Overview¶

The IronPLC runtime is a virtual machine that executes compiled IEC 61131-3 programs. It reads bytecode container (.iplc) files produced by the ironplcc compiler and runs them according to the task configuration defined in the source program.

Execution Model¶

The runtime follows the IEC 61131-3 execution model. A compiled program contains one or more tasks, each with an associated interval and priority. The runtime runs a continuous loop called the scan cycle. Each pass through the loop is one scheduling round.

On each round, the runtime reads a monotonic clock, collects all tasks whose interval has elapsed, sorts them by priority (0 is highest), and executes them in order. Within a task, program instances run in the order they are declared in the source code. All tasks execute cooperatively in a single thread — the runtime does not preempt a running task.

If a task takes longer than its interval, the runtime skips the missed cycle and realigns the next-due time forward. This prevents a cascade of back-to-back executions. The runtime tracks an overrun counter for each task so you can detect when this happens.

Each task can also have a watchdog timeout. If a single execution exceeds the watchdog duration, the runtime triggers a fault and the VM stops. Unlike an overrun (which skips a cycle and continues), a watchdog timeout halts execution to protect against runaway logic.

By default, ironplcvm runs continuously, executing scheduling rounds until interrupted with Ctrl+C. You can also limit execution to a fixed number of rounds using the --scans option.

For a detailed explanation of the scan cycle, overruns, watchdog behavior, and the VM lifecycle, see Execution Cycle.

Variable Inspection¶

The runtime can dump all variable values to a file after execution completes. This is useful for verifying program behavior and debugging. Use the --dump-vars option to specify an output file.

See the ironplcvm command reference for complete usage details.