By pairing a boolean variable defaulted to TRUE with an immediate reset to FALSE inside the code, you create an automatic, single-cycle trigger. Structured Text (ST) Implementation
One of the most critical uses is to reset latched alarms or safety circuits at startup. This prevents a machine from automatically restarting a potentially dangerous process after a power cycle.
// Check if this is the first cycle IF _TaskInfo[fbGetCurTaskIdx.index].firstCycle THEN // Place your one-time initialization logic here // For example: // - Clear global variables or states // - Set initial machine parameters // - Validate persistent memory // - Perform hardware checks // - Send start-up messages to an HMI END_IF
If initialization logic runs continuously instead of only on the first scan, it can cause severe runtime bugs: Overwriting live HMI setpoints with default values. Perpetually resetting runtime timers and counters. beckhoff first scan bit
PROGRAM MAIN VAR fbFirstScan : FB_FirstScan; bInitDone : BOOL; END_VAR
| Variable Type | Behavior on Power Cycle | Behavior on Program Download | Interaction with First Scan | |---|---|---|---| | | Re-initializes to declared start value | Re-initializes | First scan can set custom initial values | | RETAIN | Retains last value from before power loss | Retains last value (by default) | Useful for detecting first scan without losing value | | PERSISTENT | Retains value across program changes | Retains value across program downloads | Often used with first scan to determine if first run after major update |
In the world of Beckhoff TwinCAT and industrial automation, the "First Scan Bit" is a fundamental tool for ensuring your PLC starts in a predictable, safe state. If you’ve ever worked with Siemens (where it’s a system bit like FirstScan ) or Allen-Bradley (using the S:FS bit), you know how vital this is. By pairing a boolean variable defaulted to TRUE
If you have multiple programs ( MAIN , Safety , HMI ), ensure you know which program runs first to set the bFirstScan bit. It is usually best to handle initialization in the main task of your main program.
bFirstScan := FALSE; (* Set to false so this never runs again *)
PROGRAM MAIN VAR bFirstScan : BOOL := TRUE; // Initialize as TRUE bInitialized : BOOL; END_VAR // --- Logic --- IF bFirstScan THEN // Place all your initialization code here iSpeed := 100; bInitializeDone := TRUE; bFirstScan := FALSE; // Turn off after first run END_IF; // ... Rest of your program Use code with caution. Method B: Using PLC Attributes (TwinCAT 3 Best Practice) // Check if this is the first cycle
This ensures that regardless of any retained data from previous runs, the machine will always begin its operation logic from a defined starting point.
Ensuring your sequences (SFC) start at "Step 0."