Note: This method is more robust because it relies on the system's own cycle counter rather than a variable you might accidentally overwrite elsewhere. Best Practices
IF FirstScan THEN // Clear receive/transmit buffers receiveBuffer := ''; sendBuffer := ''; // Reset bus coupler ETC_ClearDeviceState();
📡 Triggers a "Hello" or synchronization pulse to external devices, such as HMIs or SQL databases, to signal that the PLC is back online. Best Practices and Pitfalls beckhoff first scan bit
Method 1: The Standard Structured Text Pattern (Recommended)
This is particularly useful for initializing complex data structures, allocating memory with __NEW , or passing parameters to function blocks before they are used in the program. Note: This method is more robust because it
Without FirstScan , you face several classic initialization hazards:
Why it matters
Ensuring your sequences (SFC) start at "Step 0."
// Your main cyclic program logic continues here Without FirstScan , you face several classic initialization
// Obtain the index of the current task fbGetCurTaskIdx();
Be careful when trying to clear or initialize variables marked as RETAIN or PERSISTENT using a classic first scan bit. Persistent variables are designed to survive power cycles. If your first scan bit overwrites them every time the PLC boots, you defeat the purpose of persistent storage (like saving machine calibration offsets). 3. Execution Order Matters
