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