select a mximum value

speedcat · Apr 27, 2009

i have a number of registers (vw200,vw202,vw204....)each of the contain a certain value i want to know the maximum value among these registers ,how can i do thaton s7-200?????

Roy Matson · Apr 27, 2009

What PLC are you using?
What Software are you using?
Roy

speedcat · Apr 28, 2009

siemens s7-200 cpu 222,microwin.....& i want to get the maximum of 100 variables so using compare instruction between every 2 variables will be very hard

speedcat · Apr 28, 2009

any ideas??

Stationmaster · Apr 28, 2009

I was shown just recently that the quickest way to select the register with the maximum or minimum value from a number of registers requires doing a compare between each register.

There are only two possibilities I can think of that would allow using less:

1. If the PLC you are using has a special function of some type that does sorting,

2. If there is some type of special relationship between the registers ie: grouped in some way that the highest value is certain to appear in a particular group with another characteristic in common.

I'm interested to see if there are any creative suggestions too..........

Stationmaster

L D[AR2P#0.0] · Apr 28, 2009

When you write to each of the variables, perform a comparison at that time and maintain the current max value. You will still have to perform the same number of comparisons. Are all 100 values written in one scan or what ? 99 compares is not difficult - you could train a monkey to do the typing for you (that would be difficult)

Lancie1 · Apr 28, 2009

Stationmaster,

Here is a creative suggestion just for you. This is a RSLogix routine that will sort any quantity of numbers (up to 255 numbers without any changes) with the same 15 rungs of logic.

It has two For-Next loops, and compares two numbers at a time until all are sorted. The J-loop does "For J = 2 to N Step 1", where N is the number of values to be sorted. The I-loop runs inside the J-loop and does "For I = J-1 to 1 Step -1".

The sorted numbers appear in memory locations N7:0 (smallest) to N7:x, where x is the location of the last and largest number.

Stationmaster · Apr 28, 2009

Stand by Lancie, I'm checking to see if my PLC will do for/next loops.

Lancie1 · Apr 28, 2009

Stationmaster,

If it is like most PLCs, it will do them if it can accept labels, jumps, and indirect addressing. The For-Next Loops are not built-in functions in this program, but are created using simple add, subtract, label, and jump instructions.

Off-Topic: Stationmaster, I just gave away the lay-off plow. My 87-year-old uncle called and said he heard I was moving and could he have my layoff plow since I wouldn't need it anymore and he sure could use it in his garden. What could I say except yes?

Stationmaster · Apr 28, 2009

Lancie1 said:
Stationmaster,

If it is like most PLCs, it will do them if it can accept labels, jumps, and indirect addressing. The For-Next Loops are not built-in functions in this program, but are created using simple add, subtract, label, and jump instructions.

Sorry for the delay, one of my MATER packers called an I had to switch to "packing plant" mode for a while. There are a LOT of distractions on this end.

Yes Lancie, it WILL do those functions and I see how to make the loops using counters. I may need a little help translating, but I'm pretty sure I can adapt it to IDEC. I use the IDEC Microsmart Pentra slim type. (I feel ssssooo inadequate.)

Lancie1 said:
Off-Topic: Stationmaster, I just gave away the lay-off plow. My 87-year-old uncle called and said he heard I was moving and could he have my layoff plow since I wouldn't need it anymore and he sure could use it in his garden. What could I say except yes?

Did he at least leave it BACK to you in his WILL?

I heard you were moving and I could sure use your TRACTOR......

: )

Lancie1 · Apr 28, 2009

Stationmaster,

You could use counters, but I just looked at my copy of WindLDR, and you have a JMP (Jump) and JEND (JumpEnd instead of Label) so you can almost translate the program line for line, using the JMP and JEND to define the loop boundaries, instead of Jump to Label and Label. You also have an Indirect Address Move (IMOV) so you are in business if you need a sort routine.

No, my uncle didn't mention his will. He can and got the plow just now. I will build another one if I need it. I built the old one from scrap metal from Roberson's Scrap Metal yard.

Stationmaster · Apr 28, 2009

Lancie1 said:
Stationmaster,

You could use counters, but I just looked at my copy of WindLDR, and you have a JMP (Jump) and JEND (JumpEnd) (instead of Label) so you can almost translate the program line for line, using the JMP and JEND to define the loop boundaries, instead of Jump to Label and Label.

Lancie, In WindLDR, if you select the PLC "Microsmart Pentra slim type" you'll see the instructions for branching, including LABEL and LJMP.

Den Mark · Apr 28, 2009

Word (W) registers max values :

signed:
Decimal - 0 to 65,535
Hex - 0 to ffff

unsigned:
Decimal - -32,768 to 32,767
Hec - 8000 to 7fff

Lancie1 · Apr 28, 2009

Stationmaster,

Yes, I see those. I do not see how to do indirect addressing for the comparision instructions however. That may require a work-around.

Stationmaster · Apr 28, 2009

Lancie1 said:
Stationmaster,

Yes, I see those. I do not see how to do indirect addressing for the comparison instructions however. That may require a work-around.

Okay.... pardon this next question, but by "indirect addressing" do you mean specifying a data register instead of a numerical constant? Or something else?

select a mximum value

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