The next system that we are building will be sorting through as many as 2000 barcodes. Using Control Logix, a barcode will be recieved, and that barcode will need to be found in the list. I was looking at a "Tree" structure to find the code in the least number of scanns or loops. This is normaly done in VB, but has anyone ever attempted to do this in Ladder Logic?
Using Data Tree
- Thread starter MavEric
- Start date
rdrast
Lifetime Supporting Member
Phew... that would be an interesting task for a PLC...
One big problem that I see, is that BTrees generally rely heavily on pointers and dynamic allocations. That can be rough on a PLC, but could be gotten around perhaps just by allocating some huge data tables to begin with.
Second problem would still be traversal, you will have to deal with non-contiguous iterators... Possible, maybe.
I'd offer that if you really want to try this, your best bet is to do the programming in structured text, but even so.... the potential for errors would be huge.
If you really really really want to do this integral to the PLC, I'd suggest getting one of the Prosoft Module's, and programming the app within that, and use the CIP Backplane interface to send data to the module, query it back, etc.
One big problem that I see, is that BTrees generally rely heavily on pointers and dynamic allocations. That can be rough on a PLC, but could be gotten around perhaps just by allocating some huge data tables to begin with.
Second problem would still be traversal, you will have to deal with non-contiguous iterators... Possible, maybe.
I'd offer that if you really want to try this, your best bet is to do the programming in structured text, but even so.... the potential for errors would be huge.
If you really really really want to do this integral to the PLC, I'd suggest getting one of the Prosoft Module's, and programming the app within that, and use the CIP Backplane interface to send data to the module, query it back, etc.
Ken Moore
Lifetime Supporting Member
Have to agree, a pc would be best, Prosoft has PC modules for the CLX chassis:
http://www.prosoft-technology.com/p...ockwell_automation/platform/controllogix/pc56
http://www.prosoft-technology.com/p...ockwell_automation/platform/controllogix/pc56
I disagree. A search through the 2,000 or so barcode array with a FSC instruction, can be done in one scan, and I estimate quicker than passing the search target to a co-processor, then waiting for a result.
How fast do you need the answer ?
How fast do you need the answer ?
MartB
Lifetime Supporting Member
We've done a similar thing in ladder logic but only for about 150 barcodes, so on a much smaller scale than you are needing.
We used the FSC instruction, but did come across one problem when one of our barcodes was an element shorter than all the others when it was scanned (11 instead of 12 digits on this occasion), so this is something you may want to be aware of, as it is a DIRECT comparison with whatever you compare to in your array of references.
With only 150 comparisons we didn't notice any impact on scan time, but I would imagine that 2000 compares would probably have some impact.
Just my 2c worth.
We used the FSC instruction, but did come across one problem when one of our barcodes was an element shorter than all the others when it was scanned (11 instead of 12 digits on this occasion), so this is something you may want to be aware of, as it is a DIRECT comparison with whatever you compare to in your array of references.
With only 150 comparisons we didn't notice any impact on scan time, but I would imagine that 2000 compares would probably have some impact.
Just my 2c worth.
Peter Nachtwey
Member
CS101 sorting and searching
So how many compares do you think it will take? I think this can be done on a PLC.
So how many compares do you think it will take? I think this can be done on a PLC.
MartB
Lifetime Supporting Member
My understanding was that the FSC compares until it finds a match then bails out of the instruction and tells you where it found the match (I'm going to go and double check all this in a minute).
Therefore the worst case sceanrio would be that it's the last record that you compare meaning it could go through all 2000 records, and I just assumed (yes I know how bad that is) that this will take longer with more records.
I personally agree that it can be done in a plc, I only wondered what impact it would have on scan time and if that is a critical part of the process or not.
I'm off now to read up on the FSC!
Therefore the worst case sceanrio would be that it's the last record that you compare meaning it could go through all 2000 records, and I just assumed (yes I know how bad that is) that this will take longer with more records.
I personally agree that it can be done in a plc, I only wondered what impact it would have on scan time and if that is a critical part of the process or not.
I'm off now to read up on the FSC!
rdrast
Lifetime Supporting Member
daba said:
My reply was directed at creating a BTree for the barcodes to keep them traversally sorted, NOT using an FSC on a linear data block.
Peter Nachtwey
Member
MavEric, if you keep the numbers sorted you should be able to find the correct number in 11 compares. 2^11=2048. You do this as if you were trying to gues a number between 1 and 100. Start with 50 and then try 25 or 75 depending on whether the number is higher or lower than 50. This is called a binary search.
MartB
Lifetime Supporting Member
Gerry said:
I think that's pretty quick, for a PLC, and interesting to know, thanks for that Gerry.
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