IEC 61499 is often presented as a more modular and distributed alternative to IEC 61131-3, but empirical comparisons usually conflate the effect of the standard with that of the underlying design, pitting a modular IEC 61499 application against a monolithic IEC 61131-3 baseline. We introduce a controlled, artifact-level method that separates the two. For two reference problems—a multi-floor elevator and an ISA-88 batch dosing unit—we build three functionally equivalent implementations: a monolithic IEC 61131-3 program ( \(I_L\) ), a modular IEC 61131-3 design ( \(I_M\) ) in the same engineering tool, and an IEC 61499 design ( \(I_E\) ). The step \(I_L\rightarrow I_M\) isolates modular design; \(I_M\rightarrow I_E\) evaluates the additional effect of adopting IEC 61499. Each version undergoes two classes of change—a parametric extension and a cross-cutting modification—measured by a reproducible canonical-diff protocol, without physical hardware. Shared-state coupling between functional units drops from 6 and 2 variables in \(I_L\) to 0 in both modular designs, yet change impact differs by at most one unit between \(I_M\) and \(I_E\) , and the monolith is least affected in two of the four changes. For the studied cases, source-level maintainability is driven mainly by modular design rather than by the standard alone, and depends on the shape of the change. The contribution is a reproducible method for separating design effects from standard effects, not a claim of IEC 61499 superiority.
