Beckhoff First Scan Bit -
Let me know in the comments — I’ve debugged many tricky startup issues and can help!
The First Scan Bit is a digital output that is automatically set by the PLC during its startup sequence. When the PLC is powered on or reset, it executes a series of internal checks and initializations before starting to execute the user program. During this first scan cycle, the First Scan Bit is set to TRUE (or 1).
For a machine to operate safely, it must always start from a well-defined "home" state. The first scan bit is the ideal place to force a state machine to begin at its initial state, such as ST_IDLE or ST_STOPPED . This prevents the machine from trying to resume a partially completed operation after a power cycle, which could be hazardous. beckhoff first scan bit
IF bFirstScan THEN // Perform initialization (e.g., setting default values) END_IF
TwinCAT provides system variables via its standard libraries that can detect system startup conditions. Implementation Let me know in the comments — I’ve
To use the First Scan Bit in a Beckhoff PLC, you need to access the PLC's system variables. The First Scan Bit is typically represented by a specific system variable, such as FirstScan or InitDone .
Understanding and Implementing the Beckhoff First Scan Bit in TwinCAT During this first scan cycle, the First Scan
The most straightforward way is to declare a global variable that resets itself after the first cycle.
Without a initialization bit, your PLC logic simply resumes from its last state or starts with default values that might not be appropriate for a running machine. Common use cases include:
The first scan bit is a versatile tool used in nearly every professional PLC project.