<9. Advanced Engineering >
● Concept of Reference Position
The reference position used for determining relative heights of POUs within a code
worksheet differs between FBD and LD.
• Reference position for FBD
Within a code worksheet created using only FBD, the highest point of each
function, function block and NPAS POU, as indicated in the figure below, is used
as the reference position for deciding relative heights.
Functions, function blocks and NPAS POUs are executed from top to bottom
according to their reference positions.
POUs whose reference positions are at the same height are executed from left
to right.
• Reference position for LD
Within a code worksheet created using only LD, the reference position for
deciding relative heights is the lowest line connecting the left and right power
rails.
In the figure below, two LD logics are coded. It looks like the left LD is higher and
would be executed first but in terms of the reference position, which is the lowest
line connecting the power rails, the left LD is actually lower and is therefore
executed after the right LD.
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