Napoleonic Big Battles Rules' system for 2/6mm.
Regiment/brigade base units. Variable length bound and interesting CoS (Change of Situation) system. A CoS may be enemy forces appear, you are fired upon, you receive a message etc.
CoS happen at every level, so a CoS for a brigade is also a CoS for every higher level formation. The rules list 13 kinds of CoS and provide for each a set of possible responses.
Responses include sending a message, taking defensive action etc.
Movement is simultaneous. There is no "movement phase". Players simply begin acting on orders until a COS occurs.
Free to Download.
Peter Dennis