Architecture

plcopen is an embeddable C++17 motion-control kernel. Its production core is organized as an inward-only dependency ladder, with support libraries and two outer consumer surfaces.

Layer map

Layer	Responsibility
L0 rt	Bounded real-time primitives, queues, time, and errors
L1 otg	Jerk-limited online trajectory generation
L2 geom	Geometry, paths, splines, and frames
L3 plan	Look-ahead and blending
L4 exec	Cyclic sampling and execution
L5 axis	Axis and group state machines
L6 fb	PLCopen-style function-block facades
L7 adapters	Narrow hardware and protocol adapters

kin and stream are support libraries consumed by L5. dyn is a caller-owned pure-math support library for fixed-base RNEA. The IEC 61131-3 st compiler and VM form an outer consumer surface beside L7.

Load-bearing invariants

• Dependencies point inward; the production graph is a DAG.
• L0–L4 plus kin, stream, and dyn contain no PLCopen semantics.
• The planning domain owns planning state and commits trajectory frames.
• The real-time domain consumes one committed frame per tick.
• Hardware access stays behind the narrow Servo boundary.

These rules matter more than the visual layer numbers. A layer number does not imply that every layer includes the one immediately below it.

Canonical design record

The complete diagrams, current implementation cross-check, and invariant table are maintained in the canonical architecture document. Architecture rationale and rejected alternatives are organized in the architecture-decision guide, with each entry linked to its canonical ADR.