I/O mapping?

[robert_ 1]

hello guys! I have a question about programming techniques: why I/O mapping is used? ie why carry the signals from input modules to a internal bits? why not use directly bits from input modules?

[JRoss 84]

Two reasons that I know: 1. Makes changes and updates easier. For example, if an input goes bad, you only have to change it in one place in the program. I do some work for an equipment builder that occasionally has to add options to their machines that requires reassigning the I/O, and updating an IO map routine is much easier and more fool-proof than doing a search and replace. 2. The newer Allen-Bradley processors have an asynchronous I/O update, which means it could update an input in the middle of a scan. This makes troubleshooting more difficult. Unless you need the speed, mapping the I/O makes things simpler.

[BobB 58]

I find it unnecessary to be honest. I use Omron and if you change an I/O number in the symbol table it automatically changes it for you right through the program. You just have to stop the PLC to download the changes in that instance that is all. If the I/O numbers you wish to use are already active, just change them to somewhere out of the way and then change the I/O numbers you wish to have there to there new location and then change the ones you have put out of the way to their new location.

[bartolim 0]

You can look at it both ways as stated here. Personally, I use I/O mapping... as stated, if you have an input point go bad, you can just assign the mapped point to the new input point at one place and off you go. Works the same for outputs. An additional benefit that I find is that my equipment will not aloow you to force an input. With the input point mapped to an internal point, you can force that point to test logic. Just my $0.02

[BobB 58]

Then you have a good reason - I do not have that restriction with Omron at all - forcing inputs is OK. Forcing outputs is OK too.