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IEC 61131-3 ST — 30-minute journey

Use the built-in ST runtime when you want PLC-style sequencing on top of the same motion kernel. The host still owns memory, binding, and scan scheduling; the VM only executes the program.

In this journey you will compile one ST program, bind an AXIS_REF, drive MC_Power and MC_MoveAbsolute, then observe completion through a host-side symbol read. The exact host below is compiled and run in CI.

Requirements

  • C++17 compiler (GCC 9+, Clang 10+, MSVC 2022+)
  • CMake 3.21+
  • An installed plcopen package or a local build prefix

0–10 minutes: understand the ownership model

The ST runtime does not hide host responsibilities:

  • compile() is load-domain work and may allocate
  • scan() is cycle-domain work and follows the RT rules
  • the caller owns the aligned byte buffer passed to Instance::load()
  • AXIS_REF / GROUP_REF objects stay in the host and are bound explicitly

That separation is deliberate. It keeps RT semantics obvious and testable.

10–20 minutes: build the canonical ST consumer

Create a directory containing this CMakeLists.txt:

cmake_minimum_required(VERSION 3.21)
project(plcopen_st_journey LANGUAGES CXX)

find_package(plcopen CONFIG REQUIRED)
add_executable(st_motion st_motion.cpp)
target_compile_features(st_motion PRIVATE cxx_std_17)
target_link_libraries(st_motion PRIVATE plcopen::plcopen)

Copy this host program to st_motion.cpp:

--8<-- "core/demo/st_motion.cpp"

Configure, build, and run it. Replace /opt/plcopen with your install prefix:

cmake -S . -B build -DCMAKE_PREFIX_PATH=/opt/plcopen
cmake --build build --config Release
./build/st_motion

Exit code 0 means:

  • the ST source compiled
  • the program loaded into a caller-owned buffer
  • bind_axis("AxisX", &axis) succeeded
  • repeated scan() + axis.cycle() drove the axis to position 1.25

20–25 minutes: read the key boundary

The ST source embedded in the host uses per-cycle motion values:

  • Velocity := 1.0
  • Acceleration := 2.0
  • Deceleration := 2.0
  • Jerk := 10.0

Those values are interpreted as per-cycle quantities in the 1 kHz task passed to Instance::load(). If your human-facing config is in SI units, convert once in the host with rt::CycleConfig, then generate or inject the ST inputs accordingly.

25–30 minutes: decide when ST is the right surface

Choose ST when:

  • the machine logic should stay PLC-shaped
  • sequencing belongs with IEC 61131-3 function blocks
  • the hardware bridge still lives in a host application

Choose direct C++ when you want every motion call site under native code control. You can also mix both: ST for sequencing, C++ for adapters.

More detail