When you start a new project, or add I/O to an existing project, the tags that you need to address for the I/O simply do not exist, so it's a non-starter.
To create the I/O data tags that you need, you have to put the I/O modules into the I/O Configuration (bottom folder in the project view). When you add the modules you need to use, the tags appear automatically.
Example : suppose I want to use slot 2 for a 1756-IB16 module. I add the module to the configuration, and it automatically creates 2 tags in the controller tag database.
IO tags are always created in the form "Location:Slot:Type".
Location is always "Local" for modules in the same chassis as the controller. For Remote IO, the location is the name assigned in the IO configuration for the comms module in the remote chassis.
The tags you would get from putting an input module in slot 2 are...
Local:2:I
Local:2:C
The "C" tag (Configuration) contains the data needed by the module to describe it's functionality. It is setup by going through the module configuration dialog tabs (wizard).
The "I" tag contains the data that is Input to the controller from the module, one of its elements is .Data, which is a 32-bit DINT containing the input bits for (upto) 32 inputs.
Now you want a normally open start pushbutton - let's say it is wired to channel 7 of the input module in slot 2. The tag address for that button would then be Local:2:I.Data.7
You can certainly use that address in your code, but most people want a tag address that means something, like you want "start_pb".
Logix5000 has no concept of using a symbolic address that substitutes for the physical address. Instead it uses alias tags.
An Alias tag is simply another tag that you can create which redirects the controller to the actual data location. An alias tag holds no data itself.
So you would create an alias tag named "start_pb" and configure it as an alias for Local:2:I.Data.7. Use "start_pb" in your code and it indirectly reads the data from Local:2:I.Data.7
The programming software (default install, but can be turned off/on) will display your tag, and underneath it will display what it is an alias for.
