Beckhoff First Scan Bit !!install!!
A common point of confusion during commissioning revolves around what actually triggers a "first scan" in TwinCAT. _TaskInfo[x].FirstCycle Behavior Custom Software Flag Behavior (Switching Config -> Run) Triggers TRUE Triggers TRUE Cold Reset / PLC Download Triggers TRUE Triggers TRUE Stop & Start PLC Code (Red Square / Green Triangle in VS) Remains FALSE Triggers TRUE (Variables re-initialize) Online Change (Modifying code on the fly) Remains FALSE Remains FALSE (Memory is preserved) The Stop/Start Runtime Trap
A standard Programmable Logic Controller (PLC) operates on a continuous loop: it reads physical inputs, executes user program logic, and updates physical outputs.
// In the main PLC program IF SystemTaskInfoArr[1].firstCycle THEN // Force the state machine to start in a known, safe state currentState := E_MachineState.STATE_HOMING; ELSE // Normal state transition logic CASE currentState OF // ... handle state transitions ... END_CASE END_IF
// Directly accessing the system array (often index 1 is the default PLC task) bFirstScan := SystemTaskInfoArr[1].firstCycle; Use code with caution.
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IF _TaskInfo[1].bFirstCycle THEN // Logic here only runs once InitialSetpoint := 50.0; SystemReady := FALSE; END_IF Use code with caution. Copied to clipboard Best Practices and Pitfalls The primary risk with first scan logic is dependency loops
But here’s the catch: unlike traditional PLCs (e.g., Siemens with OB100 or Rockwell with FirstScan ), Beckhoff’s approach is more flexible—but also more confusing for newcomers. This article will dissect every method to detect and utilize the first scan cycle in TwinCAT, from standard PLC libraries to advanced object-oriented techniques.
Implementing first-cycle logic ensures deterministic machine behaviors and prevents runtime faults. Beckhoff CX1010 first scan | PLCtalk - Interactive Q & A
: Setting non-persistent variables to a known starting state. beckhoff first scan bit
Creating a simple counter:
PROGRAM MAIN VAR bInit : BOOL := TRUE; // Our first scan flag bFirstScanDone : BOOL; fbFirstScan : F_TRIG; fbEcMaster : FB_EcCoEADsRead; // EtherCAT utility nState : INT; END_VAR
The first scan bit is a fundamental concept for writing robust and predictable PLC programs in Beckhoff TwinCAT. Whether you choose the system variable SystemTaskInfoArr[1].firstCycle for standard cold-start initialization or a custom RETAIN flag for more control, understanding this feature will help you build more reliable automation systems.
: Your initialization code depends on another part of the program that hasn't been processed yet. Common Cause : The execution order of POUs (Program Organization Units) within your task. Solution : Use the Task Configuration in TwinCAT to specify the exact execution order of your programs. Initialize critical systems in programs that execute first in the task cycle. A common point of confusion during commissioning revolves
IF NOT bInitDone THEN // Do startup logic here bInitDone := TRUE; END_IF Use code with caution. Copied to clipboard
VAR fbGetCurTaskIdx : GETCURTASKINDEX; // Function block to find the current task ID bFirstScan : BOOL; END_VAR fbGetCurTaskIdx(); // Call the FB // Access the system's internal task info array bFirstScan := _TaskInfo[fbGetCurTaskIdx.index].FirstCycle; IF bFirstScan THEN // Insert initialization logic here END_IF Use code with caution. Copied to clipboard
Populating operational variables with safe baseline data before the operator inputs custom values.
Remember: A well-implemented first scan routine separates unreliable prototypes from industrial-grade automation. Take the time to initialize deliberately—your machine's safe operation depends on it. handle state transitions