Either one should work. Personally, I prefer to avoid using Interrupts unless absolutely necessary.
The potential weakness of the "polling" method in the main SPS Loop is that, if your SPS was very large, had many branches, or contained "halting" commands like WAIT or WAIT FOR, the check for the Input could be delayed. However, a small, tidy SPS should have no trouble keeping up.
either one could be made to work...
the first one can work if program is properly structured. but i would say just be careful and avoid situations that may be difficult to debug or gracefully recover from. and why it needs to be global when SPS.SUB is the top level program? in this case, this only adds restrictions on other things like RESUME.
the second one is not really ok. entire loop only processes YOUR code. any techpackages or SPS code normally there will not be processed.
"But without WAIT in SPS. Only timer. Which solution would be appropriate?"
so if you need timer, then just ... use timer!
perhaps:
IF WATER_PRESSURE_MISSING THEN
$TIMER_STOP[1]=$TIMER[1]>=3000
IF $TIMER_STOP[1] THEN
; alarm, really need that water...
ELSE
; warning, low on water...
END
ELSE
$TIMER_STOP[1]=TRUE
$TIMER[1]=0
ENDIF 
