CLX 1756-L61 high scan time

Hi All ,

I have around 27 units talking to CLX PLC over Modbus . From each unit i am getting temperature and pressure .

Based on the temperature and pressure i need to calculate the density , for which the client has given me a density table of around 6000 points , wherein for a specific temperature and pressure there is a specific density. We have also kept a band for each pressure and temperature reading.

To calculate the density i have used FOR loop . My ladder in ST is as follows

-------------------------------------------------------------------------
For x = 1 to 27 (*X is temp.variable in PLC and 27 is no of instrument *)
For x1 = 0 to 6000 ( x1 is temp variable in PLC and 6000 is counts in table*)
temp_array[x1] = Press_Array[x1]-Current_Press[x]
temp_array1[x1] = Temp_Array[x1]-Current_Temp[x]
if abs(temp_array[x1])< 0.1 and abs(temp_array1[x1])< 0.1 then
Density [x] = Density_Array[x1]
end if
end for
end for
--------------------------------------------------------------------------
Above example works fine but the scan time has increased to around 20 seconds .
All the logic has been kept in the continous task ...
Is there any way by which i can reduce the time

A few suggestions right off the bat:

First off, anything in the continuous task will scan a lot slower than something in a periodic or event task. The difference is dramatic enough that some programmers delete the continuous task.

Second, instead of sequencing through the whole array, check the middle first. If you are less than the middle value then check in the first half. If greater than the middle value then check in the second half. This is known as a half interval search. You can speed things up even more if you check the middle of each half, effectively dividing the array in quarters. If your programing skill are up to it you can take this search method even farther, finding the correct result in less than 13 comparisons. More info here.

Third, once you have found the matching result exit the loop. There is no need to keep searching.

I don't know what your table looks like but I have to believe that you have 6000 ordered pairs that are really a list of two lists that are likely better expressed as a two-dimensional array. For example, if your list contains either temperatures or pressures at fixed deltas you could index into your arrays at more logical locations.

Also, unless you are trying to maintain a history of your calculations to see if they work, there is no reason to use an array for your intermediate calculations (the temp_array values). You could just use a single tag of the correct data type. It would be recalculated and overwritten every loop cycle.

Keith