typically used with 16 point cards. Each physical slot in a rack corresonds to a logical group and both of its logical slots. If a 16 slot rack is used in this mode, it occupies 2 logical racks. As an example, the first card would be I:000/00-17, the last card would be I:017/00-17.

They also have 1/2 and I think 1/4 slot addressing as well to accomodate 32 and 64 point cards. the addressing schemes can get very confusing, and are set by dip switches on the I/O racks and RIO adapter modules. The easiest solution with PLC5s is to use single slot addressing. Its the least confusing.

You can also address I/O in what they call complementary I/O. This is where you have two physical racks with the same logical rack number, where (for instance) one is all inputs and the second rack is all outputs.

It would take a LOT more then a few paragraphs to describe PLC5 addressing schemes.

One thing about SLCs though. Remote I/O on an SLC is addressed according to the slot number of the scanner module. If the scanner is in slot 1, the first point would be I:01.0/00. The last input would be I:01.31/15. You could (theoretically anyway) have 32 16 point I/O cards on each scanner. And (again theoretically) you could have 30 scanner cards (one in each I/O slot). So you can have a LOT of digital I/O that way. These scanners also support single slot, 2 slot , and half slot addressing, along with complementary I/O, so it can get a little strange. But if you stick to all local I/O, the SLCs are much easier to address.

Analog is also supported on RIO links with SLCs but is VERY slow, and a bit convoluted to use, so if you can it is best to put analog I/O in the local rack.

Bob Peterson

What sort of software are you using to program this? If you will be using an RSLogix software, configuration is automatic. The software will
make the right addresses for your IO. AB is using I for the discreet input and O for the discreet output (this is also true for the analog
in SLCs), like I:00/0, O:00/1, etc. If you have this software, look for the help file within the software.