Configuration: CONFIGURATION, RESOURCE, TASK

The three layers

Layer	What it represents	Plural?
`CONFIGURATION`	One PLC system	One per project, typically
`RESOURCE`	One CPU within the PLC	Many — multi-CPU PLCs
`TASK`	One scheduling slot on a resource	Many per resource
`PROGRAM` instance	An instantiated PROGRAM POU bound to a task	Many per task (rare)

A PROGRAM POU type is just a class — it doesn’t run on its own. It runs only when a PROGRAM <name> WITH <task>; statement inside a RESOURCE block instantiates it and binds it to a task.

Syntax

CONFIGURATION MyConfig
    RESOURCE MainCPU ON CPU01
        TASK fastTask(INTERVAL := T#10ms, PRIORITY := 1);
        TASK slowTask(INTERVAL := T#1s,  PRIORITY := 5);

        PROGRAM ioCycle WITH fastTask : PLC_PRG();
        PROGRAM stats   WITH slowTask : PRG_Stats();
    END_RESOURCE
END_CONFIGURATION

This says:

One PLC, one CPU.
Two tasks: fastTask runs every 10 ms, slowTask every 1 second. Lower priority number = higher priority.
Two PROGRAM instances: ioCycle of type PLC_PRG runs on fastTask; stats of type PRG_Stats runs on slowTask.

TASK parameters

TASK <name>(INTERVAL := <time>, PRIORITY := <int>);
TASK <name>(SINGLE := <bool_var>, PRIORITY := <int>);

INTERVAL — periodic task; runs every N time units. Most common.
SINGLE — event-triggered task; runs once each time the named BOOL variable rises FALSE→TRUE. Used for interrupt-style work.
PRIORITY — integer 0..N where 0 is highest. The runtime scheduler preempts a lower-priority task when a higher- priority task becomes ready.

PROGRAM instances and the task scheduler

Each PROGRAM instance is a separate state-bearing incarnation of the PROGRAM POU type. Two instances of the same PROGRAM type bound to two different tasks have their own VAR storage, run their own timer FB instances etc.

In ForgeIEC, the configuration block is rarely written by hand — the editor manages it through the “Task Configuration” dialog and emits the IEC syntax at save-time. The MCP tasks.list_resources / tasks.write.create_task / tasks.write.add_program_instance tools cover the same ground programmatically.

Cycle time and budgeting

A periodic task with INTERVAL := T#20ms is expected to finish its work in less than 20 ms, every cycle, every time. If it doesn’t, the runtime flags an overrun and the behaviour is implementation-defined (some runtimes skip the next cycle, some signal a fault, some keep running with ever-growing lag).

Two consequences for ST code:

Loops in PROGRAM bodies must be bounded — see the iteration scan-cycle warning.
Timer FBs (TON, TOF, TP) tick at task-cycle granularity. A TON(PT := T#1ms) in a 20 ms task fires after one full task cycle, not after the nominal 1 ms.

VAR_GLOBAL access scope

VAR_GLOBAL declared inside a RESOURCE is scoped to that resource. VAR_GLOBAL at CONFIGURATION level is visible to all resources. Cross-resource sharing requires VAR_ACCESS declarations — rare and seldom needed in single-CPU PLC projects.

IEC reference

IEC 61131-3 third edition (2013), clause 6.8 — “Configuration and resource elements”.

matiec conformance

matiec implements the full configuration syntax and the priority-based scheduler model. ForgeIEC’s anvild runtime honours INTERVAL and PRIORITY on POSIX targets via pthread_setschedparam.

The SINGLE (event-triggered) task variant is supported by matiec but uncommon in ForgeIEC projects — most production work uses INTERVAL cyclic tasks.