RS5000 FFU instruction

DairyBoy

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Join Date
Jun 2007
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UK
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Hello all. As I don't have anything to test this on I'll ask the question, when used without FFL, is FFU just a parallel in/serial out shift register? The local help file shows blank data (not zeros) in the unused or post-clocked array locations. I'd expect these to return to zero when the unload function vacates them but it's not obvious in the example. Thank you.
 
Yes, I just checked, and as a FIFO register unloads, 0 is inserted where the number was shifted from.
 
Dairy Boy - although the Logix5000 series uses a tag database, there is absolutely no concept (as in other databases) of "blank data", or "null".

A tag element must contain something.
An numeric tag element will contain zero when it is created.
A string tag will be set to zero length, and all its character array elements will be zero.

You cannot "empty" a numeric tag, you can only clear it's contents (to zero).

I think the help is trying to convey the meaning that the unused or post-clocked array locations simply contain "unwritten" or "uninitialised" data, and showing them as non-zero just confuses the issue.

Generally, though, I think the help is excellent, well-written, comprehensive, and a great self-learning tool.
 
Thanks for your replies (and trial). Daba, my "early days" were spent working with 7400 and 4000 series logic and as with eveything else it's either a 1 or 0, so the "blank" locations threw me a bit and as it's a system, and not just an ic, you never know what's lurking...
 
Finally got to site today and guess what? FFU doesn't want to play. I'm loading non-zero values directly into the array via the panelview and can see those values online and on the PV (the label for each numeric entry field). The FFU Empty flag remains on, even when the FFU is clocked, and the values don't shift in the array as shown in the help file. A bit of searching here leads me to think that I'll be using a copy function instead.

So, if there are 5 numeric inputs and the copy function shifts the data "down" by one element, should I set the array dimension to 6 and load only into the lower 5 elements so that it fills with zeros from the top as it empties?

Thanks.
 
When used without the FFL, I'm sure the FFU instruction still uses the Position data from the control. So if you manually load data into the array for the FFU (without using the FFL), you still have to change to Position so that it knows something is there.

I don't think the instruction determines Full or Empty based on data in the array, but instead by the current position.
 
Thanks, Tharon. So the position is incremented by the FFL and decremented by FFU? When you say "...change to position..." do you mean load a value equal to the number of nonzero elements in the array? If that's the case, there'll need to be more support logic around the FFU to tell it how many entries the operator has loaded. It's looking like COPY to me. Thanks.

Edit: At the end of the day, a shift register is just a copy with offset. I should have kept it simple.
 
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Ya, that was supposed to be "Load the position" not "to position". I think in order to use it the way you want, you would have to do some support logic to put the FFU at a non-empty position.

I have not done it before, but you might be able to just load 1 into the position, and see if it still shifts the whole array or if it only does what position it is currently at? If it does the whole array, then you won't have to worry about what position it should be in, only that it is in a position so the FFU doesn't read empty? I'd imagine it'd work if you just made it always think the FIFO was full as well, since you aren't using a FFL.
 

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