TwinCAT 3 fully supports Object-Oriented Programming (OOP). If you are encapsulating your machine parts inside custom Function Blocks (FBs), you shouldn’t rely on a global first scan bit to initialize them. Instead, use the built-in system method FB_init .
State machines are a staple of PLC programming. Using the first scan bit to set the initial state ensures a predictable and safe start-up.
: Declare a global or local boolean variable initialized to TRUE . beckhoff first scan bit
⚠️ This method can fail if the PLC is stopped/started without power cycle. Always prefer the system library method.
Method 3: Using the FB_init Method (TwinCAT 3 Object-Oriented Approach) TwinCAT 3 fully supports Object-Oriented Programming (OOP)
a global or local variable: bInitialized : BOOL := FALSE; Logic :
This system variable is automatically managed by the TwinCAT runtime. State machines are a staple of PLC programming
: Highly accurate; resets every time the PLC transitions from STOP to RUN or after a power cycle. Method 2: Manual Flag (The "Standard" Way)
Clear old error flags that might have occurred during the previous shutdown. Establish Communication:
If you are working on older legacy controllers (such as the Beckhoff CX1010 or CX1020 series) or maintaining an legacy system, you may find the global system array variable explicit: