It uses 'separation of concerns' or SoC (google for this). There is a much better way of doing this. But in the end, it's a good thing and I suggest you adopt it too. I'm sure there were lots of heated debates about doing this. PLC manufacturers quit using 'I/O maps' (input/output image tables) as a paradigm for lots of good reasons. No matter how many times I've seen programmers promote this as a good thing, it isn't. This gives electricians huge problems when they try to troubleshoot it because there's always an extra level of indirection in troubleshooting it because they always have to figure out the 'I/O map', and then go find the real input/output. You just destroyed the link between the real I/O location and the internal states. I've dealt with an entire system that the programmer did this with it. There is a huge reason for NOT doing this. The real solution is to program your system in such a way that there are no race conditions like this. Simply copying the entire bit or word 'solves' the problem by making a modern PLC act the same way as an older PLC. This sets up a race (which happens first), which can lead to all kinds of strange errors in the sequencing of a program, if you are not cognizant of it. If you have sequential logic that depends on the inputs and you read the same input from different locations in your program, you cannot assume that the input bit will remain the same value. However, this super-fast I/O sets up an issue that you need to be very aware of as a programmer.

Second, it allows your program to get an updated value and react to it as fast as possible, frequently achieving sub-millisecond response times for very fast processes such as high speed (10,000 piece per second or higher) equipment or motion control. First, quite often 90% of your I/O isn't changing state at all and it wastes time and resources to constantly update a value that doesn't actually change. There are a couple reasons for doing this. I/O is updated independent of logic solving. Finally, it writes the data in the output image table into the output devices.Race conditions dealing with I/O in a single scan can't happen because from a program point of view, all I/O is updated 'simultaneously'.

Then it solves the logic (aka 'runs the program'). First, the PLC reads all the inputs into the input image table. On older PLC's, there are 3 phases in each scan. The biggest reason that programmers do this has to do with having race conditions in their logic. Then when you go to transfer the data into the I/O card, you treat it as a full word and write it to the card, or vice versa, you read an entire word at once.

How you do it is that you have say a 16-bit word, or you create a 16 bit array with individual bits.

Logixpro door simulation exercise 3