SCANWAIT:    JNB   FLG_SCAN, $            ; Wait for 50ms tick from ISR. 
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...

WAITLUP:     CLR   FLG_SYSOK
             CALL  MSG_NOTOK              ; VFD message =SYSTEM NOT READY= 
             JNB   DIN_SYSOK, EXITSCAN    ; DIN_SYSOK will be set by input from PLC.  
             SETB  FLG_SYSOK              ; System OK to start.
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...
                    
WAIT4START:  JNB   FLG_SYSOK, EXITSCAN
             CALL  MSG_STRPB              ; VFD message =AWAITING START SIG.= 
             JNB   FLG_START, EXITSCAN    ; Await user  start command.

STARTSEQ:    (code for start cycle )
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EXITSCAN:    CLR   FLG_SCAN
             JMP   SCANWAIT               ; Go await next 50ms ticker from ISR.