Alias IO in User Data Type in ControlLogix/Studio 5000

I'm new to AB and I'm running into a lot of things people tell me can't be done in AB that actually CAN be done, so I thought I'd ask here if anyone knows of a way to point to IO points from a structure (specifically ControlLogix with Studio 5000).

To be clear, I know that the editor won't let you make a structure member an alias tag, but this seems like a very arbitrary limitation and I thought there might be a way around it.

For now, I have to make a routine to copy in all the input values and then another to copy the output values out. AB gives me the distinct feeling I've had my tools taken away and I've been given a pair of safety scissors and a helmet.

CapinWinky said:

I'm new to AB and I'm running into a lot of things people tell me can't be done in AB that actually CAN be done, so I thought I'd ask here if anyone knows of a way to point to IO points from a structure (specifically ControlLogix with Studio 5000).

To be clear, I know that the editor won't let you make a structure member an alias tag, but this seems like a very arbitrary limitation and I thought there might be a way around it.

For now, I have to make a routine to copy in all the input values and then another to copy the output values out. AB gives me the distinct feeling I've had my tools taken away and I've been given a pair of safety scissors and a helmet.

Click to expand...

I feel your pain.... You'd have thought it ought to be possible for a structure member to be an alias to another tag, but not so...

One way round this is to make all the I/O points go through handling function code, or AOIs, then they can just be specified as "Required" Input or Output parameters. Most I/O will be for field devices like pumps/valves/limit switches/level probes etc., which would usually have a standard "handler".

Even if it's a simple pushbutton, there's no harm in using a "debounce" handler (with zero times set), to allow the input to be mapped into the UDT.

There's a TN on this:

29568 - FAQ: Why cant I put an alias in a UDT or pass a tag to a subroutine by reference?

Basically it has to do with where in memory the UDT is stored and the individual elements. It would take too long to resolve all this.

Essentially that tech note says aliases aren't pointers and AB doesn't support pointers; they're just replaced by the compiler with the base tag name and are not a location in memory that contains the memory address of a base tag. While that does explain why you can't dynamically change where an alias points, it is pretty weak on the reason you can't have them in a UDT.

They're saying they can't just do the find and replace thing and leave the alias out of the memory footprint because they can't figure out a good way to do it. Instead, they'll just claim their only option would be to completely bog down the system with half assed pointer support. No wonder RSLogix/Studio 5000 has basically been the same for more than a decade.