UDFB output function value doesn't match the connected variable

[Adamkse]

Hi,
I have a PID function block  with an embedded PWM  function which should turn a heater on and off with a duty cycle. However it is always set on (1). When I monitor the PID UDFB the output from the PWM is 0 but the main ladder has it as a 1. If i drill down further in the PWM UDFB then that is also 0. I don't  understand what is happening.

Any help would be great.

[Adamkse]

Noticed that my PID_HLT output was called "PID Output bit" and not "PID Output" as I had.

Still the same problem but the oposite way round "HLT1 OUT" is always 0 even if "PID Output bit" is changing.

 

 

[Adamkse]

No ideas on this?

[Saragani]

Without having the project and debugging it, and seeing on own eyes, no one will be able to help you.

[Ausman]

What's happening in the real world at the actual heater?  Perhaps it is pulsing very rapidly and the "limited" bandwidth for monitoring is not keeping up?

Aus

[Adamkse]

Hi all

I figured it out. In real life the heater was off as the code suggested. The issue was related to the set/reset coils not functioning properly. The do not work as local, function in or function out tags. Only global tags.

[Saragani]

Hi.

Value types, like integers, Bits, Real, are being passed by value (Meaning, their value is being passed to the function, and not the pointer to the value) when used in a function-in.

So any attempt to change their value within a function code will only affect the value inside the function.

 

Arrays and structs are being passed by Reference, so if you change a value of a bit or a number inside it from a function code, then it will affect the value outside the function.

 

You have 2 options: Passing an array/ struct, or pass the number / bit as a function-out (which means that it will be passed as a reference).

I hope this information helps.