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Is there a First Scan flag in the IEC PLC

Products: ACR9600,9630,9640

The IEC PLC does not have a built-in first scan bit. Firmware version 2.30 adds a function that provides a one-shot output based on the first scan of the PLC program: ACR_First_Scan. 

When the EN input is TRUE, the Q output will be TRUE for the initial scan of the PLC program. After the first scan is complete, Q will revert to false. 


Prior to firmware update 2.30,there are a few ways this functionality can be programmed.

The IEC PLC System flag Bit16909 indicates that tasks are running. A rising  edge trigger can be tied to this flag as shown below. Place this in the first rung of the program, then use the output Q from the R_Trig FB. Q will only stay one for one scan.

A user FB is attached that reads this flag and could be used in place of the code above.


updated 15-Jul-2014  jw