The same ST on every platform: SCL, Studio 5000, CODESYS, TwinCAT

One standard, many implementations

IEC 61131-3 describes the language, but every vendor builds its own compiler, library and editor. The result: the idea travels about 90%, the details need review. This line runs unchanged everywhere:

Pump := (Start OR Pump) AND NOT Stop AND Level < 95.0;

The line that calls a timer is the one that changes.

Siemens SCL in TIA Portal

Siemens calls the language SCL (Structured Control Language). Three things matter:

• A # in front of local variables: #Pump, #T1. Global tags go in quotes: "MotorDB".Running.
• The IEC timer is TON_TIME, and it needs an instance in the block's data.
• Optimized block access: memory has no fixed address, so you reach a variable by name and not by %MD20. Faster and safer, but absolute access is gone.

// Siemens SCL
#T1(IN := #Level >= 60.0, PT := T#3s);
#Soaked := #T1.Q;
IF #Soaked THEN
    #Valve := TRUE;
END_IF;

Note the semicolon after END_IF: Siemens wants it.

Rockwell Studio 5000 ST

Closest to standard ST in shape, furthest in timers:

• No #. Tag names are used directly, and . reaches a member of a UDT.
• A timer is not a standard function block but an instruction acting on a TIMER structure with PRE, ACC, DN, EN and TT. The preset is an integer in milliseconds, not a TIME.
• Reusable logic is wrapped in an AOI (Add-On Instruction), Rockwell's equivalent of FUNCTION_BLOCK.

(* Rockwell Studio 5000 *)
T1.PRE := 3000;
TONR(T1);
Soaked := T1.DN;
IF Soaked THEN
    Valve := 1;
END_IF;

Here T1.ACC is T1.ET and T1.DN is T1.Q. TONR does reset its accumulator when the enable drops, which is standard TON behaviour, but the member names differ, so code does not copy across literally.

CODESYS

The practical reference implementation. The Standard library gives you TON, TOF, TP and CTU with the member names from the standard, and TIME written as T#500ms. If your code runs in CODESYS without vendor tricks, it is about as portable as PLC code gets.

Beckhoff TwinCAT 3

Built on CODESYS, so everything above applies, plus two things:

• A Visual Studio environment, with a build process that feels like software engineering.
• Full OOP support: METHOD, PROPERTY, INTERFACE, EXTENDS. Advanced TwinCAT code therefore does not move to a controller that does not implement the third edition.

Schneider EcoStruxure / Machine Expert

Machine Expert is also CODESYS-based, so it behaves in the standard way. Older Control Expert (Unity Pro) has its own library and the %M / %MW direct addressing style, and needs more care.

Omron Sysmac Studio

Follows the standard unusually closely and uses names like TON. The practical difference is the variable table: everything is declared as Internal, Input, Output or External in one place, direct addresses carry a % prefix, and Retain is a checkbox per variable.

Mitsubishi GX Works3

ST is supported, but the platform culture is ladder and FBD. Native device names (D100, M0, T5) show up constantly in older code, and the traditional timer is a T device with a 100 ms time base. Standard TON is available but is not the convention in legacy projects.

What changes

What stays the same

• := for assignment and = for comparison.
• IF / ELSIF / ELSE / END_IF, CASE OF, FOR, WHILE.
• AND OR XOR NOT MOD, the arithmetic and the comparisons.
• The scan cycle, the last assignment winning, and a timer needing a call every scan.
• State machines, filters, scaling and alarm logic: they move with a rename.

A practical porting rule

Write the logic in your own names and push everything that touches a vendor library into small blocks. Porting then means rewriting the small blocks while 90% of the program stays put. That is also why the lab on this site is useful: you learn the core here and export to SCL when you need to.