Step5 prgram help

[srik]

Dear all,

We are using Siemens PLC S5 135U. I need to generate a digital output from an analog input, low set value and high set value. I know there is a functional block, FB112 for high /low selections. But it's output is also analog value.Can I use any other functional block for this purpose?

 

[moggie]

Compare

Just use the compare function.

If the actual analog input is less than a "fixed value" the turn on a bit.

If the actual analog input value is greater than a "fixed value" turn on a bit.

L Analog in
L KF 50
<F
= F10.0

The above is just an example.

 

[srik]

Can I use direct Digital output, instead of flag?

 

[ivo.maenen]

Yes.

L Analog in
L KF 50
<F
= Q10.0

 

[JvdV]

There is one thing to keep in mind when you do an analoge compare function.
If the value is not stable the output will go on and off depending on the analoge value. If that is not a problem you can use it. If it is a prolem when the output goes on and off rapidely (most of the time and aplication I know), you have two options. The easiest way is to build in a time delay for on off.
The best way (programming wise) is to make an hysteresis. So the switch on an off points are different from each other. The size of the gap will depent on the apllication and type of output.

Perhaps you can give us some more information on the aplication you are playing with. I bet there are a lot of people out there that could give you more information based on the aplication.

 

[504bloke]

!=F Compare for Equal to

><F Compare for not equal to

>F Compare for Greater Than

<F Compare for Less Than

<=F Compare for Less Than or Equal to

 

[srik]

Ok,

Thanks

I will try with SET/RESET functions

 

[JvdV]

With set reset you will have the same question. When do you set and when do you reset.

Programming wise it's not very nice to use set reset. Normally you only use set/reset when you absolutly need to and don't have an other option.
The danger is in the part that you get a situation that reset is forgotten, jumped over. You will end up with a set status when you don't want it.
So in my opinion you can beter go for the safe option !

If you give some more information on the aplication I (or one of the fellow programmers) can try to point you in the correct direction.
Begin with:
- What input signal do you have and where does it come from.
- What are you trying to do with the output.
- What is the output switching on and off.