"Once" pascal statement

I don't quite get the meaning of the "once" statement which is part of the programming language within the PAC. It isn't standard Pascal, and the explanation provided by the system states that any commands which are given with the "once" statement run just once rather than all the time. Can anybody please provide any kind of explanation? Presumably the Pascal program, once uploaded, becomes resident in the PAC (or is it stored somewhere else?), and the whole program must be continually looping, otherwise it would never see the change of state of whatever is contained within the "once" statement, and thus wouldn't work as expected. If this is correct, the commands thus declared within the "once" statement are therefore running continually, not just once. If anybody could throw some light on this, I'd be very grateful.

 

Here is my attempt

Code:

SetLightingState("[COLOR="Red"]Light A[/COLOR]",ON)
once ((GetLightingState("[COLOR="Blue"]Light B[/COLOR]") = ON) AND (GetLightingState("[COLOR="blue"]Light C[/COLOR]") = ON)) then
begin
SetLightingState("Light D",ON)
end
if(GetlightingState("[COLOR="Lime"]Light E[/COLOR]") = ON)
begin
SetLightingState("Light F",ON)
end;

In the above example let assume the logic is running on PAC and when it is turned on, the Initialize turns "Light B" and "Light C" OFF.

In every loop "Light A" will be turned on. "Light D" will only be turned once Light B and C turns on.

1. if between the loops, a light A is turned off using a keypad, the light will turn on again whent the PAC loops through the code.

2. If Light B and Light C are ON, Light D will turn ON, if Light D is turned off by a keypad as well at the end of the loop or before the begining of the next loop, it will not turn on again until either lights(B and/or C) turn off and on again.

3. if light E is ON, light F will turn on, if light F is turned off by a keypad as well (like 2), it will turn on again as long as E is ON.

Basically ONCE means the code within the ONCE begin ..... end will only be executed the initial time the condition turns true, and it does not matter how long the condition stays true, the code within the ONCE will not be executed until the condition goes false first and then true again. You must also not that the condition needs to turn false while the PAC is online before the first true condition will be executed.

It is like when someone says "Now listen carefully, as I will only say this once", same principle.

Hope that helps.

 

Neil,

Check out this thread - http://www.cbusforums.com/forums/showthread.php?t=3364

 

Yeah, not so hot on the name myself. I prefer to think of it as a transition trigger. That is when the condition defined in the once declaration transitions from false to true then the code in the block gets executed. Next pass around the code doesn't get executed because there is no transition, the condition is already true. So for the block to execute a second time the condition must first become false and then transition to true again.

 

That's only half the story guys...

You have focused on the positive transition in your once descriptions. Don't forget negative transition or negative edge is just as relevant to all considerations eg

Code:

once GetLightingState("LightsZone1")= OFF then
begin
   {do something}
end;

The PAC loops it's code approx 5 times a second so a traditional "if" statement would result in "doing something" 5 times a second (not good in the C-Bus world when obviously we only what it to happen "once" on the transition positive or negative (true or false). It was a very clever man who made "once" availabe in the Clipsal logic platform

 

I agree, I use Once a lot ("more than once" ) in my projects and I think it is a nifty feature and saves a lot of coding, imagine having to use ?if? and cater for the unnecessary repeats along the way.

 

The once statement only has one transition and that is from the condition being false to being true and that is what I described. Your example of checking for a light turning off is still a check for the condition (light being off) transitioning from being false to true. This is exactly what was described.

Are you suggesting we need a trailingonce() method?

Just putting the negated condition in a normal once should achieve the desired result, exactly as you described. What was erroneous with the description?

Code:

once GetLightingState("LightsZone1")= ON then
begin
   {do something when the light comes on}
end;

once GetLightingState("LightsZone1")= OFF then
begin
   {do something when the light goes off}
end;

 

The context of this thread was a description of once v's if. I have previously and successfully described this operation via a description of positive / negative or leading / trailing edge transitions. Departing from this, you are correct, from a "code" perspective it is all a statement of false to true.

I spose it depends what direction you are looking from

 

Arr .... OK. I have a personality flaw where I see everything as code. Doesn't work so well with my wife and 2 year old either. Although seems to work remarkably well with the dog.

 

Gee Richo, you wouldn't happen to have worked as the Visual FX guy for a few rather popular movies now would you?

 

You must have a smart dog

Come to think of it my dog sits around all day, gets fed regularly, allowed to stay inside when it is cold, rainy, stormy, noisy, or any other reason. When I tell her to do something she looks at me with that "yeh right - whatever you reckon" look. I work my back side off to pay for it all and clean up the back yard after the mut. How smart must I be, NOT !

 

ONCE an event occurs do something once.
IF an event occurs do something repeatively until that event changes. (Looping)

Well that's my attempt to explain it.

so to sum up, open IF statements are Terribad when placed in open modules that don't get disabled at the end coz they constaintly loop and will flood your processes. So you must code it wisely.

 

Josh Said:

SetLightingState("Light A",ON)
once ((GetLightingState("Light B") = ON) AND (GetLightingState("Light C") = ON)) then
begin
SetLightingState("Light D",ON)
end
if(GetlightingState("Light E") = ON)
begin
SetLightingState("Light F",ON)
end;


Josh, your explanation is good, even I can follow it. However in point 2 you state that

"If Light B and Light C are ON, Light D will turn ON, if Light D is turned off by a keypad as well at the end of the loop or before the begining of the next loop, it will not turn on again until either lights(B and/or C) turn off and on again."

Should that read "both lights(B and C) turn off and on again.", as you have an 'AND' between the tests in the once line?

Regards,
Nigel Todd

 

You are right Nigel.

If you need to get around this put the 2 GA's (or more) into a scene and test the scene.

 

No.

If light B or C turns off, the condition will be false (B & C), remember

B............... C............(B and C)
false..........false..........false
false..........true.......... false
true ..........false..........false
true ..........true.......... true

So in short, B and C do not need to be both off in this case to create a false condition. They would however need to be both on to create the true condition.

Hope that helps clears the condition.