1. An automation control system comprising: a sink controller having a network variable input; a plurality of source controllers, at least two of which have a network variable output coupled to the network variable input of the sink controller to provide an update value to the network variable input; and a data structure in the sink controller for storing update values received from the network variable outputs of the source controllers that are coupled to the network variable input so the update values for each source controller are persistently stored in the sink controller with reference to the source controller that provided the update value.