Logix 5000 FAL Read Every Other Member

[AMarks95]

Is there a way to use the FAL instruction to do +=2 instead of +=1? I have an array that is organized with alternating "data" and "flag" values.

If not, I'll just have to use a FOR loop.

 

[drbitboy]

Set the Length parameter value to (array_length / 2), and use the expression

the_array[control.POS * 2]

or

the_array[(control.POS * 2) + 1]

in the FAL instruction?

 

[AMarks95]

Set the Length parameter value to (array_length / 2), and use the expression

the_array[control.POS * 2]

or

the_array[(control.POS * 2) + 1]

in the FAL instruction?

Thanks, that definitely seems like it would work, but I ended up figuring out the structured text for loop instead and it's much more readable. Just a quick JSR and it's done.

SIZE( Array_Raw, 0, length );

FOR i := 0 to ( length - 1 ) by 2 DO

    Array := Array_Raw * ( 16.0 / 100.0 );

END_FOR;

 

[Corsair]

Everybody on this forum has their pet soapbox - I'm no exception to that.

My soapbox is to avoid brand-specific instructions with complex behaviors like FAL and its cousins. They came about because of the perception of needing to do those sorts of things in ladder. The ST is easier to understand, it is much more processor brand independent, and there is no need to research how the instructions work and their curious side-effects. If someone does not understand ST they really don't understand LD either.

Shouldn't the code inside your loop have some reference to the 'i' index?

 

[robertmee]

Everybody on this forum has their pet soapbox - I'm no exception to that.

My soapbox is to avoid brand-specific instructions with complex behaviors like FAL and its cousins. They came about because of the perception of needing to do those sorts of things in ladder. The ST is easier to understand, it is much more processor brand independent, and there is no need to research how the instructions work and their curious side-effects. If someone does not understand ST they really don't understand LD either.

Shouldn't the code inside your loop have some reference to the 'i' index?

Devils advocate and the counterpoint, in the case of Rockwell, STX often requires a different license that not everyone wants to pay for. And most technicians who support the PLC when you walk out the door, hate STX and Function Blocks.

 

[AMarks95]

Shouldn't the code inside your loop have some reference to the 'i' index?

Oops

SIZE( Array_Raw, 0, length );

FOR i := 0 to ( length - 1 ) by 2 DO

    Array[i] := Array_Raw[i] * ( 16.0 / 100.0 );

END_FOR;

Devils advocate and the counterpoint, in the case of Rockwell, STX often requires a different license that not everyone wants to pay for. And most technicians who support the PLC when you walk out the door, hate STX and Function Blocks.

And yes, I agree FB is terrible. I love ST though. If they can read they can handle ST, especially something this simple. There are also comments that I left out as the code is a generic example.