Posts posted by pmhpostman
There are a lot of instructions in function block that are missing the enable input that is present when programming in ladder logic, such as ADD or MOV. It seems to be very limiting. The only way around it that I know of is to write your own function block in ladder logic or statement list and use that. I seems rather limiting of a language if it relies on other languages for it to be usable.
Does anyone know why the enable bit was left out in many function blocks when it is so useful in ladder logic?
I did find a SEL (Select statement) in RSLogix 5000, but it is not present in Connected components. It would do what I'm looking for though, thanks.
I could write my own function block in statement list (and one in ladder logic), which I have done, but it seems like if you are programming in function block, you shouldn't need to be writing statement list. If the enable input were there, it would be a lot quicker/easier. Where's the logic in function block?
I am going to post a more general question about the enable input on function block.
I'm having trouble with some very basic programming using function blocks. Specifically Connect Components FB programming, but I think it is a general, common application.
Using ladder logic, it is easy to set a value based on the status of a bit. If the bit is false, move a zero into the variable. If the bit is true, move and 88.0 into the value. This relies on the enable bit of the mov instruction in ladder logic. Many function block instructions do not have enable bits. How would I change the value of a variable based on the status of a bit using function block programming?
A real world example:
You have a two position selector switch. If it is turned to the left, the drive runs at 0 Hz. (off) if it is to the right, it runs at 88.0 Hz. Or if you have a 4 position selector and want to select between 4 speeds. This should be a simple task, right? I've called the tech connect support line a few times and sent them a few emails and they haven't been able to figure it out. This is such a basic task.
Enable bit in function block
in Allen Bradley / Rockwell Automation
Yes, this is an Allen-Bradley issue. I'm using Connected Components for this project, not RSLogix 5000. I was unaware of the ellipsis in 5000, but I don't do a lot of function block with it.
Besides this issue, I do like the Connected Components environment. You can program in ladder, statement and function block like the big Allen-Bradley PLCs for a low price. But it does have some flaws...