AB Compute Instruction

On an AB compute instruction-
if I see

Dest F8:26

Expression (I:12.8 * 1000) + I:12.9

What does the I:12.8 represent?? This cant be a bit- can it??
That wouldnt make much sense. I understand the Destination part.

Thanks

I believe I:12.8 would be an input. I:12.9 would also be another input.

SLC-500 nomenclature for Input words from modules in the controller chassis is like this:

I:S.W/B

Where it's I for Input and O for Output.
S = Slot
W = Word
B = Bit

I'll bet you're addressing input channels from an analog module or a network module. There's no Bit designator, so you're addressing the whole Word.

I:12.8 is Word 8 of the Input image for the module in Slot 12
I:12.9 is Word 9 of the Input image for the module in Slot 12

So the question is... what's in Slot 12 ?