ken,
I'm not really that sharp on NFPA 72 ( I do have my 8 hrs to do in Dec) , but here it differes as to the occupancy. i.e.--multi-unit, school , etc.

For our multi-unit's it is a matter of new construction vs. fire code retrofit, as the existing stuff may not always get an interconnect.

In VT if we are using line voltage, it must be with battery BU, the systems must have notification, it gets deeper as U go...

Case in point, a recent inspection had the notification per third party alarm co. timed to a response ( NFPA 25...dunno...) which was not to the satisfaction of the local Fire Chief or State Fire Marshal.... [Linked Image]