Many Copy instruction VS 2 copy instructions

Darkzadow · Jul 17, 2012

Hi Forum,

I am shifting an array of 50 UDT so each element is one position lower than it used to be, and the final element is overwritten. I was wondering how the PLC 5000 would handle 49 copy instructions vs 2 large copy instruction(to a temp array and then back). Aside from using extra memory for a temporary array would the copy to temp array be faster slower than the 49 single copies. And any other differences the 2 methods would have.

Thanks
Dz

DamianInRochester · Jul 17, 2012

I would use the temp array. I would expect the 49 single copies to be significantly slower, especially depending on how you implement it. Were you thinking of doing a For loop in one single can, or brute force them out? Sometimes these things are more easily answered by making some test code and running it. Have it do each 10 to 100 times in a row to make the timing differences more pronounced.

TConnolly · Jul 17, 2012

If you are copying down then you should be able to do it with a single copy.

COP MYTAG[1] MYTAG[0] 49

Mytag[1] is copied to Mytag[0]
Mytag[2] is copied to Mytag[1]
.
.
Mytag[49] is copied to Mytag[48]

Oldest data is at Mytag[0] and newest data goes to Mytag[49]

When you copy up as in
COP MYTAG[0] MYTAG[1] 49
then you end up overwriting the array with a copy of MYTAG[0].

If you are concerned about performance use the single copy method and put your newest data at the end of the array instead of at the beginning.

Darkzadow · Jul 17, 2012

I am copying up.

the overwrite issue was annoying. The single copies are indeed faster. I am not trying to complete in a single scan. just before I insert new data.

I was wondering if there were any other nuances such as interrupted execution issues, command load and execution time, and memory allocation problems.

I am moving to the single copy cause it is much easier to read, and is faster. I would emulate a queue with a moving start pointer but too many other systems point to the array that I don't have access to.

gscure · Jul 17, 2012

Synchronized Copy

I would suggest using the synchronized Copy (CPS) to make sure the integrity of the data remains. During a CPS the processor cannot interrupt the process until the copy has completed. Otherwise the data could be updated in the middle of the copy and get corrupted.

DamianInRochester · Jul 17, 2012

TConnolly said:
If you are copying down then you should be able to do it with a single copy.

COP MYTAG[1] MYTAG[0] 49

Mytag[1] is copied to Mytag[0]
Mytag[2] is copied to Mytag[1]
.
.
Mytag[49] is copied to Mytag[48]

Oldest data is at Mytag[0] and newest data goes to Mytag[49]

When you copy up as in
COP MYTAG[0] MYTAG[1] 49
then you end up overwriting the array with a copy of MYTAG[0].

If you are concerned about performance use the single copy method and put your newest data at the end of the array instead of at the beginning.

I have actually had cases where copying an array to itself in this manner has resulted in some values being misplaced. I would still recommend copying to a temp array first whether you are going top down or bottom up. In my case the array was about 100 elements and it was often that about 30% of them would not get shifted.

gscure · Jul 17, 2012

If this was being done on a controllogix then the loss of data could have been due to updating of information in the middle of the copy. The CPS instruction is great for preventing that from happening.

Darkzadow · Jul 17, 2012

Since it does not allow the processor to interrupt the copy does that action monopolize the PLC? How does the plc handle actions (IO, MSG instructions) that would happen or be recieved in that time frame?

gscure · Jul 17, 2012

That is correct it will halt other operations from happening such as messaging (which is how the I/O is process with the controllogix processor) and periodic tasks will be delayed. This is a very small amount of time which I have not seen any negative impact on performance.

Many Copy instruction VS 2 copy instructions

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