If you have a case where you have to initialise a set of tags, flags or registers on your first run though the routine or rung, then you will have to use the first scan flag or first scan status flag. It will be energized on the first normal scan of the routines in the current program. I have used it in many capacity during the system initialiation or power loss situation. It can also be used when the key switch status changes in the PLC.

The First Scan Flag will be triggered after the following process upsets.

  • Change CPU mode using key switch; for example key were to be changed from Run to Program and Run mode again (see illistration below).
  • Power loss
Xybernetics Allen-Bradley CPU Key Switch

How To Use It For PLC5 and SLC 500 Processors
In the context of PLC5 and Micrologix (using RSLogix 5 and RSLogix 500 respectively), you will be using the status file of “S:1/15”. It is accessible via Data Files — S2 – Status (from the Project Properties tree). See screenshot below.

Xybernetics Allen-Bradley First Scan Status File For PLC5 and MicroLogix SLC500 in RSLogix 5 and RSLogix 500

This is typically how you would use the first scan status file (S:1/15) in your logic (in both RSLogix 5 and RSLogix 500).
Xybernetics Allen-Bradley First Scan Status File How To Use It In Logic Example

How To Use It For Logix Processors
S:FS is a first scan flag for Logix processors. It only energizes during the first normal scan of the routines in the current program.
This first scan flag works on all Logix processors such as listed below.

  • 1756 ControlLogix
  • 1756 GuardLogix
  • 1769 CompactLogix
  • 1769 Compact GuardLogix
  • 1789 SoftLogix
  • 5069 CompactLogix
  • 5069 Compact GuardLogix
  • Micrologix

But keep in mind that there are 2 software packages in Rockwell (RSLogix 5000 and Studio 5000. Rockwell RSLogix 5000 can support for CPU with firmware of up until version 20, and if your have CPU firmware of 21 and above you will have to use Studio 5000.

If you are interested, here are additional Access Runtime Controller Configuration and Status from the same family as the First Scan flag.
S:N Negative flag
S:Z Zero flag
S:V Overflow flag
S:C Carry flag
S:MINOR Minor Fault flag

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