Indexed Addressing Allen Bradley

Ive just been looking through a program that utilises indirect addressing. now i understand the indirect addressing just fine, but whilst discussing with my colleague it has arisen that there is something called indexed addressing. Now having read the help file in RS Logix i understand how it works i think, but i dont really see the point of it. Surely it would be easier to use an integer file to store and deal with the number/s or am i missing the point?

Any words of wisdom about this would make a bunch of blokes here sleep a little easier.

Cheers

A number of instructions, block moves etc, use the indexed method normally. Indexed instructions operate faster than indirect instructions but take more to set up (you have to specifically put the offset into S:24)

On rslogix500 indexing is done thru S:24, please look at manual 1747-RM001 , LOOK AT PAGES E-11, E12, E14.

another consideration: some of the smaller SLC/Micro processors don't support the indirect method ... in those cases indexed is the only way to get the job done ...

Ah ok i see, that sounds good. well i had a play on some training equipment and i can see it all very clearly. the only question i have is, why in a copy function are all of the addresses prefixed with the # symbol. i thought that this denoted it was set through the index register. the index register value is added to the value of your address and it works on that particular data file.

Is this correct or again am i missing the point?

please post an example of a rung that is confusing you ...

The copy command internally uses the index register. While it probably could have been drawn without the '#' it is a good reminder that, after execution of this command, the index register (S:24) is in an unknown state. You would not be able to depend on it remaining constant from scan to scan if you are using it that way. In other words, before any use of the index register other than in these instructions which show a '#' you must deliberately load S:24 with an appropriate value.

bernie_carlton said:

In other words, before any use of the index register other than in these instructions which show a '#' you must deliberately load S:24 with an appropriate value.

Click to expand...

And, save the final value somewhere if you're going to need that value later.