PLC programming - Adding numbers question

programist · Oct 23, 2015

All,

I was wondering. How do you add a number to another number(say level input) only once, to keep it constant.

Example: You have Num1, which is some kind of input reading from a sensor. You want to add 100 to that number. So instead of X it will show X+100.

ADD instruction works every scan. What's the best way without using ONS.

Thanks!

bernie_carlton · Oct 23, 2015

Use OSR?

Seriously, the one-shot instructions are there for precisely that purpose, to set a bit on the edge of an event which may last for more than one scan.

programist · Oct 23, 2015

I meant OSR. But is there any other way?

Paullys50 · Oct 23, 2015

Your example doesn't make sense.

If you are reading from a sensor, and want to put an offset of 100 on it, you'll have to do that every scan as the sensor reading will constantly change, so you'll need to constantly add 100 to it to maintain it.

If you just needed an initial sensor input + offset as a reference, then yes an ONS/OSR would be what you would use. Of course you could always add a latch to ensure it only happens once, but the ONS/OSR is simple and obvious.

Steve Bailey · Oct 23, 2015

Assuming Num1 is from an analog input module and it gets updated every scan, put the result of adding 100 to Num1 in a different location than Num1 and let the instruction run continuously.

osmanmom · Oct 23, 2015

programist said:
All,

(say level input)

Thanks!

probably fix with timer or compare instruction set

AGill · Oct 24, 2015

Not sure we have enough info of your application. If you're continuously monitoring Num1 and you always want an offset of 100, you'd need a second variable, say Num2 (Num2=Num1+100). If your capture of Num1 is event driven, then again you'd use a second variable Num2 but only add the 100 at the time of capturing Num1's value. Do this with a oneshot (ONS or OSR as already stated).

RET · Oct 24, 2015

Just set the sum to zero in the rung just before the add instruction.

Mickey · Oct 24, 2015

I was wondering. How do you add a number to another number(say level input) only once, to keep it constant.

Once?? Once a day,month,year,hour?

programist · Oct 26, 2015

To clarify, I want to put an offset on analog input (num1). I thought there might be a function already build it for that. (example: Num2=Num1+100 with one shot).

harryting · Oct 26, 2015

Like Paully and Steve said, just use ADD since your input change with every scan you need to add this offset every scan, that's the very definition of doing an offset

programist · Oct 26, 2015

A simple ADD works well.

Thanks all!

PLC programming - Adding numbers question

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