the "big picture" approach ...
the absolute best way to wrap your head around the whole "addressing" idea is this:
most people START with the hardware – and THEN try to come up with a series of "addresses" to tie that hardware to the PLC's input and output data tables ...
that's backwards ...
actually it's best to START with the data tables FIRST (a paper printout is often very handy) and THEN see how the PLC's hardware devices "plug in" to those tables ...
in other words, you can take the printouts of the input and the output data tables – and then pencil-in ALL of the various "racks" and "module groups" even BEFORE you start messing around with ANY of the hardware ... this is pretty much like laying out the map for a new section of land – before you start building any roads or houses on it ...
so the "higher level" approach to the addressing problem is to realize that the PLC-5 processor originally left the factory with ALL of its "racks" - and ALL of its "module groups" - already pre-defined and permanently "mapped out" in its little electronic brain ... and now that the processor has been installed in your plant, it's just a matter of figuring out where all of your field hardware devices "land" on that pre-existing map ... the "trick" is that (unlike a map of a new subdivision) we don't use "streets" and "lot numbers" to refer to the various locations – instead we use "racks" and "module groups" to identify specific areas of the map ...
so yes, it's completely natural for someone to point to a specific piece of hardware and ask questions such as: "What rack or chassis is THIS?" – or – "What group is THIS module in?" ...
but ...
a much better approach would be to point to the same piece of hardware and ask questions like this instead: "Where on my data table does THIS particular device land?" ... once you start looking at it THAT way, then a lot of things that never made sense before – suddenly fall right into place and become totally obvious ...
I've lost count of how many grown men over the years have gotten tears in their eyes when I've explained these "addressing concepts" this way ... keep in mind that most of my students are technicians with years of PLC experience behind them – and many of them will tell me that this "PLC-5 addressing thing" is a topic that they've never really gotten a handle on – regardless of how many "other" classes they've gone through ... sometimes just realigning their point-of-view is all that it takes to relieve years of frustration and confusion ...
side trip: think of how easy this "map first" approach makes addressing devices such as PanelViews and drives ... the fact that these types of hardware don't really fit into "racks" - and that they don't have any "modules" - is thoroughly confusing to many people ... but once you ask the question: "Where does this PanelView land on the data table?" then BINGO! – things suddenly start to fall into place ... now the terms "racks" and "module groups" can be seen for what they REALLY are ... specifically, they're just the terms that we use to identify the "real estate" located on our data tables ...
the fact that we happen to be plugging a "full rack" of our "rackless" PanelView into the specific segment of our real estate identified as "rack 01" starting at "module group 0" now makes perfect sense ... just circle that part of the data tables and pencil in "PanelView" and you're done ...
on the other hand, if you're still trying to picture that same PanelView as being a "rack/chassis/module" type of thing, then the statement above comes out as pure gibberish ...
in simplest terms, it all boils down to this:
once you learn to recognize the "racks" and the "module groups" which are already permanently mapped out on your PLC-5's data tables - then the "racks" and the "module groups" in the field hardware will pretty much take care of themselves ...
