Function and FB calls

Structured Text has two different call mechanisms that are easy to confuse and that produce two of the most common beginner-and-LLM compile errors. Distinguishing them before writing any call is the single most useful habit you can build:

Aspect	Function (FUN)	Function Block (FB)
Has internal state?	No — pure function	Yes — instance carries state across cycles
Declared with	`FUNCTION foo : RETURN_TYPE`	`FUNCTION_BLOCK foo`
Used in code by	direct call: `foo(IN := x)`	first declare instance, then call the instance: `foo_inst(IN := x)`
Returns	a value via the function name	values via output pins of the instance
Example	`ABS(-3) → 3`, `SQRT(2.0) → 1.41…`	`TON`, `CTU`, `R_TRIG`, `RS`, `SR` and any user-written `FUNCTION_BLOCK`
Storage	none — call disappears after the cycle	one struct per declared instance, lives for the program’s life

Quick decision tree

Does the operation need to remember something between one scan cycle and the next (an elapsed time, a counter value, a previous edge)? → It is a Function Block. Declare an instance variable first; then call the instance.
Otherwise it is probably a Function. Call it directly.
If you don’t know which one a name is, look it up: library.read_block("name") — the response carries kind: "FB" or kind: "FUN".

Sub-pages

FB instances vs. function calls — the canonical pattern for declaring an FB instance, plus the most common LLM error “X is a Function Block — declare instance first” and its one-line fix via project.write.add_variable.
Positional vs. named parameters — both forms are legal IEC 61131-3 but the rules for which is allowed where (functions vs FBs, libraries vs project) are subtle and produce “has no declared parameter list” errors when mixed up.

ForgeIEC notes

The library catalogue (library.list_blocks MCP tool) marks every entry as either FB or FUN; never guess based on the name alone. A user-written FB_MANUAL_TIME and a built-in TON are both FBs; a user-written FC_GetKcValue and a built-in ABS are both functions. The FB_ / FC_ prefix is project convention only.
The MCP tool project.write.add_variable with iecType: "TON" (or any other FB type) creates the instance variable. The tool returns FORGE_ERR_FB_INSTANCE_AS_LOCAL when an FB instance is declared at the wrong scope (e.g. as a function-local in a FUNCTION POU, which has no persistent storage).