Retrieve Values from fifo

[PLCwannabe]

What is the best way to retrieve the 20 newest values from a fifo data block with 1000 values. It seems like i can't use fifo.head or .tail as a reference.

[BobO]

What is the best way to retrieve the 20 newest values from a fifo data block with 1000 values. It seems like i can't use fifo.head or .tail as a reference.

Look at? Or remove from the queue?

[Controls Guy]

I ended up invoking NBH (Not Built Here), and did my own.   I always use the front of the range rather than rotating loading and unloading positions through the block and physically move the values, and I also include bubbling up past empty positions, so you can introduce new entries at any empty location and it will find the end of the line and stop (I do try to load directly to the correct location to minimize/eliminate the amount of required packing/bubbling)

With auto-bubbling enabled on detection of unpackedness, you can remove as many records as you want and the rest will just deal with it.

[Controls Guy]

Unpackedness is detected like this:

Assume you have a 100-slot FIFO, FIFODATA

Then V0 = COUNTIFNE(0, FIFODATA0, 100)

and  V1 = COUNTIFNE(0, FIFODATA0, V0)

If V0 == V1, then it's packed.

[PLCwannabe]

I want to store the last 1000 values at all times, but then i need to display the 10 newest values on Hmi. So i don't want to remove the values, I just want to view them.

[Controls Guy]

I think Host's FIFO has a public member variable that tells you where the current load point is.   IIRC, .head and .tail are fixed at the limits of the range (I think).   Seems like that should work for you.   Look at the long form display of the FIFO control structure in Data View, and I think it will show the unload position, whatever the name is.

Sounds like for what you're doing, my way is more work than it would be worth.

[Bolt]

I want to store the last 1000 values at all times, but then i need to display the 10 newest values on Hmi. So i don't want to remove the values, I just want to view them.

Your Memory Block "FIFOValues" would be 1000 in size.  When the FIFO needs to load a new value, you will want to FIFOUNLOAD a value out (and "throw it away"), and then FIFOLOAD the new value in there.  Your most recent values should all be at FIFOValues0 ... FIFOValues10 if I see your scenario correctly?  These values can be displayed on your HMI.

[Controls Guy]

That's what I'd have assumed too, but I don't think that's how it works.  I believe that instead of moving the values they keep track of the offsets to the newest and oldest values (and the current count of values)

[Bolt]

I stand corrected, I just checked.  Once the FIFO is .Full, you can reference FIFOValues989...999?  If not .Full, then some MATH V0 equals FIFO.Head-10, then FIFOValues[V0], followed by V0+1, etc.

[PLCwannabe]

I was trying to indirectly address FifoValues[Fifo.head], but that didn't work. But I guess I can move the Fifo.head value to a v location and then use that as indirect reference.

[BobO]

I was trying to indirectly address FifoValues[Fifo.head], but that didn't work. But I guess I can move the Fifo.head value to a v location and then use that as indirect reference.

It works in a MATH box.

[BobO]

A few points about our implementation...

For LIFOs, the Tail always remains at 0.

FIFO's are ring buffers, so both Head and Tail move. Performance. I worry about it so you don't have to.

When FIFO is empty, Head/Tail always reset to 0. In a random Add/Remove FIFO, Head and Tail can wrap, but in a batch Add/Remove, it won't wrap.