S:24 Displays Zero Always

Just curious about something with the index register bit S:24. I noticed that in our programs we use it multiple times however no matter where I look at it, it's value appears as zero to me.

I'm assuming it's still working however, not sure what's going on though. If anyone understands this better it would be greatly appreciated if this could be explained to me. Thanks!

S:24 is a word, rather than a bit. It's function is to keep track of the element offset used in indexed addressing.

Bit_Bucket_07 said:

S:24 is a word, rather than a bit. It's function is to keep track of the element offset used in indexed addressing.

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Shouldn't it have some sort of value in it though when it's being used? I'm assuming it's just a 16 bit register.

PlastekGroup said:

Shouldn't it have some sort of value in it though when it's being used? I'm assuming it's just a 16 bit register.

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The value can change rapidly, and you aren't likely to see that while viewing the program logic.

Correct it will change anytime the PLC does any kind of COP or other type instruction.

You can use it with a MOV for example. If you MOV a 2 into S:24 then the very next rung you do a MOV of #N7:0 into N10:0 it will move the value of N7:2 into N10:0. Basically indexing starting at N7:0 + 2 where the # tells the PLC to offset the address the number in S:24.

It's really another way to do indirect addressing but they call it indexed addressing. The same MOV using N7:[N20:0] to N10:0 if N20:0 has the value of 2 would do the same thing.