FUP to SCL Fundamentals

What Changes From FUP

In FUP/FBD, the execution order is often visible through networks and signal lines. In SCL, the execution order is the order of the statements in the file.

xboA := TRUE;
xboB := xboA;
xboA := FALSE;

After this code runs, xboB is TRUE and xboA is FALSE. The second assignment used the value that existed at that point in the cycle.

Think in three layers:

FUP/FBD habit	SCL equivalent
Network with contacts and coils	Boolean expression and assignment
Set/reset coil	Explicit IF ... THEN ... ELSE logic
Function block box	FB call with named parameters
Data structure view	STRUCT, UDT, DB, or FB static variables
Network comments	Short comments and REGION sections

The PLC Scan

Most PCS logic runs cyclically:

1. Inputs and internal state already have a value at the start of the block call.
2. The block executes from top to bottom.
3. Assignments update variables immediately for the following statements in the same call.
4. Static variables keep their values for the next cycle.
5. Temporary variables are recalculated every call.

This matters because SCL is not a drawing. If a variable is assigned twice, the later assignment wins for the final value.

xboValveOpenCommand := xboOperatorRequest;

IF xboEmergencyStopActive THEN
    xboValveOpenCommand := FALSE;
END_IF;

This is a common and useful pattern: calculate the normal command first, then apply safety or interlock overrides later.

Block Interface

An SCL block usually has declaration sections before BEGIN.

FUNCTION_BLOCK "FB_SimplePumpControl"
{ S7_Optimized_Access := 'FALSE' }
VERSION : 0.1

VAR_INPUT
    xboStartRequest : Bool;
    xboStopRequest : Bool;
    xboPumpHealthy : Bool;
END_VAR

VAR_OUTPUT
    xboPumpRunCommand : Bool;
END_VAR

VAR
    _xboRunLatched : Bool;
END_VAR

BEGIN
    // Logic goes here.
END_FUNCTION_BLOCK

Section	Lifetime	Typical use
VAR_INPUT	Provided by caller	Commands, feedback, configuration inputs
VAR_OUTPUT	Written by block	Commands, status, diagnostics
VAR_IN_OUT	Shared with caller	Larger structures that are read and written
VAR	Static, remembered	State, latches, timers, filters
VAR_TEMP	Temporary, one call	Intermediate calculations

Do not store state in VAR_TEMP

A temporary variable is not a memory. Use VAR for values that must survive into the next PLC cycle, such as state numbers, latched alarms, and timer instances.