ST Error Check Assistant — CODESYS

A VS Code extension for validating Structured Text (ST / IEC 61131-3) code targeting CODESYS 3.5.18.

Features

• Real-time diagnostics in the Problems panel (red/yellow squiggles)
• On-save validation (configurable)
• Batch validation — all .st files in the workspace
• Parameter name completion when typing ( for Standard / Util function blocks outside VAR blocks
• Ctrl+Shift+K shortcut to validate the active file
• Configurable via VS Code settings

Commands

Checks Performed

Syntax

• Missing semicolons on assignments, function calls, and END_* closers
• = used instead of := for assignment
• IF without THEN; ELSE/ELSIF with trailing ;

Data types

• REAL/LREAL literal without decimal point (42 → should be 42.0)
• Variable declaration missing type annotation inside VAR blocks
• VAR CONSTANT without initialiser (:=)
• Unknown element type in ARRAY … OF <type>

Block structure

• Unmatched block openers/closers (across the whole document)
• Mismatched pairs (e.g. END_WHILE inside a FOR block)

Loops & control flow

• FOR loop missing :=, TO, or DO
• WHILE loop missing DO
• UNTIL missing trailing ;
• CASE missing OF

Function blocks

• Standard and Util FB / function parameter validation
• Standard FB calls missing required parameters (IN/PT, CU/CD/PV)
• PID call missing ACTUAL/SET_POINT
• Util.BLINK call missing ENABLE/EN, TIMELOW/TLOW, TIMEHIGH/THIGH, OUT/Q
• Exact parameter type strings preserved from workbook definitions (STRING (255), INT, TIME, etc.)
• Expected parameter style and type hints for common Standard and Util FBs (TIMELOW : TIME, OUT : BOOL, ACTUAL : REAL, etc.)
• ENABLE => used instead of EN/ENO

Examples of Issues Detected

(* Array bounds *)
myArray : ARRAY[5..0] OF INT;      (* ERROR: start > end *)

(* REAL literal *)
myVal : REAL := 42;                (* WARNING: should be 42.0 *)

(* FOR loop *)
FOR i = 0 TO 9 DO                  (* ERROR: use := not = *)
FOR i := 0 TO 9                    (* WARNING: missing DO *)

(* CASE *)
CASE myVar                          (* ERROR: missing OF *)

(* Constant *)
VAR CONSTANT pi : REAL; END_VAR    (* ERROR: missing := *)

(* Assignment *)
myVar = 5;                          (* WARNING: use := *)

(* Timer call *)
myTon(Q => result);                (* WARNING: missing IN and PT *)

(* Util.BLINK call *)
myBlink(ENABLE := start, TIMELOW := T#100ms, TIMEHIGH := T#200ms, OUT => led);
                                    (* WARNING: missing expected BLINK parameters or incorrect parameter styles *)

(* Block mismatch *)
IF cond THEN
  ...
END_FOR;                           (* ERROR: END_FOR closes FOR, not IF *)

Project Structure

.
├── src/
│   ├── extension.ts       # Extension entry point
│   └── validator.ts       # IEC 61131-3 / CODESYS validation logic
├── package.json           # Extension manifest
└── tsconfig.json          # TypeScript configuration

License

MIT

v0.0.3 improvements

• Numeric range validation for IEC integer types (USINT, UINT, SINT, INT, DINT, etc.).
• TIME literals now require units (ms, s, m, h, d).
• New workbook-driven Standard/Util library validation.
• Parameter name completion while typing ( outside VAR blocks for Standard and Util FBs.
• Improved Function Block parameter checks with exact type text from workbook definitions.
• Detects wrong use of => for FB inputs (expects :=).