Patent ID: 8255086

Claim:
A method of using a heartbeat in an HVAC network that includes a first subnet controller and a second subnet controller on a subnet, comprising: said first subnet controller providing a heartbeat message via said network; said second subnet controller in an inactive state determining if said first subnet controller has provided said heartbeat message within a first specified time period; said second subnet controller remaining in said inactive state in the event that said first subnet controller provides said heartbeat message within said first specified time period; said second subnet controller switching to an active state in the event that said first subnet controller does not provide said heartbeat message within said first specified time period; and a device on said subnet determining the presence or absence of said heartbeat within a second specified time period; wherein: in the event that said device detects said heartbeat message within said second specified time period, said device continues to operate in a prior mode; and in the event that said device fails to detect said heartbeat message within said second specified time period, said device a) issues a heartbeat-message-missing alarm; b) ceases to operate in a communication mode; and c) continues to generate device status messages via said subnet.